Entomology 3rd edition - C.Gillott - Chapter 16 potx

16
F
ood Uptake and Utilizatio
n
1
. Intr
oduc
t
ion
I
nsects feed on a wide ran
g
eofor
g
anic materials. About 7
5
% of all species are ph
y-
t
opha
g
ous, and these form an important link in the transfer of ener
gy
from primar
y
produc-
ers to second-order consumers. Others are carnivorous, omnivorous, or
p
arasitic on othe
r
an

i
ma
l
s. In accor
d
w
i
t
h
t
h
e
di
vers
i
ty o
ff
ee
di
ng
h
a
bi
ts, t
h
e means
b
yw
hi
c

hi
nsects
l
ocate
th
e
i
r
f
oo
d
,t
h
e structure an
d
p
h
ys
i
o
l
ogy o
f
t
h
e
i
r
di
gest

i
ve system, an
d
t
h
e
i
r meta
b
o
li
sm ar
e
highly
var
i
e
d
.
The feedin
g
habits of insects take on special si
g
nificance for humans, on the one hand
,
b
ecause of the enormous dama
g
e that feedin
g

insects do to our food, clothin
g
, and health,
an
d
,ont
h
eot
h
er,
b
ecause o
f
t
h
e mass
i
ve
b
ene
fi
ts t
h
at
i
nsects
p
rov
id
eas

pl
ant
p
o
lli
nators
d
ur
i
ng t
h
e
i
r searc
hf
or
f
oo
d
(see a
l
so C
h
apter 24). In a
ddi
t
i
on,
b
ecause many spec

i
es ar
e
eas
ily
an
d
c
h
eap
ly
mass-cu
l
ture
di
nt
h
e
l
a
b
orator
y
,t
h
e
yh
ave
b
een use

d
w
id
e
ly i
n researc
h
on di
g
estion and absorption, as well as in the elucidation of basic biochemical pathwa
y
s,
t
he role of s
p
ecific nutrients, and other as
p
ects of animal metabolism
.
2. Food
S
election and Feedin
g
D
i
st
i
nct v
i
sua

l
,c
h
em
i
ca
l
,an
d
mec
h
an
i
ca
l
cues act at eac
h
step o
f
t
h
e
f
oo
dl
ocat
i
on
and in
g

estion process. These steps include attraction to food, arrest of movement, tastin
g
,
b
itin
g
, further tastin
g
as in
g
estion be
g
ins, continued in
g
estion, and termination of feedin
g
.
T
he sensitivity of the insect to these cues varies with its physiological state. For example
,
a starve
di
nsect may
b
ecome
hi
g
hl
y sens
i

t
i
ve to o
d
ors or tastes assoc
i
ate
d
w
i
t
hi
ts norma
l
f
oo
d
,an
di
n extreme cases may
b
ecome qu
i
te
i
n
di
scr
i
m

i
nate
i
n terms o
f
w
h
at
i
t
i
ngests.
On the other hand, a female whose abdomen is full of e
gg
s is normall
y
“uninterested” in
feedin
g.
I
n some plant-feeding (phytophagous) species, visual stimuli such as particular pat
-
t
erns (espec
i
a
ll
y str
i
pes) or co

l
ors may serve to
i
n
i
t
i
a
ll
y attract an
i
nsect to a potent
i
a
lf
oo
d
source. Usua
ll
y,
h
owever, t
h
e
i
n
i
t
i
a

l
or
i
entat
i
on, w
h
ere t
hi
s occurs,
i
s
d
epen
d
ent on o
lf
ac-
t
or
y
stimuli. In man
y
larval forms there appear to be no specific orientin
g
stimuli because,
u
nder normal circumstances, larvae remain on the food plant selected b
y
the mother prior

4
8
7
488
CHAPTER 1
6
to oviposition. In the mi
g
rator
y
locust, on which much work has been done, olfaction is of
primar
y
importance in food location. Once the insect makes contact with the ve
g
etation,
tarsal chemosensilla initiate a reflex that results in the stoppa
g
e of movement. Sensilla on
t
h
e
l
a
bi
a
l
an
d
max

ill
ary pa
l
ps t
h
en taste t
h
e sur
f
ace waxes o
f
t
h
ep
l
ant, a
f
ter w
hi
c
h
t
h
e
l
ocus
t
ta
k
es a sma

ll bi
te. W
h
et
h
er
f
ee
di
ng cont
i
nues
i
s somet
i
mes
d
eterm
i
ne
db
y mec
h
anosens
ill
ar
responses to p
hy
s
i

ca
l
st
i
mu
li
suc
h
as t
h
e
h
ar
d
ness, tou
gh
ness, s
h
ape, an
dh
a
i
r
i
ness o
f
t
he
f
ood. More commonl

y
, it is substances in the released sap that, b
y
stimulatin
g
chemosensilla
i
n the cibarial cavit
y
,re
g
ulate the continuation or arrest of feedin
g
(Chapman, 2003). These
su
b
stances are ca
ll
e
d
“p
h
agost
i
mu
l
ants” or “
d
eterrents,” respect
i

ve
l
y. T
h
esu
b
stances may
h
ave nutr
i
t
i
ona
l
va
l
ue to t
h
e
i
nsect or may
b
e nutr
i
t
i
ona
ll
yun
i

mportant (“to
k
en st
i
mu
li
”).
Nutritional factors are almost alwa
y
s stimulatin
g
in effect. Su
g
ars, especiall
y
sucrose, ar
e
i
mportant pha
g
ostimulants for most ph
y
topha
g
ous insects. Amino acids, in contrast, are
g
en
-
e
rall

y
b
y
themselves weakl
y
stimulatin
g
or non-stimulatin
g
, thou
g
hma
y
act s
y
ner
g
isticall
y
w
ith certain sugars or token stimuli. For example, Heron (1965) showed in the spruce bud
-
w
orm
(
C
h
oristoneura
f
umi

f
eran
a
)t
h
at, w
h
ereas sucrose an
d
l
-pro
li
ne
i
n
l
ow concentrat
i
on
w
ere
i
n
di
v
id
ua
lly
on
ly

wea
k
p
h
a
g
ost
i
mu
l
ants, a m
i
xture o
f
t
h
etwosu
b
stances was
highly
stimulatin
g
. In addition to su
g
ars and amino acids, other specific nutrients ma
y
stimulat
e
f
eedin

g
in a
g
iven species. Such nutrients include vitamins, phospholipids, and steroids.
T
o
ke
nst
i
mu
li
may e
i
t
h
er st
i
mu
l
ate or
i
n
hibi
t
f
ee
di
ng. T
h
us,

d
er
i
vat
i
ves o
f
mustar
d
o
il,
p
ro
d
uce
db
y cruc
if
erous p
l
ants,
i
nc
l
u
di
ng ca
bb
age an
di

ts re
l
at
i
ves, are
i
mportant p
h
agos-
t
i
mu
l
ants
f
oravar
i
et
y
o
fi
nsects t
h
at norma
lly f
ee
d
on t
h
ese p

l
ants,
f
or examp
l
e,
l
arva
e
o
f the diamondback moth
(
Plutella xylostella), the cabba
g
e aphid
(
B
revicoryne brassicae
)
,
and the mustard beetle
(
Phaedon cochlearia
e
)
. Indeed
,
Plutella
w
ill feed naturall

y
onl
y
on
p
lants that contain mustard oil compounds. Many secondary plant metabolites, includin
g
a
lk
a
l
o
id
s, terpeno
id
s, p
h
eno
li
cs, an
d
g
l
ycos
id
es, are
f
ee
di
ng

d
eterrents
f
or p
h
ytop
h
agou
s
i
nsects. In a g
i
ven
f
oo
d
source t
h
ere w
ill
pro
b
a
bl
y
b
eam
i
xture o
f

p
h
agost
i
mu
l
ants an
d
deterrents, and the balance of this sensor
y
input, inte
g
rated throu
g
h the central nervou
s
sy
stem, determines the overall palatabilit
y
of the food
.
Species whose choice of food is limited are said to be oligophagous. In extreme cases, an
i
nsect may
b
e restr
i
cte
d
to

f
ee
di
ng on a s
i
ng
l
ep
l
ant spec
i
es an
di
s
d
escr
ib
e
d
as monop
h
agous.
S
pec
i
es t
h
at may
f
ee

d
onaw
id
evar
i
ety o
f
p
l
ants are po
l
yp
h
agous, t
h
oug
hi
t must
b
e note
d
that even these exhibit selectivit
y
when
g
iven a choice. Not surprisin
g
l
y
, monopha

g
ous
and oli
g
opha
g
ous species are especiall
y
sensitive to the presence of deterrents in non-host
p
lants.
In many pre
d
aceous
i
nsects, espec
i
a
ll
yt
h
ose t
h
at act
i
ve
l
y pursue prey, v
i
s

i
on
i
so
f
p
r
i
mary
i
mportance
i
n
l
ocat
i
ng an
d
captur
i
ng
f
oo
d
. As note
di
nC
h
apter 12 (Sect
i

on 7.1.2)
,
s
ome pre
d
aceous
i
nsects
h
ave
bi
nocu
l
ar v
i
s
i
on t
h
at ena
bl
es t
h
em to
d
eterm
i
ne w
h
en pre

yi
s
within catchin
g
distance. Carnivorous species, especiall
y
larval forms, whose visual sens
e
i
s less well developed, depend on chemical or tactile stimuli to find pre
y
. For example
,
m
any
b
eet
l
e
l
arvae t
h
at
li
ve on or
i
nt
h
e groun
dl

ocate prey
b
yt
h
e
i
r scent. Spec
i
es paras
i
t
i
c
o
not
h
er an
i
ma
l
s usua
ll
y
l
ocate a
h
ost
b
y
i

ts scent, t
h
oug
h
tsetse
fli
es may
i
n
i
t
i
a
ll
yor
i
ent
by
v
i
sua
l
means to a potent
i
a
lh
ost. For man
y
spec
i

es t
h
at
f
ee
d
on t
h
e
bl
oo
d
o
fbi
r
d
san
d
m
ammals, temperature and/or humidit
yg
radients are important in determinin
g
the precis
e
l
ocation at which an insect ali
g
hts on a host and be
g

ins to feed
.
T
he extent of food specificity for carnivorous insects is varied. Many insects are quite
n
on-spec
ifi
can
d
w
ill
attempt to capture an
d
eat any organ
i
sm t
h
at
f
a
ll
sw
i
t
hi
nag
i
ven s
i
ze

r
ange (even to t
h
e extent o
fb
e
i
ng cann
ib
a
li
st
i
c). Ot
h
ers are more se
l
ect
i
ve;
f
or examp
l
e
,
s
pider wasps (Pompilidae), as their name indicates, capture onl
y
spiders for provisionin
g

489
FOO
D
U
PT
A
KE
A
ND
UTILIZATION
t
heir nest. Parasitic insects, too, exhibit various de
g
rees of host specificit
y
. Thus, cer
-
t
ain sarcopha
g
id flies parasitize a ran
g
eof
g
rasshopper species; the common cattle
g
rub
(
H
ypoderma lineatum)ist

y
picall
y
found on cattle or bison, rarel
y
on horses and humans;
li
ce are extreme
l
y
h
ost-spec
ifi
c, as wou
ld b
e expecte
d
o
f
se
d
entary spec
i
es
.
T
h
e term
i
nat

i
on o
ff
ee
di
ng (assum
i
ng t
h
at
f
oo
d
supp
l
y
i
s not
li
m
i
t
i
ng)
i
s
l
arge
l
yre

l
ate
d
t
o the amount of food in
g
ested and the stimulation of strate
g
icall
y
located stretch receptors.
I
n locusts, for example, the fillin
g
of the crop is measured b
y
receptors at the anterio
r
end that send si
g
nals to the brain via the stomato
g
astric s
y
stem. In flies both crop- an
d
esop
h
agus-
filli

ng are
i
mportant
i
n
b
r
i
ng
i
ng
f
ee
di
ng to a c
l
ose, w
hil
e
i
n
f
ema
l
e mosqu
i
toe
s
an
d

, pro
b
a
bl
y
,
Rh
o
d
nius
p
ro
l
ixu
s
th
es
i
gna
l
sar
i
se
f
rom stretc
h
receptors
l
ocate
di

nt
he
abdominal wall, which is
g
reatl
y
distended after a blood meal. Other factors that ma
y
pla
y
a minor role in terminatin
g
feedin
g
are adaptation of the chemosensilla on the mouthpart
s
and a chan
g
e in the osmotic pressure or composition of the hemol
y
mph as absorption of
d
igested materials occurs (see chapters in Chapman and de Boer, 199
5
).
Apart
f
rom t
h
e spec

ifi
c cues out
li
ne
d
a
b
ove t
h
at
f
ac
ili
tate
l
ocat
i
on an
d
se
l
ect
i
on o
f
f
oo
d
,t
h

ere are ot
h
er
f
actors t
h
at
i
n
fl
uence
f
ee
di
n
g
act
i
v
i
t
y
.T
y
p
i
ca
lly
,
i

nsects
d
o not
f
ee
d
shortl
y
before and after a molt, or when there are mature e
gg
s in the abdomen. In addition
,
a
diurnal rh
y
thm of feedin
g
activit
y
ma
y
occur, in response to a specific li
g
ht, temperature
,
or
h
um
idi
ty st

i
mu
l
us. For examp
l
e, t
h
ere
dl
ocust (
N
oma
d
acris septem
f
asciata)
f
ee
d
s
i
n
th
e morn
i
ng an
d
even
i
ng, an

d
many mosqu
i
toes
f
ee
dd
ur
i
ng t
h
e ear
l
yeven
i
ng (t
h
oug
h
thi
sma
y
c
h
an
g
e
i
n
diff

erent
h
a
bi
tats). Pupae, most
di
apaus
i
n
gi
nsects, an
d
some a
d
u
l
t
E
phemeroptera, Lepidoptera, and Diptera do not feed
.
3
. The Alimentar
y
S
y
stem
The
g
ut and its associated
g

lands (Fi
g
ure 1
6
.1) triturate, lubricate, store, di
g
est, and
absorb food material and expel the undi
g
ested remains. Structural differences throu
g
hout the
system reflect regional specialization for performance of these functions and are correlate
d
a
l
so w
i
t
hf
ee
di
ng
h
a
bi
ts an
d
t
h

e nature o
f
norma
lf
oo
d
mater
i
a
l
.T
h
e structure o
f
t
h
e system
ma
y
var
y
at
diff
erent sta
g
es o
f
t
h
e

lif
e
hi
stor
yb
ecause o
f
t
h
e
diff
erent
f
ee
di
n
gh
a
bi
ts o
f
t
he larva and adult of a species. The
g
ut normall
y
occurs as a continuous tube betwee
n
t
he mouth and anus, and its len

g
th is broadl
y
correlated with feedin
g
habits, bein
g
short
in carnivorous forms where digestion and absorption occur relatively rapidly, and longer
(o
f
ten convo
l
ute
d
)
i
np
h
ytop
h
agous
f
orms. In a
f
ew spec
i
es t
h
at

f
ee
d
on
fl
u
id
s, suc
h
a
s
l
arvae o
f
Neuroptera an
d
Hymenoptera-Apocr
i
ta, an
d
some a
d
u
l
t Heteroptera t
h
ere
i
s
li

tt
le
or no solid waste in the food, and the
j
unction between the mid
g
ut and hind
g
ut is occluded.
As Fi
g
ure 16.1 indicates, food first enters the buccal cavit
y
, which is enclosed b
y
the
mouthparts and is not strictly part of the gut. It is into the buccal cavity that the salivar
y
g
l
an
d
sre
l
ease t
h
e
i
r pro
d

ucts. T
h
e gut proper compr
i
ses t
h
ree ma
i
nreg
i
ons: t
h
e
f
oregut,
i
nw
hi
c
h
t
h
e
f
oo
d
may
b
e store
d

