Entomology 3rd edition - C.Gillott - Chapter 1 pptx

I
E
volution and Diversit
y
1
A
rthropod Evolution
1
. Intr
oduc
t
ion
Despite their remarkable diversit
y
of form and habits, insects possess several commo
n
features b
y
which the
g
roup as a whole can be distin
g
uished. The
y
are
g
enerall
y
small
arthropods whose bodies are divisible into cephalic, thoracic, and abdominal re
g

ions. The
h
ea
d
carr
i
es one
p
a
i
ro
f
antennae, one
p
a
i
ro
f
man
dibl
es, an
d
two
p
a
i
rs o
f
max
ill

ae (t
h
e
hi
n
d
pa
i
r
f
use
d
to
f
orm t
h
e
l
a
bi
um). Eac
h
o
f
t
h
ree t
h
orac
i

c segments
b
ears a pa
i
ro
fl
egs
an
d
,
i
nt
h
ea
d
u
l
t, t
h
e meso- an
d
/or metat
h
orac
i
cse
g
ments usua
lly h
aveapa

i
ro
f
w
i
n
g
s.
Abdominal appenda
g
es, when present,
g
enerall
y
do not have a locomotor
y
function. The
g
enital aperture is located posteriorl
y
on the abdomen. With few exceptions e
gg
s are laid
,
an
d
t
h
e young
f

orm may
b
equ
i
te
diff
erent
f
rom t
h
ea
d
u
l
t; most
i
nsects un
d
ergo some
d
egre
e
o
f
metamorp
h
os
i
s
.

A
l
t
h
ou
gh
t
h
ese ma
y
seem
i
n
i
t
i
a
lly
to
b
ean
i
nausp
i
c
i
ous set o
f
c
h

aracters, w
h
en t
h
e
y
are examined in relation to the environment it can be seen quite readil
y
wh
y
the Insect
a
h
ave become the most successful
g
roup of livin
g
or
g
anisms. This aspect will be discusse
d
i
nC
h
a
p
ter 2
.
I
nt

h
e present c
h
apter we s
h
a
ll
exam
i
ne t
h
e poss
ibl
eor
i
g
i
ns o
f
t
h
e Insecta, t
h
at
i
s, t
h
e
ev
ol

ut
i
onar
y
re
l
at
i
ons
hi
ps o
f
t
hi
s
g
roup w
i
t
h
ot
h
er art
h
ropo
d
s. In or
d
er to
d

ot
hi
s mean-
in
g
full
y
it is useful first to review the features of the other
g
roups of arthropods. As wil
l
b
ecome apparent below, the question of arthropod ph
y
lo
g
en
y
is controversial, and variou
s
th
eor
i
es
h
ave
b
een
p
ro

p
ose
d
.
2. Arthropod Diversit
y
Art
h
ropo
d
ss
h
are certa
i
n
f
eatures w
i
t
h
w
hi
c
h
t
h
ey can
b
e
d

e
fi
ne
d
.T
h
ese
f
eatures are
:
se
g
mented bod
y
covered with a chitinous exoskeleton that ma
y
be locall
y
hardened and
is periodicall
y
shed, ta
g
mosis (the
g
roupin
g
of se
g
ments into functional units, for exam

-
ple, head, thorax, and abdomen in insects), presence of preoral segments, paired jointed
appen
d
ages on a var
i
e
d
num
b
er o
f
segments,
h
emocoe
li
c
b
o
d
ycav
i
ty conta
i
n
i
ng ost
i
ate
h

eart enc
l
ose
d
w
i
t
hi
n a per
i
car
di
um, nervous system compr
i
s
i
ng
d
orsa
lb
ra
i
nan
d
ventra
l
g
an
g
lionated nerve cord, muscles almost alwa

y
s striated, and epithelial tissue almost alwa
ys
non-ciliated
.
3
4
CHAPTER
1
T
hou
g
h the “true” arthropods fit readil
y
within this definition, three small
g
roups,
the On
y
chophora, Tardi
g
rada, and Pentastoma, whose members are soft-bodied, wormlike
animals with un
j
ointed appenda
g
es, are less obviousl
y
arthropodan and each is usuall
y

g
i
ven separate p
h
y
l
um status
.
2.1.
O
nychophora, Tard
ig
rada, and Pentastoma
T
he approximatel
y
200 extant species of On
y
chophora (Fi
g
ure 1.lA) are terrestria
l
animals living on land masses derived from the Gondwanan supercontinent: Africa, Cen
-
tra
l
an
d
Sout
h