,
fil
tere
d
,an
d
part
i
a
ll
y
di
geste
d
;t
h
em
id
gut, w
hi
c
hi
st
h
e
pr
i
mar
y
s

i
te
f
or
dig
est
i
on an
d
a
b
sorpt
i
on o
ff
oo
d
;an
d
t
h
e
hi
n
dg
ut, w
h
ere some a
b
sorpt

i
o
n
and feces formation occur
.
3.1. Salivar
y
Glands
S
a
li
var
ygl
an
d
s are present
i
n most
i
nsects, t
h
ou
gh
t
h
e
i
r
f
orm an

df
unct
i
on are extreme
ly
varied, and the
y
ma
y
or ma
y
not be innervated (Ribeiro, 1995). Frequentl
y
the
y
are known
490
CHAPTER 1
6
F
I
GU
RE 16.1. Alimentary canal and associated structures of a locust. [After C. Hodge, 1939, The anatomy an
d
hi
sto
l
ogy o
f
t

h
ea
li
mentary tract o
f
Locusta migratoria L. (Ort
h
optera: Acr
idid
ae)
,
J.
Morp
h
o
l
.
64
:
37
5
–399. By
p
erm
i
ss
i
on o
f
t

h
eW
i
star Press.
]
b
yot
h
er names accor
di
ng to e
i
t
h
er t
h
es
i
te at w
hi
c
h
t
h
e
i
r
d
uct enters t
h

e
b
ucca
l
cav
i
ty,
f
o
r
e
xample, labial
g
lands and mandibular
g
lands, or their function, for example, silk
g
lands
and venom
g
lands
.
T
ypically, saliva is a watery, enzyme-containing fluid that serves to lubricate the
f
oo
d
an
di
n

i
t
i
ate
i
ts
di
gest
i
on. L
ik
et
h
at o
fh
umans, t
h
esa
li
va genera
ll
y conta
i
ns on
l
y
c
ar
b
o

h
y
d
rate-
di
gest
i
ng enzymes (amy
l
ase an
di
nvertase), t
h
oug
h
t
h
ere are except
i
ons t
o
this statement. For example, the saliva of some carnivorous species contains protein- and/o
r
f
at-di
g
estin
g
enz
y

mes onl
y
; that of bloodsuckin
g
species has no enz
y
mes. In termite saliva
there are cellulose-di
g
estin
g
enz
y
mes: a
β
-1-4-
g
lucanase that brin
g
s about the initial split-
t
i
ng o
f
t
h
epo
l
ymer, an
d

β
-g
l
ucos
id
ase t
h
at
d
egra
d
es t
h
e resu
l
t
i
ng ce
ll
o
bi
ose to g
l
ucose
(
Na
k
as
hi
ma et a

l.
,
2002
;
To
k
u
d
a
e
ta
l
,
2002)
.
(See a
l
so Sect
i
on 4.2.4.
)
In t
h
e
i
nnervate
dgl
an
d
so

f
coc
k
roac
h
es an
dl
ocusts, re
l
ease o
f
sa
li
va
i
s
i
n
d
uce
d
w
h
e
n
f
ood stimulates mechano- and chemosensilla on the mouth
p
arts and antennae. The informa-
tion travels to the subesopha

g
eal
g
an
g
lion and then alon
g
aminer
g
ic or peptider
g
ic neuron
s
to t
h
eg
l
an
d
sw
h
ere
i
t
i
n
d
uces re
l
axat

i
on o
f
t
h
e musc
l
es t
h
at norma
ll
yc
l
ose o
ff
t
h
e open
i
ng
of
t
h
esa
li
vary g
l
an
dd
uct (A

li
, 1997). In contrast, t
h
e non-
i
nnervate
d
g
l
an
d
so
f
Ca
ll
i
ph
ora
er
y
t
h
rocep
h
a
la
a
re st
i
mu

l
ate
d
to re
l
ease sa
li
va
by
a
h
emo
ly
mp
hf
actor, poss
ibly
seroton
i
n
(
Trimmer, 1985
).
491
FOO
D
U
PT
A
KE

A
ND
UTILIZ
A
TION
Other substances that ma
y
occur in saliva, thou
g
h havin
g
no direct role in di
g
estion, ar
e
im
p
ortant in food ac
q
uisition. For exam
p
le, the saliva of a
p
hids has a viscous com
p
onent,
released durin
g
penetration of the st
y

lets, which hardens to form a leakproof seal around
th
e mout
hp
arts. A
phid
sa
li
va a
l
so conta
i
ns
p
ect
i
nase an
dp
erox
id
ase. T
h
e
f
ormer
f
ac
ili
tate
s

penetrat
i
on o
f
t
h
e sty
l
ets t
h
roug
h
t
h
e
i
nterce
ll
u
l
ar spaces o
f
p
l
ant t
i
ssues w
hil
et
h

e
l
atter
ma
yi
nact
i
vate tox
i
cp
hy
toc
h
em
i
ca
l
s(M
il
es, 1999). H
y
a
l
uron
id
ase, w
hi
c
hb
rea

k
s
d
ow
n
connective tissue, is secreted b
y
some insects that suck animal tissue fluids. A spectrum
of compounds that assist feedin
g
is present in the saliva of bloodsuckin
g
species. These
i
nc
l
u
d
e ant
i
coagu
l
ants,
i
n
hibi
tors o
f
p
l

ate
l
et
di
s
i
ntegrat
i
on, pyrase (an enzyme t
h
at
b
rea
ks
d
own ADP, to prevent p
l
ate
l
et aggregat
i
on), an
d
vaso
dil
ators suc
h
as n
i
tr

i
cox
id
e(R
ib
e
i
ro
,
199
5
; Ribeiro and Francischetti, 2003
)
. The nitric oxide is carried to the host’s skin o
n
h
eme-containin
g
proteins (nitrophorins) (Valenzuela and Ribeiro, 1998). The nitrophorins
also stron
g
l
y
bind histamine, released b
y
the host to induce wound healin
g
(Weichsel
et al.
,

1998). Tox
i
ns (venoms), w
hi
c
h
para
l
yze or
kill
t
h
e prey, occur
i
nt
h
esa
li
va o
f
some assass
in
b
ugs (Re
d
uv
iid
ae) an
d
ro

bb
er
fli
es (As
ilid
ae). It
i
sa
l
so reporte
d
t
h
at su
b
stances t
h
at
i
n
d
uce
g
a
ll f
ormat
i
on
by
st

i
mu
l
at
i
n
g
ce
ll di
v
i
s
i
on an
d
e
l
on
g
at
i
on are present
i
nt
h
esa
li
va o
f
som

e
g
all-inhabitin
g
species. Larvae of black flies and chironomid mid
g
es secrete lar
g
e amount
s
of viscous saliva, formin
g
nets that capture food particles.
I
n some spec
i
es t
h
eg
l
an
d
s
h
ave ta
k
en on
f
unct
i

ons qu
i
te unre
l
ate
d
to
f
ee
di
ng,
f
or
examp
l
e, pro
d
uct
i
on o
f
cocoon s
ilk b
yt
h
e
l
a
bi
a

l
g
l
an
d
so
f
caterp
ill
ars an
d
ca
ddi
s
fl
y
l
arvae,
an
d
p
h
eromone pro
d
uct
i
on
by
t
h

e man
dib
u
l
ar
gl
an
d
so
f
t
h
e queen
h
one
yb
ee.
3
.
2
. Fore
g
u
t
T
h
e
f
oregut,
f

orme
dd
ur
i
ng em
b
ryogenes
i
s
b
y
i
nvag
i
nat
i
on o
f
t
h
e
i
ntegument,
i
s
li
ne
d
wi
t

h
cut
i
c
l
e(t
h
e
i
nt
i
ma) t
h
at
i
ss
h
e
d
at eac
h
mo
l
t. Surroun
di
ng t
h
e
i
nt

i
ma, w
hi
c
h
may
b
e folded to enable the
g
ut to stretch when filled, is a thin epidermis, small bundles o
f
lon
g
itudinal muscle, a thick la
y
er of circular muscle, and a la
y
er of connective tissue throu
g
h
w
hich run nerves and tracheae (Figure 16.2). The foregut is generally differentiated into
p
h
arynx, esop
h
agus, crop, an
d
proventr
i

cu
l
us. Attac
h
e
d
to t
h
ep
h
aryngea
li
nt
i
ma are
dil
ato
r
musc
l
es. T
h
ese are espec
i
a
ll
ywe
ll d
eve
l

ope
di
n suc
ki
ng
i
nsects an
df
orm t
h
ep
h
aryngea
l
pump (Chapter 3, Section 3.2.2). The esopha
g
us is usuall
y
narrow but posteriorl
y
ma
y
b
e
d
ilated to form the cro
p
where food is stored. In Di
p
tera and Le

p
ido
p
tera, however, the cro
p
is actuall
y
a diverticulum off the esopha
g
us. Durin
g
stora
g
e the food ma
y
under
g
o some
di
gest
i
on
i
n
i
nsects w
h
ose sa
li
va conta

i
ns enzymes or t
h
at regurg
i
tate
di
gest
i
ve
fl
u
id f
rom
th
em
id
gut. In some spec
i
es t
h
e
i
nt
i
ma o
f
t
h
e crop

f
orms sp
i
nes or r
id
ges t
h
at pro
b
a
bl
ya
id i
n
b
reakin
g
up solid food into smaller particles and mixin
g
in the di
g
estive fluid (Fi
g
ure 1
6
.2A).
T
he hindmost re
g
ion of the fore

g
ut is the proventriculus, which ma
y
serve as a valve
re
g
ulatin
g
the rate at which food enters the mid
g
ut, as a filter separatin
g
liquid and soli
d
components, or as a gr
i
n
d
er to
f
urt
h
er
b
rea
k
up so
lid
mater
i

a
l
. Its structure
i
s, accor
di
ng
l
y,
qu
i
te var
i
e
d
. In spec
i
es w
h
ere
i
t acts as a va
l
ve t
h
e
i
nt
i
ma o

f
t
h
e proventr
i
cu
l
us may
f
orm
l
ong
i
tu
di
na
lf
o
ld
san
d
t
h
ec
i
rcu
l
ar musc
l
e

l
ayer
i
st
hi
c
k
ene
d
to
f
ormasp
hi
ncter. W
h
en a
fi
lter, the proventriculus contains spines that hold back the solid material, permittin
g
onl
y
liquids to move posteriorl
y
. Where the proventriculus acts as a
g
izzard,
g
rindin
g
up food,

t
he intima is formed into strong, radially arranged teeth, and a thick layer of circular muscle
covers the entire structure (Figure 16.2B)
.
Poster
i
or
l
yt
h
e
f
oregut
i
s
i
nvag
i
nate
d
s
li
g
h
t
l
y
i
nto t
h

em
id
gut to
f
orm t
h
e esop
h
agea
l
(
=
s
tomodeal) inva
g
ination (Fi
g
ure 1
6
.3). Its function is to ensure that food enters the
492
CHAPTER 1
6
F
I
GU
RE 16.2
.
T
ransverse sections throu

g
h (A) crop and (B) proventriculus of a locust. [After C. Hod
g
e, 1939,
T
he anatomy and histology of the alimentary tract o
f
L
ocusta mi
g
ratoria L. (Orthoptera: Acrididae)
,
J
.
M
or
p
hol
.
64
:37
5
–399. B
y
permission of Wistar Press.
]
m
idgut within the peritrophic matrix. It also appears to assist in molding the peritrophic
m
atr

i
x
i
nto t
h
e correct s
h
ape
i
n some
i
nsects.
3
.3. Midgut
Th
em
id
gut (= ventr
i
cu
l
u
s
=
m
esenteron)
i
so
f
en

d
o
d
erma
l
or
i
g
i
nan
d
,t
h
ere
f
ore,
has no cuticular linin
g
. In most insects, however, it is lined b
y
a thin peritrophic matrix
(
PM) composed of proteins bound to a meshwork of chitin fibrils (Fi
g
ure 1
6
.4). Some PM
proteins, the peritrophins, are heavil
yg
l

y
cos
y
lated like mucus in the intestine of vertebrates.
T
h
e
f
unct
i
ons o
f
t
h
e PM are to prevent mec
h
an
i
ca
ld
amage to t
h
em
id
gut ep
i
t
h
e
li

um, t
o
prevent entry o
f
m
i
croorgan
i
sms
i
nto t
h
e
b
o
d
ycav
i
ty, to
bi
n
d
potent
i
a
l
tox
i
ns an
d

ot
h
e
r
d
ama
gi
n
g
c
h
em
i
ca
l
s, an
d
to compartmenta
li
ze t
h
em
idg
ut
l
umen, t
h
at
i
s, to

di
v
id
e
i
t
i
nt
o
an endoperitrophic space (within the matrix) and an ectoperitrophic space (ad
j
acent to the
493
FOO
D
U
PT
A
KE
A
ND
UTILIZ
A
TION
F
IGURE 16.3.
L
ong
i
tu

di
na
l
sect
i
on t
h
roug
h
crop,
p
roventr
i
cu
l
us, an
d
anter
i
or m
idg
ut o
f
a coc
k
roac
h.
[
From R. E. Snodgrass
,

P
rinciples o
f
Insect Morphol-
ogy
.
C
opyright 1935 by McGraw-Hill, Inc. Used wit
h
p
erm
i
ss
i
on o
f
McGraw-H
ill
Boo
k
Compan
y
.
]
midgut epithelium) (Terra, 199
6
; Lehane, 1997). This separation of the epithelium from th
e
f
oo

di
mproves
dig
est
i
ve e
ffi
c
i
enc
yby
se
g
re
g
at
i
n
g
enz
y
mes
b
etween t
h
e spaces an
d
ena
bli
n

g
some enz
y
mes to be rec
y
cled (Section 4.2.1).
The PM is
g
enerall
y
absent in fluid-feedin
g
insects, for example, Hemiptera, adult
L
epidoptera, and bloodsucking Diptera. However, some insects produce the PM only at
certa
i
nt
i
mes (e.g.,
f
ema
l
e mosqu
i
toes a
f
ter a
bl
oo

d
mea
l
). Furt
h
er, as
d
escr
ib
e
db
e
l
ow , t
h
e
F
I
G
URE 16.4. Transverse sect
i
on t
h
rou
gh
m
idg
ut o
f
a

l
ocust. [A
f
ter C. Ho
dg
e, 1939, T
h
e anatom
y
an
dhi
sto
l
o
gy
of the alimentar
y
tract o
f
L
ocusta m
ig
rator
i
a L. (Ortho
p
tera: Acrididae),
J.
Mor
p

hol
.
64
:
37
5
–399. B
y
permission
of Wistar Press.
]
494
CHAPTER 1
6
t
y
pe of PM, whether or not a PM is produced, and the manner in which it is produced, ma
y
v
ar
y
between life sta
g
es (Lehane, 1997).
T
he PM is formed in two principal wa
y
s. In T
y
pe I PM delamination of successiv

e
c
oncentr
i
c
l
ame
ll
ae occurs a
l
ong t
h
em
id
gut (
i
nO
d
onata, Ep
h
emeroptera, P
h
asm
id
a, som
e
O
rt
h
optera, some Co

l
eoptera, an
dl
arva
l
Lep
id
optera). T
h
e Type II PM
f
orms
b
y secret
i
o
n
f
rom a spec
i
a
l
zone o
f
ce
ll
s (car
di
a) at t
h

e anter
i
or en
d
o
f
t
h
em
idg
ut (
i
nD
i
ptera, Dermaptera
,
Isoptera, Embioptera, and some Lepidoptera). In this method the esopha
g
eal inva
g
ination
presses firml
y
a
g
ainst the anterior wall of the mid
g
ut so that the ori
g
inall

y
viscous secretio
n
of
t
h
e PM-pro
d
uc
i
ng ce
ll
s, as
i
t
h
ar
d
ens,
i
s squeeze
d
to
f
orm t
h
etu
b
u
l

ar mem
b
rane. In D
i
c-
tyoptera, ot
h
er Ort
h
optera an
d
Lep
id
optera, Hymenoptera, an
d
Neuroptera, a com
bi
nat
i
on
o
f both methods seems to be used. In mosquitoes, larvae produce a T
y
pe II PM, whereas
the adults have a T
y
peIPM
.
T
he PM is made u

p
of a meshwork of microfibrils between which is a thin
p
roteinaceous
film. The microfibrils ha
v
e a constant
60
◦
or
i
entat
i
on to eac
h
ot
h
er
i
n Type I PM, t
h
oug
h
t
to resu
l
t
f
rom t
h

e
i
r secret
i
on
b
yt
h
e
h
exagona
ll
yc
l
ose-pac
k
e
d
m
i
crov
illi
o
f
t
h
eep
i
t
h

e
li
a
l
c
e
ll
s. In T
y
pe II PM t
h
eor
i
entat
i
on o
f
t
h
em
i
cro
fib
r
il
s
i
s ran
d
om. T

h
ePM
i
s permea
bl
eto
the products of di
g
estion and to certain di
g
estive enz
y
mes released from the epithelial cells
(
Section 4.2.1). However, it is not permeable to other lar
g
e molecules, such as undi
g
ested
p
rote
i
ns an
d
po
l
ysacc
h
ar
id

es,
i
n
di
cat
i
ng t
h
at t
h
ePM
h
as a
di
st
i
nct po
l
ar
i
ty an
di
s not mere
ly
a
nu
l
tra
fil
ter (R

i
c
h
ar
d
san
d
R
i
c
h
ar
d
s, 1977; Le
h
ane, 1997).
Th
em
id
gut
i
s usua
ll
y not
diff
erent
i
ate
di
nto structura

ll
y
di
st
i
nct reg
i
ons apart
f
rom t
h
e
development, at the anterior end, of a varied number of blindl
y
endin
g
ceca, which serve to
i
ncrease the surface area available for enz
y
me secretion and absorption of di
g
ested material.
In many Heteroptera, however, the midgut is divided into three or four easily visible regions.
In t
h
ec
hi
nc
hb

ug
(
Bl
issus
l
eucopterus) four such regions occur (Figure 16.
5
). The anterior
r
eg
i
on
i
s
l
arge an
d
sac
lik
e, an
d
serves as a storage reg
i
on (no crop
i
s present). T
h
e secon
d
r

e
g
ion serves as a valve to re
g
ulate the flow of material into the third re
g
ion where di
g
estio
n
F
IGURE 16.5. A
li
mentar
y
cana
l
o
f
c
hi
nc
hb
u
g
(
Bl
issus
l
euco

p
terus
)
showin
g
re
g
ional differentiation of mid
g
ut.
[
After H. Glasgow, 1914, The gastric caeca and the caeca
l
b
acter
i
ao
f
t
h
e Heteroptera
,
B
io
l
.Bu
ll
.
26
:

101
–
1
7
0
.
]
495
FOO
D
U
PT
A
KE
A
ND
UTILIZ
A
TION
F
IGURE 16.6
.
Ali
mentar
y
cana
l
o
f
cercopid (Cercopoidea) showin

g
filter
chamber arrangement. [From R. E. Snod-
g
rass,
P
rincip
l
es of Insect Morp
h
o
l
ogy .
Cop
y
ri
g
ht 193
5
b
y
McGraw-Hill, Inc
.
U
sed with permission of McGraw-Hil
l
B
oo
k
Compan

y
.
]
pro
b
a
bly
occurs. Ten
fi
n
g
er
lik
e ceca
fill
e
d
w
i
t
hb
acter
i
a are attac
h
e
d
to t
h
e

f
ourt
h
re
gi
on
,
w
hich ma
y
be absorptive in function. The role of the bacteria is not known.
I
n man
y
homopterans, which feed on plant sap, the mid
g
ut is modified both morpho-
logically and anatomically so that excess water present in the food can be removed, thus
prevent
i
ng
dil
ut
i
on o
f
t
h
e
h

emo
l
ymp
h
.T
h
oug
hd
eta
il
s vary among
diff
erent groups o
fh
o-
mopterans, t
h
e anter
i
or en
d
o
f
t
h
em
id
gut (or,
i
n some spec

i
es, t
h
e poster
i
or part o
f
t
he
esopha
g
us) is brou
g
ht into close contact with the posterior re
g
ion of the mid
g
ut (or anterior
h
ind
g
ut), and the re
g
ion of contact becomes enclosed within a sac called the “filter cham-
b
er” (Figure 16.6). Such an arrangement facilitates rapid movement of water by osmosi
s
f
rom t
h

e
l
umen o
f
t
h
e anter
i
or m
id
gut across t
h
ewa
ll
o
f
t
h
e poster
i
or m
id
gut an
d
poss
ibl
y
a
l
so t

h
eMa
l
p
i
g
hi
an tu
b
u
l
es. T
h
us, re
l
at
i
ve
l
y
li
tt
l
eo
f
t
h
eor
i
g

i
na
l
water
i
nt
h
e
f
oo
d
actua
ll
y
passes alon
g
the full len
g
th of the mid
g
ut.
The lack of morpholo
g
ical differentiation within the mid
g
ut of most species is reflecte
d
in its uniform histology. Throughout its length, the mature cells lining the lumen are identical
an
d

serve to pro
d
uce
di
gest
i
ve enzymes, to a
b
sor
b
t
h
e pro
d
ucts o
fdi
gest
i
on, an
di
n some
i
nsects secrete t
h
e Type I PM. Rep
l
acement o
fd
egenerate ce
ll

s occurs w
i
t
h
t
h
e maturat
i
on
and differentiation of re
g
enerative cells found sin
g
l
y
or in
g
roups (nidi) near the base o
f
t
he epithelium (Fi
g
ure 1
6
.4). Numerous peptide hormone-containin
g
cells also occur in the
mid
g
ut, which ma

y
pla
y
a role in modulatin
g
mid
g
ut contraction (Lan
g
e and Orchard, 1998)
.
I
n some spec
i
es
hi
sto
l
og
i
ca
l diff
erent
i
at
i
on
i
s
f

oun
d
. For examp
l
e, spec
i
a
li
zat
i
on o
f
cer
-
t
a
i
n anter
i
or ce
ll
s
f
or Type I PM pro
d
uct
i
on was note
d
ear

li
er.Ina
ddi
t
i
on,
diff
erent
i
at
i
on
i
nto
dig
est
i
ve an
d
a
b
sorpt
i
ve re
gi
ons occurs
i
n some spec
i
es. In tsetse

fli
es t
h
ece
ll
so
f
t
h
ean
-
t
erior mid
g
ut are small and are concerned with absorption of water from the in
g
ested blood.
T
he
y
produce no enz
y
mes and di
g
estion does not be
g
in until food reaches the middle re
g
ion
wh

ere t
h
ece
ll
s are
l
arge, r
i
c
hi
nr
ib
onuc
l
e
i
cac
id
,an
d
pro
d
uce enzymes. In t
h
e poster
i
o
r
m
id

gut t
h
ece
ll
s are sma
ll
er, c
l
ose
l
y pac
k
e
d
,an
d
pro
b
a
bl
y concerne
d
w
i
t
h
a
b
sorpt
i

on o
f
dig
este
df
oo
d
. In some spec
i
es
diff
erent re
gi
ons o
f
t
h
em
idg
ut are apparent
ly
a
d
apte
d
to t
h
e
absorption of particular food materials. I
n

Aedes
l
arvae the anterior mid
g
ut is concerne
d
49
6
CHAPTER 1
6
w
ith fat absorption and stora
g
e, whereas the posterior portion absorbs carboh
y
drates an
d
stores them as
g
l
y
co
g
en. In larval Lepidoptera
g
oblet cells, with a lar
g
e flask-shaped central
c
avit

y
, are scattered amon
g
the re
g
ular epithelial cells. The
y
are thou
g
ht to pla
y
a role in
t
h
eregu
l
at
i
on o
f
t
h
e potass
i
um
l
eve
l
w
i

t
hi
nt
h
e
h
emo
l
ymp
h
(C
h
apter 18, Sect
i
on 2.2).
3
.4. H
i
nd
g
ut
T
he hindgut is an ectodermal derivative and, as such, is lined with cuticle, though this
i
st
hi
nner t
h
an t
h

at o
f
t
h
e
f
oregut, a
f
eature re
l
ate
d
to t
h
ea
b
sorpt
i
ve
f
unct
i
on o
f
t
hi
sreg
i
on.
T

h
eep
i
t
h
e
li
a
l
ce
ll
st
h
at surroun
d
t
h
e cut
i
c
l
e are
fl
attene
d
except
i
nt
h
e recta

l
pa
d
s (se
e
below) where the
y
become hi
g
hl
y
columnar and filled with mitochondria. Muscles are onl
y
w
eakl
y
developed and, usuall
y
, the lon
g
itudinal strands lie outside the sheet of circular
m
uscle
.
Th
e
hi
n
d
gut usua

ll
y
h
as t
h
e
f
o
ll
ow
i
ng reg
i
ons: py
l
orus,
il
eum, an
d
rectum. T
h
epy
l
orus
m
ay
h
aveawe
ll
-

d
eve
l
ope
d
c
i
rcu
l
ar musc
l
e
l
ayer (py
l
or
i
csp
hi
ncter) an
d
regu
l
ate t
h
e move
-
m
ent o
f

mater
i
a
lf
rom m
idg
ut to
hi
n
dg
ut. A
l
so, t
h
eMa
l
p
ighi
an tu
b
u
l
es c
h
aracter
i
st
i
ca
lly

e
nter the
g
ut in this re
g
ion. The ileum (Fi
g
ure 1
6
.7A) is
g
enerall
y
a narrow tube that serve
s
to conduct undi
g
ested food to the rectum for final processin
g
. In some insects, however
,
some a
b
sorpt
i
on o
fi
ons an
d
/or water may occur

i
nt
hi
sreg
i
on. In a
f
ew spec
i
es pro
d
uct
i
on
an
d
excret
i
on o
f
n
i
trogenous wastes occur
i
nt
h
e
il
eum (C
h

apter 18, Sect
i
on 2.2). In many
w
oo
d
-eat
i
n
gi
nsects,
f
or examp
l
e, spec
i
es o
f
term
i
tes an
db
eet
l
es, t
h
e
il
eum
i

s
dil
ate
d
t
o
f
orm a fermentation pouch housin
g
bacteria or protozoa that di
g
est wood particles. The
products of di
g
estion, when liberated b
y
the microor
g
anisms, are absorbed across the wal
l
of
t
h
e
il
eum. T
h
e most poster
i
or part o

f
t
h
e gut, t
h
e rectum,
i
s
f
requent
l
y
dil
ate
d
.T
h
oug
hf
o
r
t
h
e most part t
hi
n-wa
ll
e
d
,t

h
e rectum
i
nc
l
u
d
es s
i
xtoe
i
g
h
tt
hi
c
k
-wa
ll
e
d
recta
l
pa
d
s(F
i
gure
16
.7B) whose function is to absorb ions, water, and small or

g
anic molecules (Chapter 18
,
S
ection 4). As a result, the feces of terrestrial insects are expelled as a more or less dr
y
pellet
.
Frequentl
y
, the pellets are ensheathed within the PM, which continues into the hind
g
ut.
4
.
G
ut Phys
i
ology
T
he primar
y
functions of the alimentar
y
canal are di
g
estion and absorption. For these
processes to occur e
ffi
c

i
ent
l
y,
f
oo
di
s move
d
a
l
ong t
h
e cana
l
. In some spec
i
es, enzyme
secret
i
ons are move
d
anter
i
or
l
ysot
h
at
di

gest
i
on can
b
eg
i
n some t
i
me
b
e
f
ore
f
oo
d
reac
h
e
s
t
h
ere
gi
on o
f
a
b
sorpt
i

on
.
4
.1. Gut Movement
s
Th
oug
h
t
h
ea
li
mentary cana
li
s
i
nnervate
d
, neura
l
contro
li
spr
i
nc
i
pa
ll
y assoc
i

ate
d
wi
t
h
t
h
e open
i
n
g
/c
l
os
i
n
g
o
f
va
l
ves t
h
at occur w
i
t
hi
nt
h
e cana

l
(see
b
e
l
ow). T
h
er
hy
t
h
m
ic
peristaltic muscle contractions that move food posteriorl
y
throu
g
h the
g
ut are m
y
o
g
enic; that
i
s, the
y
ori
g
inate within the muscles themselves rather than occurrin

g
as a result of nervou
s
st
i
mu
li
. Myogen
i
c centers
h
ave
b
een
l
ocate
di
nt
h
e esop
h
agus, crop, an
d
proventr
i
cu
l
us,
in
G

a
ll
eria,
f
or examp
l
e. In
i
nsects t
h
at
f
orm a Type II PM,
b
ac
k
war
d
movement o
ff
oo
di
s
a
id
e
dbyg
rowt
h
o

f
t
h
e mem
b
rane. Ant
i
per
i
sta
l
t
i
c movements a
l
so occur
i
n some spec
i
es
and serve to move di
g
estive fluid forward from the mid
g
ut into the crop.
497
FOO
D
U
PT

A
KE
A
ND
UTILIZ
A
TION
F
IGURE 16.7. Transverse sections through (A) ileum and (B) rectum of a locust. [After C. Hodge, 1939, Th
e
anatom
y
an
dhi
sto
l
o
gy
o
f
t
h
ea
li
mentar
y
tract o
f
Locusta m
i

grator
i
a L. (Ort
h
o
p
tera: Acr
idid
ae),
J.
Morp
h
o
l
.
6
4
:
375–399. B
y
permission of Wistar Press.]
T
h
e rate at w
hi
c
hf
oo
d
moves t

h
roug
h
t
h
e gut
i
s not un
if
orm. It var
i
es accor
di
ng
t
ot
h
ep
h
ys
i
o
l
og
i
ca
l
state o
f
an

i
nsect;
f
or examp
l
e,
i
t
i
s greater w
h
en an
i
nsect
h
as
b
ee
n
starved previousl
y
or is active. The rate ma
y
also differ between sexes and with a
g
e. Anothe
r
im
p
ortant variable is the nature of the food. Some insects are able to move some com

p
onents
of the diet rapidly through the gut while retaining others for considerable periods. Withi
n
th
e gut,
f
oo
d
movesatvar
i
a
bl
e rates
i
n
diff
erent reg
i
ons.
T
h
e proventr
i
cu
l
ar an
d
py
l

or
i
cva
l
ves are
i
mportant regu
l
ators o
ff
oo
d
movement
,
th
ou
gh li
tt
l
e
i
s
k
nown a
b
out
h
ow t
h
e

i
r open
i
n
g
an
d
c
l
os
i
n
g
are contro
ll
e
d
.I
n
Peri
pl
anet
a
openin
g
of the proventriculus was shown to depend on the osmotic pressure of in
g
ested fluid
(Dave
y

and Treherne, 1963). As the concentration is increased, the proventriculus open
s
l
ess o
f
ten an
dl
ess w
id
e
l
y, an
d
v
i
ce versa. Davey an
d
Tre
h
erne suggeste
d
t
h
at osmoreceptor
s
i
nt
h
ep
h

arynx prov
id
e
i
n
f
ormat
i
on on t
h
e osmot
i
c pressure o
f
t
h
e
f
oo
d
an
d
t
hi
s
i
n
f
ormat
i

on
t
rave
l
sv
i
at
h
e
f
ronta
lg
an
gli
on to t
h
e
i
n
gl
uv
i
a
lg
an
gli
on t
h
at contro
l

st
h
e proventr
i
cu
l
us
.
However, no osmoreceptor has been located and it ma
y
be that the osmotic feedback come
s
498
CHAPTER 1
6
f
rom the hemol
y
mph rather than directl
y
from the
g
ut as seems to be the case in
L
ocu
s
ta.
D
istension of the fore
g

ut or, in blood-feedin
g
species, the abdomen, is known to cause
release of neurosecretion from the corpora cardiaca which enhances
g
ut peristalsis and,
h
ence, t
h
e rate o
ff
oo
d
passage. Loca
li
ze
d
en
h
ancement o
f
per
i
sta
l
s
i
s may
b
e

i
n
d
uce
db
y
re
l
ease o
f
pept
id
e
h
ormones
f
rom ce
ll
s
i
nt
h
ewa
ll
o
f
t
h
em
id

gut (Lange an
d
Orc
h
ar
d
, 1998).
4
.2. Di
g
estion
As note
d
a
b
ove,
di
gest
i
on may
b
e
i
n
i
t
i
ate
db
y enzymes present

i
nt
h
esa
li
va e
i
t
h
er m
i
xe
d
wi
t
h
t
h
e
f
oo
d
as
i
t enters t
h
e
b
ucca
l

cav
i
ty or secrete
d
onto t
h
e
f
oo
d
pr
i
or to
i
ngest
i
on. Most
dig
est
i
on
i
s
d
epen
d
ent,
h
owever, on enz
y

mes secrete
dby
t
h
em
idg
ut ep
i
t
h
e
li
um. D
ig
est
i
on
m
ostl
y
occurs in the lumen of the mid
g
ut, thou
g
hre
g
ur
g
itation of di
g

estive fluid into the
c
rop is important in some species. In wood-eatin
g
forms, much of the di
g
estion is carried
o
ut
b
ym
i
croorgan
i
sms
i
nt
h
e
hi
n
d
gut (Sect
i
on 4.2.4)
.
4.2.1. D
i
gest
i