Amer
i
ca, an
d
Austra
l
as
i
a(Ta
i
t, 2001). T
h
ey are genera
ll
y con
fi
ne
d
to mo
i
st
h
a
bi
tats an
d
are
f
oun
db

eneat
h
stones
i
n rott
i
ng
l
ogs an
dl
ea
f
mo
ld
, etc. T
h
ey possess a com-
bination of annelidan and arthropodan characters and, as a result, are alwa
y
s prominent in
discussions of arthropod evolution. Althou
g
h covered b
y
a thin arthropodlike cuticle (com
-
prising procuticle and epicuticle, but no outer wax layer—see Chapter 11), the body wall is
anne
lid
an, as are t

h
e met
h
o
d
o
fl
ocomot
i
on, un
j
o
i
nte
dl
egs, t
h
e excretory system, an
d
t
h
e
n
ervous system. T
h
e
i
r art
h
ropo

d
an
f
eatures
i
nc
l
u
d
ea
h
emocoe
li
c
b
o
d
ycav
i
ty, t
h
e
d
eve
l
op-
m
ent and structure of the
j
aws, the possession of salivar

yg
lands, an open circulator
y
s
y
stem,
a tracheal respirator
y
s
y
stem, and claws at the tips of the le
g
s. Amon
g
livin
g
arthropods
,
m
yriapods resemble the Onychophora most closely: their body form is similar, tagmosis i
s
restr
i
cte
d
to t
h
et
h
ree-segmente

dh
ea
d
, exsert
il
eves
i
c
l
es are present
i
nD
i
p
l
opo
d
aan
d
Sym-
p
h
y
l
aaswe
ll
as
i
n some onyc
h

op
h
orans, a
di
gest
i
ve g
l
an
di
sa
b
sent, t
h
em
id
gut
i
ss
i
m
il
ar
,
the
g
enital tracts of On
y
chophora resemble those of m
y

riapods, the
g
onopore is subtermi
-
n
al, and certain features of embr
y
onic development are common to both
g
roups (Tie
g
s and
Manton, 1958). However, this resemblance is superficial. Recent on
y
chophorans are but th
e
remnants o
f
a more w
id
esprea
df
auna (
f
oss
il
s
f
rom t
h

e Car
b
on
if
erous are very s
i
m
il
ar t
o
m
o
d
ern
f
orms) t
h
at may
h
ave evo
l
ve
df
rom mar
i
ne
l
o
b
opo

d
s
i
nt
h
e Cam
b
r
i
an per
i
o
d.
Tardigrades are mostly very small (
<
(
(
0
.
5
mm lon
g
) animals, commonl
y
known as water
bears (Fi
g
ure 1.lB). The ma
j
orit

y
of the 800 extant species are found in the temporar
y
water
films that coat mosses and lichens. A few live in
p
ermanent a
q
uatic habitats, either marine
o
r
f
res
h
water, or
i
n water
fil
ms
i
nso
il
an
df
orest
li
tter (K
i
nc
hi

n, 1994; Ne
l
son, 2001)
.
T
h
e
i
r
b
o
d
y
i
s covere
d
w
i
t
h
ac
hi
t
i
n
i
ze
d
cut
i

c
l
ean
db
ears
f
our pa
i
rs o
f
un
j
o
i
nte
dl
egs, eac
h
F
I
GU
RE 1.1
.
(
A
)
P
er
ip
ato

p
s
i
s
s
p. (On
y
chophora); (B
)
P
s
eu
d
echini
s
cu
ss
uillu
s
(
Tardi
g
rada); and (C)
C
e
p
halobaena tetra
p
od
a

(Pentastomida). [A, from A. Sedgewick, 1909,
A
Student’s Textbook o
f
Zoolog
y
,V
ol.
VV
I
II
,
Swan
,
Sonnenhein and Co.
,
Ltd. B
,
C
,
from P P. Grass´e
,
19
6
8
,
Tr
ait
rr
ede