ve Enzymes
A wide variet
y
and lar
g
e number of di
g
estive enz
y
mes have been reported for insects
.
In many
i
nstances,
h
owever, enzymes
h
ave
b
een c
h
aracter
i
ze
d
(an
d
name
d
)ont

h
e
b
as
i
so
f
t
h
e
i
r act
i
v
i
ty on unnatura
l
su
b
strates, t
h
at
i
s, mater
i
a
l
st
h
at

d
o not occur
i
nt
h
e norma
ldi
et o
f
t
h
e
i
nsect. T
hi
s
i
s
b
ecause man
y dig
est
i
ve enz
y
mes, espec
i
a
lly
car

b
o
hyd
rases, are “
g
roup
-
specific”; that is, the
y
h
y
drol
y
ze an
y
substrate that includes a particular bond between
two
p
arts of the molecule. For exam
p
le,
α
-
g
lucosidase splits all
α
-g
lucosides, includin
g
sucrose, maltose, furanose, trehalose, and melezitose. Further, in preparing enzyme extracts

f
or ana
l
ys
i
s, e
i
t
h
er gut contents or m
id
gut t
i
ssue
h
omogenates are typ
i
ca
ll
y use
d
. As House
(
1974) note
d
,t
h
e
f
ormer may

i
nc
l
u
d
e enzymes
d
er
i
ve
df
rom t
h
e
f
oo
d
p
er se
,
w
hil
et
he
l
atter contains endoenz
y
mes (intracellular enz
y
mes) that have no di

g
estive function. Thus,
reports on di
g
estive enz
y
me activit
y
must be examined cautiousl
y.
As would be expected, the enz
y
mes produced reflect both qualitativel
y
and quan-
t
i
tat
i
ve
l
yt
h
e norma
l
const
i
tuents o
f
t

h
e
di
et. Omn
i
vorous spec
i
es pro
d
uce enzymes
f
or
di
gest
i
ng prote
i
ns,
f
ats, an
d
car
b
o
h
y
d
rates. Carn
i
vorous spec

i
es pro
d
uce ma
i
n
l
y
li
pase
s
and proteases; in some species these ma
y
be hi
g
hl
y
specific in action. Blow fl
y
larva
e
(
Lucilia cu
p
rin
a
)
, for example, produce lar
g
e amounts of colla

g
enase. The nature of th
e
e
nz
y
mes produced ma
y
chan
g
e at different sta
g
es of the life histor
y
as the diet of an insect
ch
anges. For examp
l
e, caterp
ill
ars
f
ee
di
ng on p
l
ant t
i
ssue secrete a spectrum o
f

enzymes,
wh
ereas nectar-
f
ee
di
ng a
d
u
l
t Lep
id
optera pro
d
uce on
l
y
i
nvertase. Interest
i
ng
l
y,
h
owever,
ev
e
n
i
nt

h
ose en
d
opter
yg
otes
i
nw
hi
c
h
t
h
e
l
arvae an
d
a
d
u
l
ts ut
ili
ze t
h
e same
f
oo
d
t

h
e prop
-
e
rties of the enz
y
mes chan
g
e at metamorphosis. I
n
T
e
n
eb
r
io
,
for exam
p
le, the larval an
d
adult tr
y
psins and ch
y
motr
y
psins differ in molecular size, substrate specificit
y
, and kinetics

,
t
h
oug
h
w
h
yt
hi
ss
h
ou
ld b
e
i
s not c
l
ear
.
Insects can
di
gest a w
id
e range o
f
car
b
o
h
y

d
rates, even t
h
oug
h
on
l
ya
f
ew
di
st
i
nct en
-
z
y
mes ma
yb
e pro
d
uce
d
. As note
d
ear
li
er,
α
-gl

ucos
id
ase w
ill hyd
ro
ly
ze a
ll
α
-
gl
ucos
id
es
.
Likewise
,
β
-
g
lucosidase facilitates splittin
g
of cellobiose,
g
entiobiose, and phen
y
l
g
luco-
s

ides
;
β
-g
alactosidase h
y
drol
y
zes
β
-
g
alactosides such as lactose. In some species, however
,
there appear to be carbohydrate-digesting enzymes that exhibit absolute specificity. Thus
,
adult
Luci
l
ia cuprina pro
d
uce a
n
α
-g
l
ucos
id
ase, tre
h

a
l
ase, t
h
at sp
li
ts on
l
y tre
h
a
l
ose. T
he
n
orma
l
po
l
ysacc
h
ar
id
e-
di
gest
i
ng enzyme pro
d
uce

di
s amy
l
ase
f
or
h
y
d
ro
l
ys
i
so
f
starc
h,
thou
g
h particular species ma
y
produce enz
y
mes for di
g
estion of other pol
y
saccharides. Fo
r
499

FOO
D
U
PT
A
KE
A
ND
UTILIZ
A
TION
example, firebrats and silverfish (Z
yg
entoma), larvae of wood-borin
g
Ceramb
y
cidae and
Anobiidae (Coleoptera), as well as both lower and hi
g
her termites, have endo
g
enous cel-
lulases, thou
g
h in most insects production of this enz
y
me is restricted to microor
g
anism

s
present
i
nt
h
e
hi
n
d
gut. Cur
i
ous
l
y,
l
ower term
i
tes pro
d
uce ce
ll
u
l
ase
i
nt
h
esa
li
va, w

h
ereas
in
hi
g
h
er term
i
tes t
h
em
id
gut
i
st
h
e source o
f
t
hi
s enzyme. Sco
l
yt
i
nae (Co
l
eoptera) pro
d
uce a
h

em
i
ce
ll
u
l
ase, c
hi
t
i
nase
i
s reporte
d
to occur
i
nt
h
e
i
ntest
i
na
lj
u
i
ce o
f
Peri
pl

anet
a
,
an
d
some
h
erbivorous Ortho
p
tera
p
roduce lichenase
.
As in other or
g
anisms, the protein-di
g
estin
g
enz
y
mes produced b
y
the mid
g
ut are di
-
v
i
s

ibl
e
i
nto two types: en
d
opept
id
ases, w
hi
c
h
e
ff
ect t
h
e
i
n
i
t
i
a
l
sp
li
tt
i
ng o
f
prote

i
ns
i
nto
po
l
ypept
id
es, an
d
exopept
id
ases, w
hi
c
hb
r
i
ng a
b
out
d
egra
d
at
i
on o
f
po
l

ypept
id
es
b
yt
h
e se-
quential splittin
g
off of individual amino acids from each end of a molecule. Exopeptidase
s
can be further cate
g
orized into carbox
y
peptidases, which remove amino acids from the car-
b
ox
y
lic end of a pol
y
peptide, and aminopeptidases, which cause h
y
drol
y
sis at the amino en
d
o
f
amo

l
ecu
l
e. A
di
pept
id
ase a
l
so
i
s
f
requent
l
y present. In some spec
i
es on
l
yen
d
opept
id
ases
occur
i
nt
h
em
id

gut
l
umen (spec
ifi
ca
ll
yw
i
t
hi
nt
h
een
d
oper
i
trop
hi
c space), t
h
e exopept
i
-
d
ases
b
e
i
n
gf

oun
d
outs
id
et
h
e PM or even attac
h
e
d
to t
h
eap
i
ca
l
p
l
asma mem
b
rane o
f
t
h
e
epithelial cells. Some insects produce specific enz
y
mes for the di
g
estion of particularl

y
resistant structural proteins. Colla
g
enase has been mentioned alread
y
. Keratin, the primar
y
const
i
tuent o
f
woo
l
,
h
a
i
r, an
df
eat
h
ers,
i
sa
fib
rous prote
i
nw
h
ose po

l
ypept
id
e components
li
es
id
e
b
ys
id
e
li
n
k
e
db
y
hi
g
hl
y sta
bl
e
di
su
lfid
e
b
on

d
s
b
etween a
dj
acent su
lf
ur-conta
i
n
i
ng
am
i
no ac
id
s, suc
h
as c
y
st
i
ne an
d
met
hi
on
i
ne. A
k

erat
i
nase
h
as
b
een
id
ent
ifi
e
di
nc
l
ot
h
e
s
moth larvae
(
Ti
n
eola
) and ma
y
also occur in other keratin-di
g
estin
g
species, such as der-

mestid beetles and Mallopha
g
a. The keratinase is active onl
y
under anaerobic (reducin
g
)
conditions and, in this context, it is interesting to note that the midgut o
f
Tineol
a
i
s poorly
t
rac
h
eate
d
.
D
i
etary
f
ats o
f
e
i
t
h
er an

i
ma
l
or p
l
ant or
i
g
i
n are a
l
most a
l
ways tr
i
g
l
ycer
id
es, t
h
at
i
s
,
g
l
y
cerol in combination with three fatt
y

acid molecules. The latter ma
y
ran
g
e from unsat-
u
rated to full
y
saturated. Lipases, which h
y
drol
y
ze fats to the constituent fatt
y
acids an
d
glycerol, have low specificity. Therefore, the presence of one such enzyme will normally
sat
i
s
f
yan
i
nsect’s nee
d
s. In a
f
ew spec
i
es,

h
owever, at
l
east two
li
pases
h
ave
b
een
id
ent
ifi
e
d
,
h
av
i
ng
diff
erent pH opt
i
ma an
d
act
i
ng on tr
i
g

l
ycer
id
es o
f diff
erent s
i
zes. Fat
di
gest
i
on
i
s
g
enerall
y
somewhat slow as insects lack an
y
thin
g
comparable to the bile salts of vertebrates
t
hat would emulsif
y
and stabilize lipid droplets
.
4
.2.2. Factors A
ff

ect
i
ng Enzyme Act
i
v
i
ty
Accordin
g
to House (1974), three factors markedl
y
affect di
g
estion in insects: pH,
buf
fering capacity, and redox potential of the gut.
f
f
T
h
epH
d
eterm
i
nes not on
l
yt
h
e act
i

v
i
ty o
fdi
gest
i
ve enzymes,
b
ut a
l
so t
h
e nature an
d
e
xtent o
f
m
i
croorgan
i
sms
i
nt
h
e gut an
d
t
h
eso

l
u
bili
ty o
f
certa
i
n mater
i
a
l
s
i
nt
h
e gut
l
umen
.
T
he latter affects the osmotic pressure of the
g
ut contents and, in turn, the rate of absorption
of molecules across the
g
ut wall. Anal
y
ses of the pH in various re
g
ions of the

g
ut have been
made for a wide ran
g
e of species, and various authors have attempted to correlate thes
e
wi
t
h
t
h
e
f
ee
di
ng
h
a
bi
ts or p
h
y
l
ogenet
i
c pos
i
t
i
on o

f
an
i
nsect. At
b
est, t
h
ese corre
l
at
i
ons ar
e
on
l
y
b
roa
dl
y correct, an
d
many except
i
ons are
k
nown. In most
i
nsects t
h
e gut

i
ss
li
g
h
t
l
y
ac
id
or s
ligh
t
ly
a
lk
a
li
ne t
h
rou
gh
out
i
ts
l
en
g
t
h

. Furt
h
er, t
h
epH
g
enera
lly i
ncreases
f
rom
fore
g
ut to mid
g
ut, then decreases from mid
g
ut to hind
g
ut. Thou
g
h the latter is true for most
500
CHAPTER 1
6
ph
y
topha
g
ous species, in man

y
omnivorous and carnivorous species the pH of the hind
g
u
t
i
s
g
reater than that of the mid
g
ut
.
M
an
y
variables affect the pH of different re
g
ions of the
g
ut. Generall
y
the pH of th
e
c
rop
i
st
h
e same as t
h

at o
f
t
h
e
f
oo
d
,t
h
oug
hi
n some spec
i
es
i
t
i
s cons
i
stent
l
y
l
ess t
h
an 7
b
ecause o
f

t
h
e
di
gest
i
ve act
i
v
i
ty o
f
m
i
croorgan
i
sms or regurg
i
tat
i
on o
fdi
gest
i
ve
j
u
i
ce
f

ro
m
t
h
em
idg
ut. T
h
epHo
f
t
h
em
idg
ut
diff
ers amon
g
spec
i
es
b
ut ten
d
sto
b
e constant
f
or a
gi

ven
species because of the presence in this re
g
ion of bufferin
g
a
g
ents. In a few species there
are local variations in pH within the mid
g
ut that can be related to chan
g
es in di
g
estive
f
unct
i
on
f
rom one part to anot
h
er. For examp
l
e,
i
nt
h
e coc
k

roac
h
Naup
h
oeta cinerea t
h
epH
o
f the anterior midgut is
6
.0–7.2, which coincides with the pH optimum of the amylas
e
f
ound mainl
y
in this re
g
ion. In the posterior mid
g
ut, on the other hand, the pH is about
9
, near the o
p
timum for the
p
roteinases that are active there (El
p
idina
et al.
, 2001

)
.Th
e
hind
g
ut t
y
picall
y
has a pH sli
g
htl
y
less than 7, presumabl
y
resultin
g
from the presence of
t
h
en
i
trogenous waste pro
d
uct, ur
i
cac
id
(C
h

apter 18, Sect
i
on 3.2). T
h
e
hi
n
d
gut contents
of
some p
h
ytop
h
agous spec
i
es may
b
equ
i
te ac
idi
c as a resu
l
to
f
t
h
e
f

ormat
i
on o
f
organ
ic
ac
id
s
f
rom ce
ll
u
l
ose
by
s
y
m
bi
ot
i
cm
i
croor
g
an
i
sms
.

T
he relativel
y
constant pH found in different re
g
ions of the
g
ut results from the pres
-
e
nce in the lumen of both inor
g
anic and or
g
anic bufferin
g
a
g
ents. In some species, inor
g
ani
c
i
ons, espec
i
a
ll
yp
h
osp

h
ates,
b
ut
i
nc
l
u
di
ng a
l
um
i
num, ammon
i
um, ca
l
c
i
um,
i
ron, magne-
s
i
um, potass
i
um, so
di
um, car
b

onate, c
hl
or
id
e, an
d
n
i
trate, seem to o
ff
er su
ffi
c
i
ent
b
u
ff
er
i
n
g
c
apac
i
t
y
.Inot
h
er spec

i
es or
g
an
i
cac
id
s,
i
nc
l
u
di
n
g
am
i
no ac
id
san
d
prote
i
ns, ten
d
to supp
l
e
-
m

ent or replace the bufferin
g
effect of the inor
g
anic ions. For some of the inor
g
anic ions an
d
w
ater, active secretor
y
or resorption mechanisms are known to re
g
ulate their concentration
i
n the midgut. These mechanisms are also capable of inducing fluid flow, especially in the
e
ctoper
i
trop
hi
c space. Spec
ifi
ca
ll
y, secret
i
on o
fi
ons an

d
water across t
h
e poster
i
or m
id
gut
e
p
i
t
h
e
li
um s
i
mu
l
taneous
l
yw
i
t
h
t
h
e
i
r resorpt

i
on at t
h
e anter
i
or en
d
esta
bli
s
h
es a
f
orwar
d
flowofdi
g
estive fluid. This is thou
g
ht to conserve nutrients and rec
y
cle enz
y
mes
.
R
edox potential, which measures abilit
y
to
g

ain or lose electrons, that is, to be reduce
d
o
r oxidized, respectively, is an important factor in digestion in some insects as it affects th
e
structure o
fb
ot
hdi
etary prote
i
ns an
d
proteo
l
yt
i
c enzymes. T
h
e gut re
d
ox potent
i
a
l
,w
hi
c
h
i

sc
l
ose
l
y
li
n
k
e
d
to pH,
i
s norma
ll
y pos
i
t
i
ve,
i
n
di
cat
i
ng ox
idi
z
i
ng (aero
bi

c) con
di
t
i
ons.
H
owever, in species able to di
g
est keratin the redox potential of the mid
g
ut fluid is stron
g
l
y
n
e
g
ative. It has been su
gg
ested that such an anaerobic (reducin
g
) environment is necessar
y
to enable the keratinase to split the disulfide bonds (House, 1974). Subsequentl
y
, norma
l
proteases
h
y

d
ro
l
yze t
h
epo
l
ypept
id
es
.
4.2.3. Control of Enzyme
S
ynthesis and
S
ecretio
n
N
umerous studies have shown that enzyme activity in the midgut varies in relatio
n
to
f
oo
di
nta
k
e, t
h
oug
hi

t
i
s not a
l
ways c
l
ear w
h
et
h
er
i
t
i
s synt
h
es
i
san
d
/or re
l
ease o
f
t
he
e
nzymes t
h
at

i
s
b
e
i
ng contro
ll
e
d
. In many spec
i
es,
i
nc
l
u
di
ng Locusta mi
g
ratoria
a
n
d
Tene-
b
r
io
m
olito
r

,
enz
y
mes are not stored in the mid
g
ut cells but are liberated immediatel
y
into
the
g
ut lumen. In other insects, for example
,
S
tomoxys calcitrans
a
nd some mos
q
uitoes,
e
nz
y
mes are stored (possibl
y
in an inactive form) and released when feedin
g
occurs. The
synt
h
es
i

s/re
l
ease o
f
enzyme
i
n proport
i
on to t
h
e amount o
ff
oo
di
ngeste
d
may
b
eregu
l
ate
d
b
y secretagogue,
h
ormona
l
or neura
l
mec

h
an
i
sms, t
h
oug
h
t
h
ere
i
s very
li
tt
l
eev
id
ence
f
or t
h
e
l
atter. Un
f
ortunate
ly
,
f
or most spec

i
es, t
h
eev
id
ence presente
di
n support o
f
one mec
h
an
i
s
m
o
r another is equivocal. In a secreta
g
o
g
ue s
y
stem enz
y
mes are produced in response t
o
501
FOO
D
U

PT
A
KE
A
ND
UTILIZ
A
TION
food present in the mid
g
ut. Presumabl
y
the amount produced is directl
y
influenced b
y
the
concentration of food in the lumen. The best evidence for secreta
g
o
g
ue control of enz
y
me
activit
y
is for mosquitoes and other blood feeders. In
A
edes
a

blood meal cannulated directl
y
i
nto t
h
em
id
gut st
i
mu
l
ates pro
d
uct
i
on o
f
a proport
i
onate amount o
f
tryps
i
n (Br
i
ege
l
an
d
Lea

,
197
5
). These authors showed that a variety of components of blood could serve as secreta-
g
o
g
ues. However,
i
t
i
s notewort
hy
t
h
at t
h
e amount o
f
enz
y
me pro
d
uce
dby
t
hi
s means wa
s
reduced in insects whose median neurosecretor

y
cells had been removed. Where hormona
l
control of enz
y
me production has been proposed, the amount of food passin
g
alon
g
the
f
oregut, measure
d
as t
h
e
d
egree o
f
stretc
hi
ng o
f
t
h
e gut wa
ll
,
i
s

b
e
li
eve
d
to resu
l
t
i
nt
he
re
l
ease
f
rom t
h
e corpora car
di
aca o
f
a proport
i
onate amount o
f
neurosecret
i
on w
hi
c

h
trave
l
s
via the hemol
y
mph to the mid
g
ut cells.
At present, there is no consensus as to whether a mid
g
ut epithelial cell produces
a
complete packa
g
eofenz
y
mes or whether the proportions of different enz
y
mes can var
y
wi
t
h
c
h
anges
i
nt
h

e
di
et. Certa
i
n
l
y, a secretagogue met
h
o
df
or regu
l
at
i
ng enzyme act
i
v
i
ty
cou
ld
more eas
il
y account
f
or c
h
anges
i
nt

h
e
l
eve
l
o
f
spec
ifi
c enzymes reporte
d
to occur
wi
t
h
a
l
terat
i
ons to t
h
e
di
et o
f
some spec
i
es
.
4

.2.4. Di
g
estion by Microor
g
anism
s
M
i
croorgan
i
sms (
b
acter
i
a,
f
ung
i
,an
d
protozoa) may
b
e present
i
nt
h
e gut,
b
ut
f

or on
ly
af
ew
f
f
s
pec
i
es
h
as t
h
ere
b
een a conv
i
nc
i
n
gd
emonstrat
i
on o
f
t
h
e
i
r

i
mportance
i
n
dig
est
i
on
.
I
n man
y
insects microor
g
anisms appear to have no role, as the insects can be reared equall
y
w
ell in their absence. In other species microor
g
anisms ma
y
be more important with respec
t
t
o an insect’s nutrition than digestio
n
p
er
s
e. Where a role for microorganisms in digestio

n
h
as
b
een
d
emonstrate
d
,t
h
ere
l
at
i
ons
hi
p
b
etween t
h
em
i
croorgan
i
sms an
di
nsect
h
ost
i

sno
t
a
l
ways o
bli
gate,
b
ut may
b
e
f
acu
l
tat
i
ve or even acc
id
enta
l.
Bacteria are important cellulose-di
g
estin
g
a
g
ents in man
y
ph
y

topha
g
ous insects, es
-
peciall
y
wood-eatin
g
species whose hind
g
ut ma
y
include a fermentation pouch in which
t
he microorganisms are housed. In other species, for example, the wood-eating cockroac
h
P
ane
s
t
h
ia,
b
acter
i
a
i
nt
h
e crop are essent

i
a
lf
or ce
ll
u
l
ose
di
gest
i
on. In
l
arvae o
f
t
h
ewa
x
mot
h
G
a
ll
eria,
b
acter
i
a norma
ll

y present
i
nt
h
e gut un
d
ou
b
te
dl
ya
id i
nt
h
e
di
gest
i
on o
f
b
eeswax,
y
et bacteriolo
g
icall
y
sterile larvae produce an intrinsic lipase capable of de
g
rad-

in
g
certain wax components. Finall
y
, man
y
insects feed on deca
y
in
g
ve
g
etation and must,
t
herefore, ingest a large number of saprophytic bacteria which, temporarily at least, woul
d
cont
i
nue t
h
e
i
r
d
egra
d
at
i
ve act
i

v
i
ty
i
nt
h
e gut. In t
hi
s sense, t
h
ere
f
ore, t
h
oug
h
t
h
ere
l
at
i
ons
hip
i
s acc
id
enta
l
,t

h
em
i
croorgan
i
sms are ass
i
st
i
ng
i
n
di
gest
i
on
.
I
n
l
ower term
i
tes an
d
some pr
i
m
i
t
i

ve woo
d
-eat
i
n
g
coc
k
roac
h
es
(
C
r
y
ptocercu
s
)
,
fl
a
g
-
ellate and ciliate protozoa occur in enormous numbers in the hind
g
ut. The relationship
b
etween the insects and
p
rotozoa is mutualistic; that is, in return for a suitable, anaerobi

c
env
i
ronment
i
nw
hi
c
h
to
li
ve, t
h
e protozoa p
h
agocytose part
i
c
l
es o
f
woo
d
eaten
b
yt
he
i
nsects,
f

erment
i
ng t
h
ece
ll
u
l
ose an
d
re
l
eas
i
ng
l
arge amounts o
f
g
l
ucose (
i
n
C
r
y
ptocercu
s
)
or or

g
an
i
cac
id
s(
i
n term
i
tes)
f
or use
by
t
h
e
i
nsects. In
high
er term
i
tes (Term
i
t
id
ae) t
he
h
ind
g

ut contains bacteria, not protozoa, but there is no evidence that the bacteria produce
cellulol
y
tic enz
y
mes (and see Section 4.2.1).
Fung
i
rare
l
yp
l
ay a
di
rect ro
l
e
i
nt
h
e
di
gest
i
ve process o
fi
nsects, t
h
oug
hi

t
i
s reporte
d
th
at yeasts capa
bl
eo
fh
y
d
ro
l
yz
i
ng car
b
o
h
y
d
rates occur
i
nt
h
e gut o
f
some
l
ea

fh
opper
s
(C
i
ca
d
e
llid
ae). However, a mutua
li
st
i
cre
l
at
i
ons
hi
p
h
as evo
l
ve
db
etween man
yf
un
gi
an

d
insects whereb
y
the fun
g
i convert wood into a more usable form, while the insects serve t
o
502
CHAPTER 1
6
transport the fun
g
i to new locations. Some ants and hi
g
her termites, for example, cultur
e
ascom
y
cete or basidiom
y
cete fun
g
i in special re
g
ions of the nest called fun
g
us
g
ardens
.

Chewed wood or other ve
g
etation is brou
g
ht to the fun
g
us
g
arden and becomes the substrat
e
o
nw
hi
c
h
t
h
e
f
ung
i
grow,
f
orm
i
ng
h
yp
h
ae to

b
e eaten
b
yt
h
e
i
nsects. Certa
i
nwoo
d
-
b
or
i
n
g
i
nsects,
f
or examp
l
e,
b
ar
kb
eet
l
es (Sco
l

yt
i
nae),
i
nocu
l
ate t
h
e
i
r tunne
l
sw
i
t
hf
unga
l
ce
lls
wh
en t
h
e
yi
nva
d
e a new tree. T
h
e

f
un
g
a
l
m
y
ce
li
um t
h
at
d
eve
l
ops, a
l
on
g
w
i
t
h
part
i
a
lly
decomposed wood, can then be used as food b
y
the insects.

4
.3. Absor
p
tion
Th
ema
j
or
i
t
y
o
f
a
b
sorpt
i
on occurs
i
nt
h
em
idg
ut, espec
i
a
lly
t
h
e anter

i
or port
i
on,
i
n
-
c
ludin
g
the mesenteric ceca. A few reports have indicated that absorption of lipid materials
(
includin
g
the insecticides parathion and dieldrin) ma
y
occur across the crop wall, while
t
h
e upta
k
eo
f
a range o
f
sma
ll
organ
i
cmo

l
ecu
l
es occurs
i
nt
h
e
hi
n
d
gut. T
h
e
l
atter reg
i
o
n
i
s,
h
owever, pr
i
mar
il
yo
fi
mportance as t
h

es
i
te o
f
water or
i
on resorpt
i
on
i
n connect
i
on
wi
t
h
osmore
g
u
l
at
i
on (C
h
apter 18, Sect
i
on 4), t
h
ou
gh i

n
i
nsects t
h
at
h
ave s
y
m
bi
ot
i
cm
i-
c
roor
g
anisms in the hind
g
ut it ma
y
also be an important site for absorption of small or
g
ani
c
m
olecules, especiall
y
carbox
y

lic and amino acids.
M
ost a
b
sorpt
i
on o
f
organ
i
cmo
l
ecu
l
es across t
h
em
id
gut wa
ll i
s pass
i
ve, t
h
at
i
s,
f
rom
a

hi
g
h
er to a
l
ower concentrat
i
on, t
h
oug
h
t
h
e rap
id
rate at w
hi
c
h
some mo
l
ecu
l
es are a
b
sor
b
e
d
su

gg
ests t
h
at spec
i
a
l
carr
i
ers
f
ac
ili
tate t
h
e
i
r movement. T
h
ea
b
sorpt
i
on rate
i
sen
h
ance
dby
a steep concentration

g
radient maintained between the mid
g
ut lumen and hemol
y
mph. Thi
s
m
a
y
be achieved b
y
absorption of water from the
g
ut lumen so that the hemol
y
mph becomes
m
ore
dil
ute or
b
y rap
id
convers
i
on o
f
t
h

ea
b
sor
b
e
d
mo
l
ecu
l
es to a more comp
l
ex
f
orm. T
he
a
b
sorpt
i
on o
f
organ
i
cmo
l
ecu
l
es across t
h

e gut wa
ll in
S
c
h
istocerc
a
an
d
Perip
l
anet
a
f
orme
d
the subject of a series of papers by Treherne in the late 19
5
0s (see Treherne, 1967, for
details
)
.
Usin
g
isotopicall
y
labeled monosaccharides, Treherne demonstrated that nearl
y
all
sugars are absorbed in the anterior region of the midgut, especially the ceca. Further, th

e
m
onosacc
h
ar
id
es are converte
d
rap
idl
ytot
h
e
di
sacc
h
ar
id
e tre
h
a
l
ose
i
nt
h
e
f
at
b

o
d
y. Inter-
e
st
i
ng
l
y,
in
S
c
h
istocerc
a
muc
h
o
f
t
h
e
f
at
b
o
d
y
i
s

i
nc
l
ose prox
i
m
i
ty to t
h
em
id
gut wa
ll
.O
f
the monosaccharides studied,
g
lucose was found to be absorbed most rapidl
y
. Fructose and
m
annose are absorbed relativel
y
slowl
y
because of their accumulation in the hemol
y
mph.
The latter is related to the lower rate at which the
y

are converted to trehalose. Apparentl
y
n
o
m
ec
h
an
i
sm
f
or act
i
ve upta
k
eo
f
monosacc
h
ar
id
es occurs (or
i
s necessary)
i
n
S
c
h
istocerca

b
ecause
i
ts pr
i
nc
i
pa
l
“
bl
oo
d
sugar”
i
s tre
h
a
l
ose. In certa
i
n
i
nsects,
f
or examp
l
e, t
h
e

h
one
y
b
ee, a cons
id
era
bl
e amount o
fgl
ucose
i
s norma
lly
present
i
nt
h
e
h
emo
ly
mp
h
an
d
,
i
n suc
h

species, active transport s
y
stems ma
y
be necessar
y
for su
g
ar absorption
.
In
S
chistocerca
,
amino acids, like su
g
ars, are apparentl
y
absorbed passivel
y
throu
gh
t
h
ewa
ll
o
f
t
h

e mesenter
i
c ceca an
d
anter
i
or m
id
gut. T
h
eo
b
servat
i
on t
h
at t
h
e
i
ra
b
sorpt
i
on
i
s
pass
i
ve

i
so
fi
nterest, as
i
t
i
s
k
nown t
h
at t
h
e concentrat
i
on o
f
am
i
no ac
id
s
i
nt
h
e
h
emo
l
ymp

h
i
s norma
lly
ver
y high
.Tre
h
erne
di
scovere
d
t
h
at, pr
i
or to am
i
no ac
id
a
b
sorpt
i
on, t
h
ere
i
s rapid movement of water from the
g

ut lumen to the hemol
y
mph, which establishes
af
av
f
f
o
rable concentration
g
radient for passive absorption of amino acids. In contrast, in
le
p
ido
p
teran larvae a suite of absor
p
tion mechanisms has been found (Turunen, 1985;
Wo
lf
ers
b
erge
r
,
1996, 2000; Sacchi and Wolfersberger, 1996). In high midgut concentrations
t
h
eam
i

no ac
id
se
i
t
h
er
diff
use pass
i
ve
l
y across t
h
e gut wa
ll
or are move
d
on spec
ifi
c carr
i
ers
(
facilitated diffusion). At low concentrations, active uptake of amino acids occurs across th
e
503
FOO
D
U