´
Z
oo
l
ogi
e
,V
ol. 6. By permission of
V
V
Masson et Cie.
]
5
A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
pair bein
g
innervated from a se
g
mental

g
an
g
lion in the ventral nerve cord. The fluid-filled
,
h
emocoelic bod
y
cavit
y
serves as a h
y
drostatic skeleton. The affinities of the tardi
g
rades
remain unclear. The
y
were traditionall
y
ali
g
ned with pseudocoelomates. However, the
y
h
ave a num
b
er o
f
onyc
h

op
h
oran an
d
art
h
ropo
d
an structura
lf
eatures, an
d
t
h
emo
d
ern v
i
e
w
i
st
h
at t
h
ey are c
l
ose
l
yre

l
ate
d
to t
h
ese groups. Recent mo
l
ecu
l
ar ev
id
ence supports t
hi
s
proposa
l
.
Pentastomids (ton
g
ue worms) (Fi
g
ure l.lC), of which about 100 species are known,
are parasitic in the nasal and pulmonar
y
cavities of vertebrates, principall
y
reptiles bu
t
i
nc

l
u
di
ng
bi
r
d
san
d
mamma
l
s. T
h
e
b
o
d
yo
f
t
h
ese worms, w
hi
c
h
range
f
rom2to13cm
l
ong,

i
s covere
d
w
i
t
h
a cut
i
c
l
ean
dh
as two pa
i
rs o
f
anter
i
or un
j
o
i
nte
dl
egs. Interna
ll
y, t
h
ere

i
sa
fl
u
id
-
fill
e
d
cav
i
ty (
d
e
b
ata
bl
ye
i
t
h
er a
h
emocoe
l
or a pseu
d
ocoe
l
om) conta

i
n
i
ngapa
i
re
d
v
e
ntral nerve cord with se
g
mental
g
an
g
lia innervatin
g
each le
g
. Larval development occur
s
in the tissues of an intermediate host, which ma
y
be an omnivorous or herbivorous insect,
fi
sh, or mammal. Though pentastomids are highly modified as a result of their parasitic life,
th
ey are un
d
ou

b
te
dl
y art
h
ropo
d
s. T
h
e
i
r exact pos
i
t
i
on rema
i
ns controvers
i
a
l
,re
l
at
i
ons
hi
ps
wi
t

h
Acar
i
na, myr
i
apo
d
s, an
db
ranc
hi
uran crustaceans
h
av
i
ng
b
een suggeste
d
.
2
.2. Tr
i
l
obi
t
a
The trilobites (Fi
g
ure 1.2), of which almost 4000 species have been described, ar

e
marine fossils that reached their peak diversit
y
in the Cambrian and Ordovician peri-
ods (
5
00–600 million years ago) (Whittington, 1992). Despite their antiquity they were,
h
owever, not pr
i
m
i
t
i
ve
b
ut
hi
g
hl
y spec
i
a
li
ze
d
art
h
ropo
d

s. In contrast to mo
d
ern art
h
ropo
d
s
th
etr
il
o
bi
tes as a w
h
o
l
es
h
ow a remar
k
a
bl
eun
if
orm
i
t
y
o
fb

o
dy
structure. T
h
e
b
o
dy
, usua
lly
F
I
GU
RE 1.2.
T
riarthru
s
eaton
i
(
Trilobita). (A) Dorsal view; and (B) ventral view. [From R. D. Barnes, 1968,
Inverte
b
rate Zoo
l
ogy,2n
d
e
d
. By perm

i
ss
i
on o
f
t
h
e W. B. Saun
d
ers Co., P
hil
a
d
e
l
p
hi
a.]
6
CHAPTER
1
o
val and dorsoventrall
y
flattened, is divided transversel
y
into three ta
g
mata (head, thorax,
and p

yg
idium) and lon
g
itudinall
y
into three lobes (two lateral pleura and a median axis).
The head, which bears a pair of antennae, compound e
y
es, and four pairs of biramous ap-
pen
d
ages,
i
s covere
db
y a carapace. A pa
i
ro
fid
ent
i
ca
lbi
ramous appen
d
ages
i
s
f
oun

d
on
e
ac
h
t
h
orac
i
c segment. T
h
e
b
asa
l
segment o
f
eac
hli
m
bb
ears a sma
ll
,
i
nwar
dl
y pro
j
ect

i
n
g
e
n
di
te t
h
at
i
s use
d
to
di
rect
f
oo
d
towar
d
t
h
e mout
h.
M
uch about the habits of trilobites can be surmised from examination of their remains
and the de
p
osits in which these are found. Most trilobites lived near or on the sea floor.
W

hil
e some spec
i
es preye
d
upon sma
ll
,so
f
t-
b
o
di
e
d
an
i
ma
l
s, t
h
ema
j
or
i
ty were scavengers.
H
owever,
lik
e eart

h
worms, a
f
ew sma
ll
er tr
il
o
bi
tes too
ki
nmu
d
an
ddi
geste
d
t
h
e organ
ic
m
atter
f
rom
i
t. On t
h
e
b

as
i
so
f
X-ray stu
di
es o
f
pyr
i
t
i
ze
d
tr
il
o
bi
te spec
i
mens, w
hi
c
h
s
h
ow
that trilobites possess a combination of chelicerate and crustacean characteristics, Cisne
(
1974) concluded that the Trilobita, Chelicerata, and Crustacea form a natural

g
roup with a
c
ommon ancestry. Their ancestor would have a body form similar to that of trilobites. Most
aut
h
ors
di
spute t
h
e propose
d
tr
il
o
bi
te-crustacean
li
n
k
,an
d
some even re
j
ect t
h
e assoc
i
at
i

on
b
etween tr
il
o
bi
tes an
d
c
h
e
li
cerates. In
d
ee
d
,t
h
ere are t
h
ose w
h
o suggest t
h
at t
h
etr
il
o
bi

tes
themselves are pol
y
ph
y
letic (Willmer, 1990)
.
Althou
g
h the decline of trilobites (and their replacement b
y
the crustaceans as the
dominant aquatic arthropods) is a matter solely for speculation, Tiegs and Manton (1958)
suggeste
d
t
h
at t
h
e
i
r
b
as
i
c, rat
h
er cum
b
ersome

b
o
d
yp
l
an may
h
ave pro
hibi
te
d
t
h
eevo
l
ut
i
o
n
of f
ast movement at a t
i
me w
h
en
hi
g
hl
y mot
il

e pre
d
ators suc
h
as
fi
s
h
an
d
cep
h
a
l
opo
ds
w
ere becomin
g
common. In addition, the man
y
identical limbs presumabl
y
moved in
a
m
etachronal manner, which is a rather inefficient method in lar
g
eor
g

anisms
.
2.3. The Chelicerate Arthro
p
od
s
Th
enext
f
our groups are o
f
ten p
l
ace
d
toget
h
er un
d
er t
h
e genera
lh
ea
di
ng o
f
C
h
e

li
cerata
because their members possess a bod
y
that is divisible into cephalothorax and abdomen
,
the former usuall
y
bearin
g
a pair of chelicerae (but lackin
g
antennae), a pair of pedipalps,
and four pairs of walkin
g
le
g
s. Althou
g
h there is little doubt of the close relationshi
p
b
etween t
h
eX
i
p
h
osura, Eurypter
id

a, an
d
Arac
h
n
id
a, t
h
e pos
i
t
i
on o
f
t
h
e Pycnogon
id
a
i
s
uncerta
i
n. T
h
oug
h
t
h
ey are usua

ll
y
i
nc
l
u
d
e
d
asac
l
ass o
f
c
h
e
li
cerates, t
h
e
i
ra
ffi
n
i
t
i
es w
i
t

h
o
t
h
er mem
b
ers o
f
t
hi
s
g
roup rema
i
n unc
l
ear, an
d
t
h
ere are some aut
h
ors w
h
o cons
id
er t
h
e
y

deserve more separated status (see Kin
g
, 1973; Manton, 1978; Ed
g
ecombe, 1998; Forte
y
and Thomas, 1998
).
Xi
phosura.
L
imu
l
us po
ly
p
h
emus
,
t
h
e
ki
ng or
h
orses
h
oe cra
b
(F

i
gure 1.3),
i
son
e
of f
our surv
i
v
i
ng spec
i
es o
f
ac
l
ass o
f
art
h
ropo
d
st
h
at
fl
our
i
s
h

e
di
nt
h
eOr
d
ov
i
c
i
an-Upper
D
evonian periods. Kin
g
crabs occur in shallow water alon
g
the eastern coasts of North and
Central America. Three s
p
ecies o
f
Tac
h
ypleu
s
a
n
d
C
arcinoscor

p
ius
o
ccur alon
g
the coast
s
o
f China, Japan, and the East Indies. Like trilobites the
y
are bottom feeders, stirrin
g
up the
su
b
strate an
d
extract
i
ng t
h
e organ
i
c mater
i
a
lf
rom
i
t. I

n
Limu
l
u
s
t
h
ece
ph
a
l
ot
h
orax
i
s covere
d
wi
t
h
a
h
orses
h
oe-s
h
ape
d
carapace. T
h