PT
A
KE
A
ND
UTILIZ
A
TION
mid
g
ut. For most neutral amino acids, a s
y
mport s
y
stem operates; that is, active transport of
t
he amino acid occurs concurrentl
y
with movement of a cation, especiall
yK
+
. Some amino
acids, however, are transported in the absence of cations (the uniport s
y
stem). Within th
e
mid
gut, t
h
ere are reg

i
ona
l diff
erences
i
n upta
k
ea
bili
ty;
f
or examp
l
e,
i
n
l
arvae o
f
M
an
d
uca
s
exta t
h
e symport system
f
or
l

euc
i
ne an
d
pro
li
ne
i
s concentrate
di
nt
h
e poster
i
or m
id
gu
t
(
Wolfersber
g
er, 199
6
).
Another mechanism that ma
y
facilitate their absorption, is to convert amino acids to
a
m
ore complex stora

g
e molecule. For example, when certain amino acids were fed to starve
d
Ae
d
e
s
,g
l
ycogen rap
idl
y appeare
di
n some o
f
t
h
ece
ll
so
f
t
h
em
id
gut an
d
ceca. However,
i
t

i
s not
k
nown w
h
et
h
er t
h
eg
l
ycogen was
f
orme
ddi
rect
l
y
f
rom t
h
ese am
i
no ac
id
s.
E
ar
ly hi
stoc

h
em
i
ca
l
stu
di
es
d
emonstrate
d
t
h
at
d
rop
l
ets o
fli
p
id
are present
i
nt
h
e ep-
i
thelial cells of the crop and
g
ave support to the idea that the crop was the site of lipi

d
absorption. Thou
g
h, as noted above, a few reports indicate that certain lipoidal molecules
c
an penetrate t
h
e crop wa
ll
,Tre
h
erne’s wor
k
, aga
i
n
i
nvo
l
v
i
ng
l
a
b
e
l
e
d
compoun

d
s, s
h
owe
d
th
at
,i
n
S
c
h
istocerc
a
,
a
b
sorpt
i
on o
fli
p
id
occurs not across t
h
e crop wa
ll b
ut v
i
at

h
e anter
i
or
midg
ut an
d
ceca. In ot
h
er
i
nsects t
h
e occurrence o
fli
pop
hili
cce
ll
s
i
nt
h
em
iddl
ean
d
poste-
rior re
g

ions of the mid
g
ut su
gg
ests that these ma
y
be sites of lipid absorption. Absorptio
n
of lipids is a
g
ain a passive process and is relativel
y
slow compared to su
g
ars and amin
o
ac
id
s;
i
ts rate
i
s,
h
owever,
i
ncrease
db
yt
h

e ester
ifi
cat
i
on o
f
t
h
ea
b
sor
b
e
d
mater
i
a
l
s
i
n
di
-
an
d
tr
i
g
l
ycer

id
es an
d
ot
h
er comp
l
ex
li
p
id
s
i
nt
h
em
id
gut ep
i
t
h
e
li
um, a process ana
l
ogous
t
ot
h
es

i
tuat
i
on
i
n verte
b
rates
.
5
.M
e
t
abo
li
sm
S
u
b
stances a
b
sor
b
e
d
t
h
rou
gh
t

h
e
g
ut wa
ll
(occas
i
ona
lly
t
h
e
i
nte
g
ument; e.
g
., certa
in
insecticides) seldom remain unchan
g
ed in the hemol
y
mph for an
y
len
g
th of time but are
quickl
y

converted into other compounds. Metabolism comprises all of the chemical reac-
ti
ons t
h
at occur
i
na
li
v
i
ng organ
i
sm. It
i
nc
l
u
d
es ana
b
o
li
sm (react
i
ons t
h
at resu
l
t
i

nt
he
f
ormat
i
on o
f
more comp
l
ex mo
l
ecu
l
es an
d
are, t
h
ere
f
ore, energy-requ
i
r
i
ng) an
d
cata
b
o
li
s

m
(react
i
ons
f
rom w
hi
c
h
s
i
mp
l
er mo
l
ecu
l
es resu
l
tan
d
energy
i
sre
l
ease
d
). Ana
b
o

li
c react
i
on
s
include, for example, the formation of structural proteins or enz
y
mes from amino acids, and
t
he formation from simple su
g
ars of pol
y
saccharides that serve as an ener
gy
store. Man
y
catabolic reactions have evolved for the specific purpose of producing the large quantities
o
f
energy requ
i
re
db
yt
h
e organ
i
sm
f

or per
f
ormance o
f
wor
k.
T
h
e meta
b
o
li
sm o
fi
nsects genera
ll
y resem
bl
es t
h
at o
f
mamma
l
s,
d
eta
il
so
f

w
hi
c
h
can
b
e found in standard biochemical texts. The present account, therefore, will be lar
g
el
y
com
p
arative in nature.
5
.1.
S
ites of Metabolism
Ch
em
i
ca
l
react
i
ons are carr
i
e
d
out
by

a
ll li
v
i
n
g
ce
ll
s, t
h
ou
gh
t
h
e
y
are usua
lly li
m
i
te
d
in number and, of course, are related to the specific function of the cell in which the
y
occur
.
Fo
re
x
ample, in midgut epithelial cells, metabolism is directed largely toward synthesis o

f
spec
ifi
c prote
i
ns, t
h
e enzymes use
di
n
di
gest
i
on. Meta
b
o
li
sm
i
n musc
l
ece
ll
s
i
s spec
ifi
ca
ll
y

concerne
d
w
i
t
h
pro
d
uct
i
on o
fl
arge amounts o
f
energy,
i
nt
h
e
f
orm o
f
ATP,
f
or t
h
e contract
i
o
n

process. In ep
id
erma
l
ce
ll
s react
i
ons
l
ea
di
n
g
to t
h
e pro
d
uct
i
on o
f
c
hi
t
i
nan
d
certa
i

n prote
i
ns,
t
he com
p
onents of cuticle, are
p
redominant. Certain tissues, however, are not so s
p
ecialize
d
504
CHAPTER 1
6
and in them a multitude of biochemical reactions, involvin
g
the three ma
j
or raw materials
(
su
g
ars, amino acids, and lipids), are carried out. In vertebrates the liver performs thes
e
m
ultiple functions. The analo
g
ous tissue in insects is the fat bod
y

(Kilb
y
, 1965; Keele
y
,
1
98
5
).
5.1.1. Fat Bod
y
T
he fat bod
y
is derived durin
g
embr
y
o
g
enesis from the mesodermal walls of th
e
c
oe
l
om
i
ccav
i
t

i
es. In ot
h
er wor
d
s,
i
t
i
s
i
n
i
t
i
a
ll
y a segmenta
ll
y arrange
d
t
i
ssue t
h
oug
h
t
hi
s

b
ecomes o
b
scure
d
as t
h
e
h
emocoe
ld
eve
l
ops. Nevert
h
e
l
ess, an
d
contrary to w
h
at a casua
l
e
xam
i
nat
i
on ma
y

su
gg
est, t
h
e
f
at
b
o
dy d
oes
h
ave a
d
e
fi
n
i
te arran
g
ement
i
nt
h
e
h
emocoe
l
c
haracteristic of the species. T

y
picall
y
, there are subepidermal and perivisceral la
y
ers of
fat
bod
y
, plus sheets or cords of cells occur in other specific locations. Thus, the fat bod
y
presents a
l
arge sur
f
ace area to t
h
e
h
emo
l
ymp
h
,a
ll
ow
i
ng t
h
e rap

id
exc
h
ange o
f
meta
b
o
li
tes
(
Dean
e
ta
l
., 198
5
). Contrary to what was thought originally, the fat body is not a single,
uniform tissue (Haunerland and Shirk, 199
5
; Jensen and Bør
g
esen, 2000). Not onl
y
are there
re
g
ional and species-specific differences, but also differences between larval and adult fa
t
bod

y
, as well as in fat bod
y
cell t
y
pes, have been reported. For example, Jensen and Bør
g
ese
n
(
2000) described 11 cell types in queens of the pharaoh ant
,
M
onomorium pharaonis, based
o
nt
h
e
i
r pos
i
t
i
on,
hi
stoc
h
em
i
stry, an

d
u
l
trastructure. Groups o
f
ce
ll
so
f
eac
h
type are
l
ocate
d
i
n spec
ifi
c pos
i
t
i
ons t
h
roug
h
out t
h
e
b

o
d
y. It s
h
ou
ld b
e stresse
d
,
h
owever, t
h
at t
h
ere
i
s
li
tt
l
e
e
vidence for the functions of these man
y
histot
y
pes
.
T
he fat bod

y
is composed mainl
y
of cells called trophoc
y
tes, thou
g
h in some species
urate cells (uroc
y
tes) and/or m
y
cetoc
y
tes (Section 4.1.2) also can be seen scattered throu
g
h-
o
ut t
h
et
i
ssue. In em
b
ryos, ear
l
y postem
b
ryon
i

c stages, an
d
starve
di
nsects t
h
e
i
n
di
v
id
ua
l
trop
h
ocytes are eas
il
y
di
st
i
ngu
i
s
h
a
bl
e, t
h

e
i
r nuc
l
eus
i
s roun
d
e
d
,an
d
t
h
e
i
r cytop
l
asm con
-
ta
i
ns
f
ew
i
nc
l
us
i

ons. As suc
h
,t
h
e
y
c
l
ose
ly
resem
bl
e
h
emoc
y
tes, w
i
t
h
w
hi
c
h
t
h
e
y
pro
b

a
bly
have a close ph
y
lo
g
enetic relationship. In later larval sta
g
es and adults the trophoc
y
te
s
e
nlar
g
e and become vacuolated. The vacuoles contain reserves of fat, protein, and
g
l
y
co-
gen. T
h
e trop
h
ocyte nuc
l
e
i
are proport
i

onate
l
y
l
arge an
df
requent
l
y
b
ecome e
l
ongate an
d
m
uc
hb
ranc
h
e
d
. Dur
i
ng metamorp
h
os
i
s
i
nen

d
opterygotes, t
h
e reserves are
lib
erate
di
nto
t
h
e
h
emo
ly
mp
h
. In some D
i
ptera an
d
H
y
menoptera t
h
ema
j
or
i
t
y

o
f
trop
h
oc
y
tes a
l
so
di
s
i
n
-
te
g
rate at this time, and the fat bod
y
appears to be completel
y
re-formed in the adult from
the few cells that remain
.
In a number of species uric acid accumulates in large quantities in specific cells, th
e
urocytes, within the fat body. Cochran (198
5
) disputed the traditional view that this is a
f
orm o

f
storage excret
i
on (C
h
apter 18, Sect
i
on 3.3) an
d
propose
d
t
h
at suc
h
accumu
l
at
i
o
n
represents a mobile reserve of nitro
g
en, especiall
y
in species such as cockroaches an
d
termites whose natural diet is deficient in this element. In cockroaches the uroc
y
tes surround

m
ycetocytes (see below) and there is circumstantial evidence to suggest that the bacteria
are intimately involved in the synthesis and utilization of the uric acid (Cochran, 198
5
)
.
5.
1
.
2
.M
y
cetoc
y
te
s
M
ycetocytes (
b
acter
i
ocytes) are
f
oun
di
nw
id
e
l
y

diff
erent groups o
fi
nsects, t
h
oug
hi
n
c
ommon t
h
ese
h
ave nutr
i
t
i
ona
ll
y poor or un
b
a
l
ance
ddi
ets suc
h
as woo
d
(ants, ceram

b
y
-
c
id, and anobiid beetle larvae), phloem sap (aphids, planthoppers, meal
y
bu
g
s), and bloo
d
(
bedbu
g
s, tsetse flies, suckin
g
lice). M
y
cetoc
y
tes are
g
enerall
y
stated to be specialized fat
505
FOO
D
U
PT
A

KE
A
ND
UTILIZ
A
TION
b
od
y
cells (e.
g
., Haunerland and Shirk, 199
5
). However, Braendle
et al.
(
2003
)
showed that
in the
p
ea a
p
hid,
A
cyrthosiphon pisu
m
,
some m
y

cetoc
y
tes ori
g
inate from nuclei near the
posterior end of the embr
y
o while others ma
y
have their ori
g
in as nuclei in the middle o
f
th
eem
b
ryon
i
c centra
l
syncyt
i
um. Usua
ll
y, t
h
e mycetocytes
f
orm spec
ifi

c structures
k
nown
as mycetomes (
b
acter
i
omes)
di
st
i
nct
f
rom t
h
e
f
at
b
o
d
y. T
h
e mycetocytes conta
i
n sym
bi
ot
ic
b

acter
i
a (rare
ly
,
y
easts)
f
or w
hi
c
h
var
i
ous ro
l
es
h
ave
b
een propose
d
(Dou
gl
as, 1989, 1998;
Dixon, 1998). The m
y
cetoc
y
te s

y
stem of aphids is particularl
y
well studied. The primar
y
b
acterial s
y
mbiont is Buchnera aphidicol
a
, which is inherited transovariall
y
(from mothe
r
t
o
d
aug
h
ters v
i
at
h
e ovary). T
h
e num
b
er o
f
mycetocytes

i
nt
h
e
h
ost
i
s
fi
xe
d
at
bi
rt
h
(
i
.e., t
h
ey
d
o not
di
v
id
e), t
h
oug
h
t

h
ece
ll
sgrowtoa
b
out
f
our t
i
mes t
h
e
i
ror
i
g
i
na
l
s
i
ze
d
ur
i
ng
l
arva
l
d

evelopment as the bacteria multipl
y
. For aphids there is stron
g
evidence that the bacteria
suppl
y
the host with essential amino acids. The bacteria ma
y
also participate in nitro
g
en
rec
y
clin
g
and up
g
radin
g
(convertin
g
excretor
y
nitro
g
en to useful materials) as su
gg
ested
a

b
ove
f
or coc
k
roac
h
es. In ap
hid
st
h
eev
id
ence
i
s aga
i
nstaro
l
e
f
or t
h
e
b
acter
i
a
i
nt

h
e syn
-
th
es
i
so
f
v
i
tam
i
ns an
dli
p
id
s (Doug
l
as, 1998);
h
owever, t
hi
s poss
ibili
ty cannot
b
eru
l
e
d

out
f
or ot
h
er
i
nsect
g
roups. Cur
i
ous
ly
,an
d
contrar
y
to w
h
at
i
s
g
enera
lly
assume
d
,t
h
ere
i

sno
known benefit to the bacteria from this association (Dou
g
las, 1998).
5
.2. Carboh
y
drate Metabolis
m
A
s
i
not
h
er an
i
ma
l
s, s
i
mp
l
esu
g
ars prov
id
e a rea
dily
ava
il

a
bl
esu
b
strate t
h
at can
be
oxidized for production of ener
gy
. However, in contrast to vertebrates where
g
lucose in the
b
lood is the su
g
ar of importance as an ener
gy
source, in insect hemol
y
mph
g
lucose an
d
other monosaccharides usually are present only in minimal amounts. An exception to thi
s
statement
i
st
h

ewor
k
er
h
oney
b
ee w
h
ose
h
emo
l
ymp
h
g
l
ucose may reac
h
a concentrat
i
on
o
f
a
l
most 3 g/100 m
l
an
di
s use

d
as t
h
e energy source
d
ur
i
ng
fli
g
h
t. In most
i
nsects,
a
d
isaccharide, trehalose, is the immediate ener
gy
source. Trehalose consists of two
g
lucos
e
molecules
j
oined throu
g
han
α
1
, 1-linka

g
e. Its level in the hemol
y
mph is constant and in a
state of dynamic equilibrium with glycogen stored in the fat body (Friedman, 1978). In this
respect, t
h
ere
f
ore, t
h
es
i
tuat
i
on
i
s compara
bl
etot
h
at
i
n verte
b
rates w
h
ose
bl
oo

d
g
l
ucos
e
l
eve
li
s
i
n equ
ilib
r
i
um w
i
t
hli
ver g
l
ycogen. T
h
es
i
m
il
ar
i
ty goes
f

urt
h
er. Just as t
h
e convers
i
on
of liver
g
l
y
co
g
en to blood
g
lucose is promoted b
y
the hormone
g
luca
g
on, which stimulates
g
l
y
co
g
en phosphor
y
lase activit

y
, in insect fat bod
y
the formation of trehalose from
g
l
y
co
g
en
is promoted b
y
ah
y
per
g
l
y
cemic hormone released from the corpora cardiaca. This hormon
e
act
i
vates t
h
ep
h
osp
h
ory
l

ase, w
hi
c
h
removes a g
l
ucose un
i
t
f
rom t
h
eg
l
ycogen. Becaus
e
o
fi
ts
hi
g
hl
ypo
l
ar, po
l
y
h
y
d

roxy
l
nature, tre
h
a
l
ose
d
oes not eas
il
y penetrate t
h
e musc
le
ce
ll
mem
b
rane. T
h
ere
f
ore,
b
e
f
ore
i
t can
b

eox
idi
ze
dby
musc
l
e
i
t must
fi
rst
b
e converte
d
into
g
lucose b
y
ah
y
drol
y
zin
g
enz
y
me, trehalase, present in the muscle cell membrane
.
Hemol
y

mph trehalose is also the source of the
g
lucose that is converted b
y
epiderma
l
ce
ll
s
i
nto acety
l
g
l
ucosam
i
ne
d
ur
i
ng pro
d
uct
i
on o
f
t
h
en
i

trogenous po
l
ysacc
h
ar
id
ec
hi
t
i
n
(C
h
apter 11, Sect
i
on 3.1).
Gl
ycogen
i
san
i
mportant reserve su
b
stance
i
na
l
most a
ll i
nsects an

di
s
f
oun
di
n
hi
g
h
concentration in the fat bod
y
, with smaller amounts in muscle, especiall
y
fli
g
ht muscle, and
sometimes the mid
g
ut epithelium. In the mature bee larva, for example,
g
l
y
co
g
en makes
u
p about one-third of the dry weight. It is produced principally from glucose and othe
r
monosacc
h

ar
id
es a
b
sor
b
e
df
rom t
h
e gut
f
o
ll
ow
i
ng
di
gest
i
on. In some
i
nsects g
l
ycogen may
a
l
so
b
e synt

h
es
i
ze
df
rom am
i
no ac
id
s. As note
d
a
b
ove,
f
at
b
o
d
yg
l
ycogen (an
d
per
h
ap
s
also that stored in the mid
g
ut epithelium) is used to maintain a constant level of trehalose

506
CHAPTER 1
6
i
n the hemol
y
mph. The
g
l
y
cerol produced as an antifreeze in the hemol
y
mph of some
i
nsects that must withstand extremel
y
low winter temperatures is also derived from fat
bod
yg
l
y
co
g
en. Gl
y
co
g
en in muscle is used directl
y
as an ener

gy
source, bein
g
de
g
raded a
s
i
n mamma
li
an t
i
ssue v
i
at
h
eg
l
yco
l
yt
i
c pat
h
way, Kre
b
scyc
l
e, an
d

resp
i
ratory c
h
a
i
n, w
i
t
h
resu
l
tant pro
d
uct
i
on o
f
ATP.
G
ly
co
g
en a
l
so
i
sas
ig
n

ifi
cant component o
f
t
h
e
y
o
lk i
nt
h
ee
gg
so
f
some
i
nsects. Its us
e
as an ener
gy
source in this situation is, however, secondar
y
to its importance as a provide
r
o
f
g
lucose units for chitin s
y

nthesis in the developin
g
embr
y
o.
As
i
n verte
b
rates, t
h
ema
j
or
f
unct
i
ons o
f
t
h
e pentose cyc
l
e
i
n
i
nsects are (1) pro
d
uct

i
on
of
re
d
uc
i
ng equ
i
va
l
ents (as NADP) t
h
at are use
d
,
f
or examp
l
e,
i
n
li
p
id
synt
h
es
i
s, an

d
(2)
production of five-carbon su
g
ars for nucleic acid s
y
nthesis. In addition, throu
g
h its abilit
y
to interconvert su
g
ars containin
g
from three to seven carbon atoms, the pentose c
y
cle ca
n
c
han
g
e “unusual” su
g
ars produced durin
g
di
g
estion into six-carbon derivatives and henc
e
i

nto g
l
ycogen.
5.3. L
i
p
i
d Metabol
i
s
m
Fo
r most insects, fats stored in the fat bod
y
are the primar
y
ener
gy
reserve and, like thos
e
of
ot
h
er an
i
ma
l
s, are most
l
ytr

i
g
l
ycer
id
es. T
h
ey may
b
e
f
orme
ddi
rect
l
y
b
y com
bi
nat
i
o
n
of
t
h
e
f
atty ac
id

san
d
g
l
ycero
l
pro
d
uce
dd
ur
i
ng
di
gest
i
on or
f
rom am
i
no ac
id
san
d
s
i
mp
l
e
su

g
ars. T
y
p
i
ca
lly
,
f
at
i
s store
d
t
h
rou
gh
out t
h
e
j
uven
il
e per
i
o
d
, espec
i
a

lly i
nen
d
opter
yg
ote
s
w
here at metamorphosis it ma
y
make up between one-third and one-half of the dr
y
wei
g
h
t
o
f an insect. Lar
g
e amounts of fat also accumulate in the e
gg
durin
g
vitello
g
enesis. Fats
are used as an energy source during “long-term” energy-requiring events, for example
,
e
mbryogenesis, metamorphosis, starvation, and sustained flight (Chapter 14, Section 3.3.

5
).
O
nawe
i
g
h
t-
f
or-we
i
g
h
t
b
as
i
s,
f
ats conta
i
ntw
i
ce as muc
h
energy as car
b
o
h
y

d
rates; t
h
ey ar
e
therefore more economical to store
.
In addition to the fats
j
ust described which serve solel
y
as ener
gy
reserves or sources o
f
c
arbon, many other lipids having structural or metabolic functions occur in insects. Waxes
are m
i
xtures o
fl
ong-c
h
a
i
na
l
co
h
o

l
sorac
id
s, t
h
e
i
r esters, an
d
para
ffi
ns. T
h
e num
b
er o
f
c
arbon atoms that form the chain ranges between 12 and 3
6
, and both unsaturated an
d
saturated compounds have been identified. It seems that the various components of wax ar
e
s
y
nthesized from fatt
y
acid precursors. The paraffins and, possibl
y

, some acids are produced
b
y
oenoc
y
tes, whereas alcohol and ester s
y
nthesis occurs in the fat bod
y
. Compound lipids
are
f
atty ac
id
s com
bi
ne
d
w
i
t
h
avar
i
ety o
f
organ
i
cor
i

norgan
i
c res
id
ues,
f
or examp
l
e
,
c
ar
b
o
h
y
d
rates, n
i
trogenous
b
ases, am
i
no ac
id
s, p
h
osp
h
ate, an

d
su
lf
ate. T
h
e meta
b
o
li
sm o
f
t
h
ese
li
p
id
s
i
n
i
nsects
i
s
f
or t
h
e most part poor
ly k
nown. Certa

i
no
f
t
h
em,
f
or examp
l
e
,
c
holine, a phospholipid, cannot be s
y
nthesized b
y
insects and must be included in the diet
.
L
ikewise, sterols are essential com
p
onents of the diet in almost all insects
.
5.4. Am
i
no Ac
i
d and Prote
i
n Metabol

i
sm
In
g
rowin
g
insects a lar
g
e proportion of the amino acids that result from di
g
estion is
used directl
y
in the formation of new tissue proteins, both structural and metabolic. Withi
n
the fat body especially, but also in other tissues, a variety of transaminations also occur;
t
h
at
i
s, t
h
eam
i
no group
f
rom an am
i
no ac
id

can
b
e trans
f
erre
d
to a
k
eto ac
id
to
f
ormanew
am
i
no ac
id
. Suc
h
transam
i
nat
i
ons are espec
i
a
ll
y
i
mportant w

h
en an
i
nsect’s
di
et conta
i
ns
i
nsufficient amounts of particular amino acids. As in vertebrates, not all amino acids can
be s
y
nthesized in insect tissues. Those that cannot must be included in the diet or provided
507
FOO
D
U
PT
A
KE
A
ND
UTILIZ
A
TION
by
s
y
mbiotic microor
g

anisms (Section
5
.1.2). Durin
g
starvation or when present in excess
,
amino acids ma
y
under
g
o oxidative deamination (i.e., be oxidized and simultaneousl
y
los
e
t
h
e
i
r
α
-
amino
g
roup) within the fat bod
y
, resultin
g
in the formation of the correspondin
g
k

eto ac
id
s. T
h
e
l
atter can t
h
en
b
e
f
urt
h
er ox
idi
ze
d
v
i
at
h
e Kre
b
scyc
l
ean
d
resp
i

ratory c
h
a
in
t
o prov
id
e energy or may
b
e converte
di
nto car
b
o
h
y
d
rate or
f
at reserves. T
h
e ammon
ia
pro
d
uce
dd
ur
i
n

gd
eam
i
nat
i
on
i
s norma
lly
converte
di
nto ur
i
cac
id.
The fat bod
y
is an important site of hemol
y
mph protein s
y
nthesis, especiall
y
in the late
j
uvenile sta
g
es of endopter
yg
otes and in adult female insects. Stora

g
e hexamers (so-called
b
ecause t
h
e prote
i
ns compr
i
se s
i
x
h
omo
l
ogous su
b
un
i
ts)
h
ave
b
een c
h
aracter
i
ze
df
rom mor

e
th
an 20 spec
i
es
i
ns
i
xor
d
ers, ma
i
n
l
yD
i
ptera an
d
Lep
id
optera (Te
lf
er an
d
Kun
k
e
l
, 1991)
.

T
he
y
reach ver
y
hi
g
h concentrations in the hemol
y
mph
j
ust before metamorphosis, and ar
e
effectivel
y
servin
g
as a store of amino acids for use in adult tissue and protein formation. If
stored “individuall
y
,” the amino acids would create a severe osmotic problem for the insect
.
I
ns
ilk
mot
h
(
B
om

by
x
)
caterp
ill
ars,
f
or examp
l
e, t
h
e
h
emo
l
ymp
h
prote
i
n concentrat
i
on
i
ncreases s
i
x
f
o
ld f
rom t

h
e
f
ourt
h
to t
h
e
fi
na
li
nstar,
i
n preparat
i
on
f
or sp
i
nn
i
ng t
h
e cocoon,
metamorp
h
os
i
s, an
d

e
gg
pro
d
uct
i
on. W
h
en t
h
ea
d
u
l
t emer
g
es, t
h
e concentrat
i
on
h
as
f
a
ll
en to
about one-third the value at pupation. Durin
g
sexual maturation in females of man

y
species,
t
he fat bod
y
produces vitello
g
enins (“female-specific” proteins). These proteins, whos
e
synt
h
es
i
s
i
sregu
l
ate
db
y
j
uven
il
e
h
ormone or ec
d
ysone, are accumu
l
ate

di
n
l
arge amounts
b
yt
h
e
d
eve
l
op
i
ng oocytes (C
h
apter 19, Sect
i
on 3.1.1). In
f
ema
l
e
i
nsects t
h
at
d
o not
f
ee

d
as
a
d
u
l
ts (e.
g
., Bom
by
x), t
h
e stora
g
e
h
examers are t
h
e source o
f
t
h
eam
i
no ac
id
s
f
or v
i

te
ll
o
g
en
in
production which occurs durin
g
the pharate adult sta
g
e (Chapter 21, Section 3.3.3). In the
males of some species the fat bod
y
produces proteins that are accumulated b
y
the accessor
y
reproductive glands, probably for use in spermatophore production. Lipophorins are anothe
r
i
mportant group o
fh
emo
l
ymp
h
prote
i
ns synt
h

es
i
ze
di
nt
h
e
f
at
b
o
d
y. T
h
ese, o
f
ten very
l
arge
,
mo
l
ecu
l
es are
i
mportant
i
nt
h

ea
b
sorpt
i
on an
d
transport o
fli
p
id
san
d
,a
ddi
t
i
ona
ll
y, serve as
coa
g
ulo
g
ens in hemol
y
mph clottin
g
(Chapter 17, Section 4.2.2)
.
5

.5. Metabolism of Insecticides
Nowa
d
ays
i
nsect
i
c
id
es may
b
e regar
d
e
d
as a norma
l
env
i
ronmenta
lh
azar
df
or
i
nsects
,
survival over which has been achieved throu
g
h natural selection of resistant strains. Thou

g
h
t
he purpose of this section is to outline the biochemical pathwa
y
sb
y
which insecticides
are rendered harmless, it should be realized that resistance can also be developed, solel
y
or
part
i
a
ll
y, as a resu
l
to
f
p
h
ys
i
ca
l
rat
h
er t
h
an meta

b
o
li
cc
h
anges
i
n a spec
i
es. T
hi
s
i
s
b
ecause
,
ul
t
i
mate
l
y, t
h
e
d
egree o
f
res
i

stance
i
s
d
epen
d
ent on t
h
e rate at w
hi
c
h
an
i
nsect can
d
egra
d
e
th
e tox
i
c mater
i
a
l
so t
h
at
l

et
h
a
l
quant
i
t
i
es
d
o not accumu
l
ate at t
h
es
i
te o
f
act
i
on. Amon
g
t
h
e
ph
y
sical alterations that ma
y
lead to increased resistance are (1) a decline in the permeabilit

y
of the inte
g
ument (achieved b
y
increasin
g
cuticle thickness or the extent of tannin
g
,orb
y
mo
dif
y
i
ng t
h
e compos
i
t
i
on o
f
t
h
e cut
i
c
l
e); (2) a c

h
ange
i
nt
h
epHo
f
t
h
e gut, resu
l
t
i
ng
i
n
a
d
ecrease
i
nso
l
u
bili
ty an
d
,t
h
ere
f

ore, rate o
f
a
b
sorpt
i
on o
f
an
i
nsect
i
c
id
e; (3) an
i
ncreas
e
i
nt
h
e amount o
ff
at store
d(
most
i
nsect
i
c

id
es are
f
at-so
l
u
bl
ean
d
,t
h
ere
f
ore, accumu
l
ate
i
n
fatt
y
tissues in which the
y
are ineffective); (4) a decrease in permeabilit
y
of the membrane
s
surroundin
g
the tar
g

et tissue (usuall
y
the nervous s
y
stem); and (5) a chan
g
e in the ph
y
sica
l
structure of the target site.
I
nsects
h
ave v ar
i
ous met
h
o
d
s
f
or
d
etox
if
y
i
ng potent
i

a
ll
y
h
arm
f
u
l
su
b
stances, man
y
o
f
w
hi
c
h
para
ll
e
l
t
h
ose
f
oun
di
n verte
b

rates. Hy
d
ro
l
ys
i
s,
h
y
d
roxy
l
at
i
on, su
lf
at
i
on, met
h
y
-
lation, acet
y
lation, and con
j
u
g
ation with c
y

steine,
g
l
y
cine,
g
1ucose,
g
lucuronic acid, o
r
phosphate are examples of the methods emplo
y
ed (Perr
y
and A
g
osin, 1974; Wilkinson
,
508
CHAPTER 1
6
1
97
6
). H
y
drox
y
lation and con
j

u
g
ation are also important in makin
g
the normall
y
fat-solubl
e
i
nsecticides water-soluble so that the
y
can be excreted. Each of these processes is enz
y
-
m
aticall
y
controlled, and it is not surprisin
g
to find, therefore, that metabolic resistance
to
i
nsect
i
c
id
es most o
f
ten resu
l

ts
f
rom qua
li
tat
i
ve or quant
i
tat
i
ve c
h
anges
i
nt
h
e enzymes
c
oncerne
d
so t
h
at t
h
e rate o
fd
etox
i
cat
i

on
i
s
i
ncrease
d
. For examp
l
e,avar
i
ety o
f
esterases
b
r
i
n
g
a
b
out
hyd
ro
ly
s
i
s, m
i
xe
d

-
f
unct
i
on ox
id
ases common
ly i
n
d
uce
hyd
rox
yl
at
i
on, an
dgl
u
-
tat
hi
o
n
e
S
-
transferase is responsible for promotin
g
con

j
u
g
ation with this tripeptide. In fact
,
the involvement of these enz
y
mes in insecticide detoxication appears to be an extension of
a more genera
lf
unct
i
on. T
h
us,
i
nsects t
h
at encounter a
b
roa
d
range o
f
natura
ll
y occurr
i
n
g

p
l
ant-
d
er
i
ve
d
tox
i
cants,
f
or examp
l
e, po
l
yp
h
agous Lep
id
optera,
h
ave s
i
gn
ifi
cant
l
y
hi

g
h
e
r
m
ixed-function oxidase levels than do oli
g
opha
g
ous species (Ronis and Hod
g
son, 1989). I
n
some insecticide-resistant strains, an increase in the amount of detoxicant enz
y
me has been
o
bserved, which at the
g
ene level results from either an increase in the rate of transcription
(
mRNA pro
d
uct
i
on) or gene amp
lifi
cat
i
on (t

h
e presence o
f
many
id
ent
i
ca
l
cop
i
es o
f
t
h
e
D
NA co
di
ng
f
or t
h
e enzyme) (Devons
hi
re an
d
F
i
e

ld
, 1991). In ot
h
ers,
i
t appears t
h
at t
h
e
e
nz
y
me
h
as c
h
an
g
e
d
so t
h
at
i
t
i
s now more spec
ifi
ctowar

di
ts “new” su
b
strate, t
h
e
i
nsect
i
-
c
ide. In a few s
p
ecies, resistance seems to have develo
p
ed as a result of an increase in the
q
uantit
y
, or decrease in the sensitivit
y
, of the enz
y
me normall
y
affected b
y
the insecticide,
spec
ifi

ca
ll
yc
h
o
li
nesterase
i
nt
h
e nervous system.
M
ec
h
an
i
sms
f
or
d
etox
i
cat
i
on vary among t
h
e
diff
erent categor
i

es o
fi
nsect
i
c
id
es. It
i
s appropr
i
ate, t
h
ere
f
ore, to exam
i
ne separate
ly
t
h
e meta
b
o
li
sm o
f
compoun
d
s
i

nt
h
ese
c
ate
g
ories. Three cate
g
ories of insecticides will be considered: chlorinated h
y
drocarbons
,
o
r
g
anophosphates, and carbamates
.
Among the chlorinated hydrocarbon insecticides are DDT, lindane
(
γ
-
BHC), chlor-
d
ane,
h
eptac
hl
or, a
ld
r

i
n, an
di
so
d
r
i
n
.
∗
Th
ec
hl
or
i
nate
dh
y
d
rocar
b
ons act on t
h
e nervou
s
s
ystem, prevent
i
ng norma
li

mpu
l
se transm
i
ss
i
on
b
y
bi
n
di
ng to so
di
um or c
hl
or
id
ec
h
anne
l
p
roteins in the axonal membrane (Bloomquist, 199
6
; Zlotkin, 1999). B
y
bindin
g
to sodium