ea
bd
omen art
i
cu
l
ates
f
ree
l
yw
i
t
h
t
h
e cep
h
a
l
ot
h
orax
an
d
at
i
ts poster
i
or en

d
carr
i
es a
l
on
g
te
l
son. On t
h
e ventra
l
s
id
eo
f
t
h
e cep
h
a
l
ot
h
orax are
six pairs of limbs. The most anterior pair are the chelicerae, and these are followed b
y
fiv
e

pairs of le
g
s. Each le
g
has a lar
g
e
g
nathobase, which serves to break up food and pass it
f
orwar
d
to t
h
e mout
h
.S
i
xpa
i
rs o
f
appen
d
ages are
f
oun
d
on t
h

ea
bd
omen. T
h
e
fi
rst pa
ir
f
use me
di
a
ll
yto
f
orm t
h
e opercu
l
um. T
hi
s protects t
h
e rema
i
n
i
ng pa
i
rs, w

hi
c
hb
ear g
ill
so
n
t
h
e
i
r poster
i
or sur
f
ace.
7
A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
F

IGURE 1.3
.
T
he horseshoe crab
,
L
imulus pol
y
phemus
.
(
A) Dorsal view and (B) ventral view. [From
R
. D. Barnes
,
19
6
8
,
I
nverte
b
rate Zoo
l
ogy
,
2n
d
e
d

.B
y
perm
i
ss
i
on o
f
t
h
e W. B. Saun
d
ers Co., P
hil
a
d
e
l
p
hi
a.]
Eurypter
i
da
.
T
h
e Eurypter
id
a(g

i
ant water scorp
i
ons) (F
i
gure 1.4A) were
f
ormer
ly
i
nc
l
u
d
e
d
w
i
t
h
t
h
eX
i
p
h
osura
i
nt
h

ec
l
ass Merostomata. However, most recent stu
di
es
h
av
e
conc
l
u
d
e
d
t
h
at t
h
e two are not s
i
ster
g
roups an
d
t
h
at t
h
e Merostomata
i

s a parap
hyl
et
ic
assembla
g
e (authors in Ed
g
ecombe, 1998). More than 300 species of this entirel
y
fossil
g
roup of predator
y
arthropods, which existed from the Ordovician to the Permian periods,
are known. Because of their sometimes large size (up to 2.5 m) they are also known a
s
Gi
gantostraca. T
h
ey are
b
e
li
eve
d
to
h
ave
b

een
i
mportant pre
d
ators o
f
ear
l
y
fi
s
h
, prov
idi
n
g
se
l
ect
i
on pressure
f
or t
h
eevo
l
ut
i
on o
fd

erma
lb
one
i
nt
h
e Agnat
h
a. In
b
o
d
yp
l
an t
h
ey wer
e
rather similar to the xiphosurids. Six pairs of limbs occur on the cephalothorax, but, in
contrast to those of kin
g
crabs, the second pair is often
g
reatl
y
enlar
g
ed and chelate formin
g
pedipalps, which presumably served in defense and to capture and tear up prey. The trunk

o
f
eurypter
id
s can
b
e
di
v
id
e
di
nto an anter
i
or prea
bd
omen on w
hi
c
h
appen
d
ages (concea
l
e
d
gill
s) are reta
i
ne

d
an
d
a narrow ta
illik
e posta
bd
omen
f
rom w
hi
c
h
appen
d
a
g
es
h
ave
b
een
l
ost
.
T
hou
g
h the earl
y

eur
y
pterids were marine, adaptive radiation into freshwater and perhap
s
ev
en terrestrial habitats occurred. Indeed, it was from freshwater forms that arachnids are
b
elie
v
ed to ha
v
ee
v
ol
v
ed
.
A
r
ac
hni
da.
Scorp
i
ons, sp
id
ers, t
i
c
k

s, an
d
m
i
tes
b
e
l
ong to t
h
ec
l
ass Arac
h
n
id
aw
h
os
e
approximatel
y6
2,000 species are more easil
y
reco
g
nized than defined. Livin
g
members of
t

he
g
roup are terrestrial (althou
g
h a few mites are secondaril
y
aquatic) and have respirator
y
or
g
ans in the form of lun
g
books or tracheae. In contrast to the two aquatic chelicerate
g
roup
s
d
escr
ib
e
d
ear
li
er, most arac
h
n
id
sta
k
eon

l
y
li
qu
id f
oo
d
, extracte
df
rom t
h
e
i
r prey
b
y means
o
f
ap
h
aryngea
l
suc
ki
ng pump, o
f
ten a
f
ter extraora
ldi

gest
i
on. Scorp
i
ons, o
f
w
hi
c
h
t
h
er
e
are about 1
5
00 livin
g
species, are the oldest arachnids with fossils known from the Silurian.
Some of these fossils were aquatic (Polis, 1990). With about 35,000 species, spiders form
8
CHAPTER
1
F
IGURE 1.4. (A) Eurypterid and (B
)
Ny
mphon rubru
m
(

Pycnogonida). [A, from D. T. Anderson (ed.), 2001
,
I
nverte
b
rate Zoo
l
ogy
,
2nd ed. B
y
permission of Oxford Universit
y
Press. B, from R. D. Barnes. 19
6
8,
I
nverte
b
rate
Z
oolo
gy
, 2nd ed. B
y
permission of the W. B. Saunders Co., Philadelphia.
]
an extremely diverse group. The earliest spider fossils are from the Devonian, and by th
e
Tert

i
ary t
h
esp
id
er
f
auna was very s
i
m
il
ar to t
h
at seen to
d
ay (Foe
li
x, 1997)
.
P
ycno
g
onida
.
T
h
e approx
i
mate
ly

1000 spec
i
es o
fli
v
i
n
g
P
y
cno
g
on
id
a (Pantopo
d
a
)
are the remnants of a
g
roup that ori
g
inated in the Devonian. The
y
are commonl
y
know
n
as sea spiders because of their superficial similarit
y

to these arachnids (Fi
g
ure 1.4B). The
y
are
f
oun
d
at vary
i
ng
d
ept
h
s
i
na
ll
oceans o
f
t
h
ewor
ld
,
b
ut are part
i
cu
l

ar
l
y common
i
nt
h
e
s
h
a
ll
ower waters o
f
t
h
e Arct
i
can
d
Antarct
i
c Oceans. T
h
ey
li
ve on t
h
e sea
fl
oor an

df
ee
d
on
c
oe
l
enterates,
b
r
y
ozoans, an
d
spon
g
es. On t
h
e cep
h
a
l
ot
h
orax
i
sa
l
ar
g
e pro

b
osc
i
s,ara
i
se
d
tubercle bearin
g
four simple e
y
es, a pair of chelicerae and an associated pair of palps, an
d
five pairs of le
g
s. The le
g
s of the first pair differ from the rest in that the
y
are small and
pos
i
t
i
one
d
ventra
ll
y. T
h

ese ov
i
gerous
l
egs are use
di
nt
h
ema
l
e
f
or carry
i
ng t
h
e eggs. T
h
e
a
bd
omen
i
s very sma
ll
an
dl
ac
k
s appen

d
ages.
As note
d
a
b
ove, t
h
e prec
i
se re
l
at
i
ons
hi
ps o
f
t
h
e pycnogon
id
stoot
h
er art
h
ropo
d
s rema
i

n
c
ontroversial. Althou
g
h the presence of chelicerae, the structure of the brain, and the natur
e
o
f the sense or
g
ans are chelicerate characters, the structure and innervation of the proboscis
,
the similarity between the intestinal diverticula and those of annelids, the multiple paire
d
gonopores, an
d
t
h
e suggest
i
on t
h
at t
h
e pycnogon
id
s
h
ave a true coe
l
om s

h
ow t
h
at t
h
ey must
have left the main line of arthropod evolution at a very early date (Sharov, 19
66
). Other
n
on-chelicerate features that the
y
possess are (1) the partial se
g
mentation of the le
g
-bearin
g
part of the bod
y
, (2) the reduction of the opisthosoma to a small abdominal component, and
(
3) the presence, in the male, of ovigerous legs
.
2.4. The Mandibulate Arthro
p
ods
Th
e rema
i

n
i
ng groups o
f
art
h
ropo
d
s (crustaceans, myr
i
apo
d
san
dh
exapo
d
s) were or
i
g-
i
nall
yg
rouped to
g
ether as the Mandibulata b
y
Snod
g
rass (1938) because their members
9