-
c
hannel proteins, DDT and its analo
g
ues enable sodium ions to diffuse readil
y
across th
e
m
embrane of excitatory neurons; thus, the membrane becomes permanently depolarized.
D
DT was t
h
e
fi
rst synt
h
et
i
c
i
nsect
i
c
id
eto
b
e
d
eve

l
ope
d
an
d
“appropr
i
ate
l
y” was t
h
e
fi
rst
to w
hi
c
hi
nsects
d
eve
l
ope
d
res
i
stance. Res
i
stance to DDT an
di

ts ana
l
ogues
i
s most o
f-
ten the result of the presence of an enz
y
me that dechlorinates the compound, formin
g
the
l
ess toxic dichloroeth
y
lene derivative, DDE. Some species, however, convert DDT int
o
o
ther less harmful materials such as DDA (the acetic acid derivative), dicofol (kelthane, the
tr
i
c
hl
oroet
h
ano
ld
er
i
vat
i

ve), an
d
DDD (t
h
e
di
c
hl
oroet
h
ane
d
er
i
vat
i
ve). Many compar
i
sons
of
t
h
e act
i
v
i
ty o
f
t
h

e DDT-
d
egra
di
ng enzyme
i
n res
i
stant an
d
suscept
ibl
e stra
i
ns o
f
a spec
i
es
h
ave s
h
own
,h
owever
,
t
h
at meta
b

o
li
c res
i
stance a
l
one
i
so
f
ten
i
nsu
ffi
c
i
ent to account
f
o
r
the full extent of resistance. Specificall
y
, the known maximum rate of de
g
radation of DDT
measu
r
ed
i
nv

i
tro is not hi
g
h enou
g
h to account for the hi
g
h tolerance shown b
y
the resis-
tant stra
i
n. In suc
h
cases,
f
urt
h
er wor
kh
as usua
ll
ys
h
own t
h
e
i
mportance a
l

so o
f
p
h
ys
i
ca
l
r
es
i
stance mec
h
an
i
sms o
f
t
h
e type out
li
ne
d
ear
li
er
.
Th
ec
y

c
l
o
di
ene compoun
d
s,
h
eptac
hl
or, a
ld
r
i
n, an
di
so
d
r
i
n, are o
fi
nterest
f
rom severa
l
v
iewpoints. In themselves the
y
are not toxic but are oxidized within an insect’s tissues to

the hi
g
hl
y
toxic epox
y
derivatives, heptachlorepoxide, dieldrin, and endrin, respectivel
y,
ap
rocess known as “autointoxication.” Because of this conversion, insects treated with
∗
Th
ese are approve
d
common names. For t
h
ec
h
em
i
ca
l
names, see Perr
y
an
d
A
g
os
i

n (1974).
509
FOO
D
U
PT
A
KE
A
ND
UTILIZATION
t
hese compounds show no s
y
mptoms for 1 or 2 hours after treatment, in contrast to insect
s
t
reated with other insecticides that react within a matter of minutes. In contrast to DDT
,
t
hese insecticides block chloride channels in inhibitor
y
neurons, b
y
bindin
g
to the GABA-
receptor prote
i
n, caus

i
ng
h
yperexc
i
tat
i
on o
f
t
h
e nervous system. T
h
e res
i
stance s
h
own
by
certa
i
n stra
i
ns
i
s not
b
ecause t
h
ey no

l
onger convert an
i
nsect
i
c
id
eto
i
ts tox
i
c
f
orm a
s
mi
g
ht be anticipated. Further, the toxic derivatives appear to have
g
reat stabilit
y
, remainin
g
u
nchan
g
ed even in resistant insects for several da
y
s. Resistance is due to a simple chan
g

e
in the structure of the GABA-receptor protein, specificall
y
, substitution of alanine to serine
or g
l
yc
i
ne (
ff
renc
h
-Constant
e
ta
l.
,
1993, 2000).
Organop
h
osp
h
ates (e.g., parat
hi
on, ma
l
at
hi
on,
di

az
i
non, an
ddi
met
h
oate)
bi
n
d
cova
-
lentl
y
with and inhibit the action of cholinesterase, the enz
y
me that normall
y
de
g
rades
acet
y
lcholine at excitator
y
s
y
napses, thou
g
h there are reports that their toxicit

y
is partiall
y
related also to inhibition of other tissue esterases. As with chlorinated h
y
drocarbons, resis
-
t
ance to organop
h
osp
h
ates may
b
e
d
eve
l
ope
d
as a resu
l
to
f
p
h
ys
i
ca
l

c
h
ange,
b
ut genera
lly
i
s meta
b
o
li
c. L
ik
ecyc
l
o
di
enes, many organop
h
osp
h
ates are “act
i
vate
d
” (ren
d
ere
d
more

t
ox
i
c) as a resu
l
to
f
ox
id
at
i
on;
f
or examp
l
e, parat
hi
on
i
s converte
d
to paraoxon. T
h
us, re
-
sistance ma
y
be caused b
y
a decrease in the rate of activation and/or an increase in the

rate of conversion of the compound to a non-toxic form. (Apparentl
y
, resistance does no
t
d
eve
l
op as a resu
l
to
fd
ecrease
d
sens
i
t
i
v
i
ty o
f
t
h
ec
h
o
li
nesterase to an
i
nsect

i
c
id
e.) Many
reports
h
ave s
h
own t
h
at res
i
stant stra
i
ns are more a
bl
e to carry out con
j
ugat
i
on, espec
i
a
lly
wi
t
hgl
utat
hi
one, or

hyd
ro
ly
s
i
so
f
t
h
e
i
nsect
i
c
id
et
h
an are suscept
ibl
e
i
nsects. T
hi
sa
bili
t
y
results from the presence of either
g
reater quantities of esterif

y
in
g
enz
y
mes or enz
y
me
s
t
hat, throu
g
h mutation and natural selection, have become more specific for an insecticide.
C
arbamates, for example, furadan, sevin, pyrolan, and isolan, are substituted esters o
f
car
b
am
i
cac
id
,w
hi
c
h
,
lik
e organop
h

osp
h
ates, attac
k
c
h
o
li
nesterase. Res
i
stance to t
h
ese
i
nsect
i
c
id
es a
l
so
i
s very s
i
m
il
ar to t
h
at
f

or organop
h
osp
h
ates. Some res
i
stance can
be
achieved b
y
ph
y
sical chan
g
es, but most is the result of increased rates of de
g
radation
,
especiall
y
throu
g
h oxidation and h
y
drol
y
sis
.
S
hortly after the discovery that most resistance is metabolic, that is, results fro

m
i
ncrease
d
quant
i
t
i
es or spec
ifi
c
i
ty o
f
part
i
cu
l
ar enzymes t
h
at cause more rap
id b
rea
k
-
d
own o
f
an
i

nsect
i
c
id
e,
i
t was rea
li
ze
d
t
h
at t
h
ep
h
enomenon o
f
synerg
i
sm m
i
g
h
t
b
eex-
plored to advanta
g
e in the use of insecticides. S

y
ner
g
ism describes the situation in which
t
he combined effect of two substances is much
g
reater than the sum of their separate
effects. In
p
ractical terms, in the
p
resent context, it means that a
pp
ro
p
riate substances
(synerg
i
sts), w
h
en m
i
xe
d
w
i
t
h
an

i
nsect
i
c
id
e, wou
ld i
ncrease t
h
e
l
atter’s e
ff
ect
i
veness
by
com
bi
n
i
ng w
i
t
h
(an
di
n
hibi
t

i
ng) t
h
e enzymes t
h
at norma
ll
y
d
egra
d
et
h
e
i
nsect
i
c
id
e. T
h
e
s
y
ner
gi
sts use
d
ma
yb

equ
i
te unre
l
ate
d
c
h
em
i
ca
lly
to t
h
e
i
nsect
i
c
id
e
b
ut, most o
f
ten, are
analo
g
ues. The principle of s
y
ner

g
ism has been applied with limited success in the case
of p
y
rethrin insecticides and DDT. For example, in the earl
y
1950s DMC, the ethanol
d
er
i
vat
i
ve o
f
DDT, was
f
oun
d
to
b
eane
ff
ect
i
ve synerg
i
st
f
or DDT
i

n DDT-res
i
stant
h
ouse
-
fl
ies. However, perhaps not surprisingly, by 19
55
the flies had developed resistance to the
com
bi
nat
i
on
!
6
. Summar
y
Visual
,
tactile
,
or chemical cues stimulate food location and/or selection in most in-
sects. The stimuli ma
y
be
g
eneral, for example, color, pattern, and size, or hi
g

hl
y
specific
510
CHAPTER 1
6
such as the particular odor or taste of a chemical. The chemicals that promote feedin
g
(
pha
g
ostimulants) ma
y
have no nutritional value for an insect.
Ty
picall
y
saliva lubricates and initiates di
g
estion of the food. However, it ma
y
include
c
ompoun
d
st
h
at act
i
n

di
rect
l
yto
f
ac
ili
tate
f
oo
d
upta
k
ean
ddi
gest
i
on or t
h
at
h
ave
f
unct
i
ons
unre
l
ate
d

to
f
ee
di
ng. T
h
e gut
i
nc
l
u
d
es t
h
ree pr
i
mary su
bdi
v
i
s
i
ons,
f
oregut, m
id
gut, an
d
hind
g

ut, and these are t
y
picall
y
differentiated into re
g
ions of differin
g
function. The fore
g
u
t
i
s concerned with stora
g
e and trituration of food, the mid
g
ut with di
g
estion and absorptio
n
o
f small or
g
anic molecules, and the hind
g
ut with absorption of water and ions, thou
gh
some a
b

sorpt
i
on o
f
sma
ll
organ
i
cmo
l
ecu
l
es may occur across t
h
e
hi
n
d
gut wa
ll
, espec
i
a
lly
i
n
i
nsects w
i
t

h
sym
bi
ot
i
cm
i
croorgan
i
sms
i
nt
h
e
i
r
hi
n
d
gut
.
T
he di
g
estive enz
y
mes produced match qualitativel
y
and quantitativel
y

the normal
c
omposition of the diet. The enz
y
mes ma
y
have low specificit
y
, enablin
g
an insect to di
g
est
av
a
riet
y
of molecules of a
g
iven t
y
pe, or ma
y
be hi
g
hl
y
specific, for example, when a specie
s
f

ee
d
sso
l
e
l
y on a part
i
cu
l
ar
f
oo
d
. Gut
fl
u
id i
s
b
u
ff
ere
d
w
i
t
hi
n a narrow pH range to
f

ac
ili
tat
e
di
gest
i
on an
d
a
b
sorpt
i
on. Enzymes are re
l
ease
d
as soon as t
h
ey are synt
h
es
i
ze
d
. Synt
h
es
i
s

i
sre
g
u
l
ate
d
so t
h
at an appropr
i
ate amount o
f
enz
y
me
i
s pro
d
uce
df
or t
h
e
f
oo
d
consume
d
.

Microor
g
anisms in the
g
ut ma
y
be important in di
g
estion, especiall
y
in wood-eatin
g
species
,
w
here the
y
de
g
rade cellulose.
A
b
sorpt
i
on o
fdi
gest
i
on pro
d

ucts occurs most
l
y
i
nt
h
e anter
i
or m
id
gut an
d
mesenter
i
c
c
eca. It
i
s genera
ll
y a pass
i
ve process, t
h
oug
h
carr
i
er mo
l

ecu
l
es may
b
e use
d
to
f
ac
ili
tate t
h
e
process. T
h
e rate at w
hi
c
h
su
g
ars are a
b
sor
b
e
di
s
li
n

k
e
d
to t
h
e rate at w
hi
c
h
t
h
e
y
are converte
d
to trehalose and, hence,
g
l
y
co
g
en. Lipid absorption is
g
enerall
y
slow, with the lipids bein
g
c
onverted into di- and tri
g

l
y
cerides as the
y
move throu
g
h the mid
g
ut epithelium. Amin
o
acid absorption may be preceded by absorption of water across the midgut wall to produce
a
f
av
f
f
o
ra
bl
e gra
di
ent
f
or
diff
us
i
on. However,
f
ac

ili
tate
d diff
us
i
on an
d
act
i
ve transport systems
are use
df
or some am
i
no ac
id
s
i
n some spec
i
es.
T
he fat bod
y
is the primar
y
site of intermediar
y
metabolism as well as a site for
stora

g
e of metabolic reserves. In most insects, trehalose in the hemol
y
mph is the su
g
ar o
f
i
mportance as an energy reserve. Its concentration in the hemolymph is constant and i
s
i
n
d
ynam
i
c equ
ilib
r
i
um w
i
t
h
g
l
ycogen store
di
nt
h
e

f
at
b
o
d
y. L
i
p
id
s
i
nt
h
e
f
at
b
o
d
y
f
or
m
t
h
ema
j
or energy reserve mo
l
ecu

l
es an
d
are use
di
n
l
ong-term energy-requ
i
r
i
ng processe
s
such as fli
g
ht, metamorphosis, starvation, and embr
y
o
g
enesis. The fat bod
y
is important i
n
protein metabolism, includin
g
amino acid transamination and s
y
nthesis of some specific
p
roteins

.
Th
e
d
eve
l
opment o
f
res
i
stance to
i
nsect
i
c
id
es
i
s norma
ll
yt
h
e resu
l
to
fi
ncrease
d
a
bili

ty
of
an
i
nsect to
d
egra
d
et
h
e
i
nsect
i
c
id
es to
l
ess
h
arm
f
u
l
an
d
excreta
bl
e pro
d

ucts,
b
ut may
b
e
re
l
ate
d
a
l
so to
i
ncrease
d
p
hy
s
i
ca
l
res
i
stance, t
h
at
i
s, to structura
l
c

h
an
g
es t
h
at prevent
i
nsec
-
ticides from reachin
g
or reco
g
nizin
g
the site of action. Metabolic resistance normall
y
devel-
o
ps throu
g
h the production of more specific or
g
reater quantities of insecticide-de
g
radin
g
e
nzymes
.

7
. Literature
Av
a
st
li
terature ex
i
sts on gut p
h
ys
i
o
l
ogy an
d
meta
b
o
li
sm, an
d
t
h
e
f
o
ll
ow
i

ng
i
s
a
representat
i
ve se
l
ect
i
on. Foo
d
se
l
ect
i
on an
d
t
h
eregu
l
at
i
on o
ff
ee
di
ng are exam
i

ne
db
y Barton
Browne (197
5
), Berna
y
s (198
5
), and authors in Chapman and de Boer (199
5
). Authors i
n
511
FOO
D
U
PT
A
KE
A
ND
UTILIZ
A
TION
L
ehane and Billin
g
sle
y

(199
6
) deal with diverse aspects of mid
g
ut structure and function
.
R
ichards and Richards (1977), Terra (1996), and Lehane (1997) review the
p
eritro
p
hi
c
matrix. Di
g
estion is covered b
y
House (1974), Terra (1990), and Terra and Ferreira (1994).
B
rezna
k
(1982) an
d
Brezna
k
an
d
Brune (1994) exam
i
ne t

h
ero
l
eo
f
m
i
croorgan
i
sms
i
nt
he
d
igestion of cellulosic materials. Absorption is considered by Treherne (19
6
7), Turunen
(198
5
), Sacchi and Wolfersber
g
er (1996), and Turunen and Crailsheim (1996). Dean
et al.
(1985), Keele
y
(1985), Haunerland and Shirk (1995), and Locke (1998) review the fat bod
y.
G
ilmour (1965) summarizes
g

eneral insect metabolism, while Steele (1976, 1983), Keele
y
(1978), an
d
G¨a
d
e (2004) rev
i
ew t
h
e
h
ormona
l
contro
l
o
f
meta
b
o
li
sm. Downer’s (1981
)
t
ext
d
ea
l
sw

i
t
h
t
h
e energy meta
b
o
li
sm o
fi
nsects, emp
h
as
i
z
i
ng
h
ow
i
t
diff
ers
f
rom t
h
at o
f
other animals. The metabolism of insecticides is reviewed b

y
Perr
y
and A
g
osin (1974), and
authors in Wilkinson
(
1976
)
and Kerkut and Gilbert
(
1985
)
, Volume 10.
Ali
, D. W., 1997, T
h
eam
i
ner
gi
can
d
pept
id
er
gi
c
i

nnervat
i
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fi
nsect sa
li
var
ygl
an
d
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1
9
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Berna
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h
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i
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i
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