A
RTHR
O
P
O
D
E
VOLUTION
possess a pair of mandibles as the primar
y
masticator
y
or
g
ans. Thou
g
h this view becam
e
w
idel
y
accepted, some later authors, notabl
y
Manton, ar
g
ued forcefull
y
that the mandible of
t
he crustaceans is not homolo

g
ous with that of the m
y
riapods and insects. That is, the term
M
an
dib
u
l
ata s
h
ou
ld
not
b
e use
d
to
i
mp
l
yap
h
y
l
ogenet
i
cre
l
at

i
ons
hi
p
b
ut on
l
y a commo
n
level of advancement reached by several groups independently (Tiegs and Manton, 19
5
8;
M
anton, 1977). T
h
e
d
e
b
ate over w
h
et
h
er t
h
e Man
dib
u
l
ata const

i
tute a monop
hyl
et
i
c
g
rou
p
continues to be vi
g
orous (see chapters in Ed
g
ecombe, 1998; Forte
y
and Thomas, 1998;
also Section 3.3.1), and the conclusion reached t
y
picall
y
hin
g
es on the t
y
pe of evidenc
e
presente
d
.Ev
id

ence
f
rom comparat
i
ve morp
h
o
l
ogy,
bi
oc
h
em
i
stry, an
d
mo
l
ecu
l
ar
bi
o
l
ogy
o
fli
v
i
ng spec

i
es ten
d
s to support monop
h
y
l
y, w
h
ereas
d
ata
f
rom
f
oss
il
s genera
ll
ya
li
g
n
th
e Crustacea w
i
t
h
t
h

eC
h
e
li
cerata. W
i
t
h
t
h
e
i
rtwopa
i
rs o
f
antennae, t
h
e Crustacea wou
ld
appear ver
y
distinct from the other two
g
roups. M
y
riapods and hexapods have a sin
g
le pair
of antennae, a feature that led Sharov (1966) to unite these

g
roups in the Atelocerata. Tie
gs
and Manton (1958) and Manton (1977) went a step further, placing the two groups wit
h
th
eOnyc
h
op
h
ora
i
nt
h
eUn
i
ram
i
a. However,
l
oo
k
s can
b
e
d
ece
i
v
i

ng, an
d
many mo
d
ern
p
h
y
l
ogenet
i
c
i
sts cannot accept t
h
e Ate
l
ocerata (see Sect
i
on 3.3.1) an
d
t
h
eUn
i
ram
i
a (se
e
Sections 3.2.2 and 3.3) as monoph

y
letic taxa
.
C
rustacea
.
T
o
t
he Crustacea belong the crabs, lobsters, shrimps, prawns, barnacles,
an
d
woo
dli
ce. T
h
e Crustacea are a success
f
u
l
group o
f
art
h
ropo
d
s: some 40,000
li
v
i

ng
spec
i
es
h
ave
b
een
d
escr
ib
e
d
an
d
t
h
ere
i
sana
b
un
d
ant
f
oss
il
recor
d
.T

h
e
y
are pr
i
mar
ily
aquatic, and few have mana
g
ed to successfull
y
conquer terrestrial habitats. The
y
exhibit
a remarkable diversit
y
of form; indeed, man
y
of the parasitic forms are unreco
g
nizable i
n
t
he adult stage. Typical Crustacea, however, usually possess the following features: bod
y
di
v
id
e
di

nto cep
h
a
l
ot
h
orax an
d
a
bd
omen; cep
h
a
l
ot
h
orax w
i
t
h
two pa
i
rs o
f
antennae, t
h
re
e
pa
i

rs o
f
mout
h
parts (man
dibl
es an
dfi
rst an
d
secon
d
max
ill
ae), an
d
at
l
east
fi
ve pa
i
rs o
fl
egs;
b
iramous appenda
g
es
.

The reason for the success of Crustacea (and perhaps the reason wh
y
the
y
replace
d
t
rilobites as the dominant aquatic arthropods) is their adaptability. Like their terrestrial
counterparts, t
h
e
i
nsects, crustaceans
h
ave exp
l
o
i
te
d
to t
h
e
f
u
ll
t
h
ea
d

vantages con
f
erre
d
b
y possess
i
on o
f
a segmente
db
o
d
yan
dj
o
i
nte
dli
m
b
s. Pr
i
m
i
t
i
ve crustaceans,
f
or examp

l
e
,
t
he fair
y
shrimp (Fi
g
ure 1.
5
), have a bod
y
that shows little si
g
nofta
g
mosis and limb
specialization. In contrast, in a hi
g
hl
y
or
g
anized crustacean such as the cra
y
fish (Fi
g
ure 1.
6)
t

he appenda
g
es have become specialized so that each performs onl
y
one or two functions,
an
d
t
h
e
b
o
d
y
i
sc
l
ear
l
y
di
v
id
e
di
nto tagmata. In t
h
e
l
arger (

b
ottom-
d
we
lli
ng) Crustacea
spec
i
a
li
ze
dd
e
f
ens
i
ve weapons
h
ave evo
l
ve
d
(e.g., c
h
e
l
ae, t
h
ea
bili

ty to c
h
ange co
l
or
i
n
re
l
at
i
on to t
h
eenv
i
ronment, an
d
t
h
ea
bili
t
y
to move at
high
spee
d
over s
h
ort

di
stances
by
snappin
g
the flexible abdomen under the thorax). B
y
contrast, smaller, planktonic Crustacea
are often transparent and have evolved hi
g
h reproductive capacities and short life c
y
cles to
f
ac
ili
tate sur
viv
a
l
.
My
ria
p
oda
.
T
h
e mem
b

ers o
ff
our groups o
f
man
dib
u
l
ate art
h
ropo
d
s(C
hil
opo
d
a,
Diplopoda, Pauropoda, and S
y
mph
y
la) share the followin
g
features: five- or six-se
g
mente
d
h
ead, unique mandibular bitin
g

mechanism, sin
g
le pair of antennae, absence of compound
F
I
GU
RE 1.5
.
B
r
a
n
c
h
i
n
ecta
s
p., a fairy shrimp
.
[
From R. D. Barnes, 19
6
8, Inverte
b
rate Zoo
l
ogy
,
2n

d
e
d
.B
y
perm
i
ss
i
on o
f
t
h
e W. B. Saun
d
ers Co.,
P
hiladelphia.]
10
CHAPTER
1
FI
GU
RE 1.6. Cra
yfi
s
h
. Ventra
l
v

i
ew o
f
one s
id
etos
h
ow
diff
erent
i
at
i
on o
f
appen
d
a
g
es.
e
y
es, e
l
ongate trun
k
t
h
at
b

ears many pa
i
rs o
fl
egs, art
i
cu
l
at
i
on o
f
t
h
e coxa w
i
t
h
t
h
e sternu
m
(
rat
h
er t
h
an t
h
ep

l
euron as
i
n
h
exapo
d
s), trac
h
ea
l
resp
i
ratory system, Ma
l
p
i
g
hi
an tu
b
u
l
es
f
or excretion, absence of mesenteric ceca, and distinctive mechanism b
y
which the anima
l
e

xits the old cuticle durin
g
ecd
y
sis. Further, the
y
are found in similar habitats (e.
g
., leaf
m
old, loose soil, rottin
g
lo
g
s).
Fo
rt
h
ese reasons, t
h
ey were tra
di
t
i
ona
ll
yp
l
ace
di

nas
i
ng
l
e
l
arge taxon, t
h
e Myr
i
apo
d
a
.
T
h
e monop
h
y
l
et
i
c nature o
f
t
h
e myr
i
apo
d

s
h
as
b
een supporte
db
y some,
b
ut not a
ll
,c
l
a
di
st
ic
ana
ly
ses o
fl
ar
g
e
d
ata sets w
i
t
h
a com
bi

nat
i
on o
f
morp
h
o
l
o
gi
ca
l
an
d
mo
l
ecu
l
ar c
h
aracters
o
f livin
g
species (Wheeler
et al
., 1993; authors in Forte
y
and Thomas, 1998). Yet other
m

orpholo
g
ical and molecular studies indicate that the m
y
riapods constitute a paraph
y
leti
c
o
revenpo
l
yp
h
y
l
et
i
c group. Determ
i
nat
i
on o
f
t
h
ere
l
at
i
ons

hi
ps w
i
t
hi
nt
h
e Myr
i
apo
d
a
h
a
s
prove
d diffi
cu
l
t
b
ecause potent
i
a
ll
y
h
omo
l
ogous c

h
aracters are s
h
are
db
y
diff
erent pa
i
rs o
f
g
roups. For examp
l
e, D
i
p
l
opo
d
aan
d
Pauropo
d
a
h
ave t
h
e same num
b

er o
fh
ea
d
se
g
ments
and one pair of maxillae; Diplopoda and Chilopoda have se
g
mental tracheae; and S
y
mph
y
l
a
11
A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
F
IGURE 1.7.

M
yriapoda. (A)
L
ithobiu
s
sp. (centipede), (B) Julus terrestri
s
(
millipede), (C
)
Pauro
p
us silvaticus
(p
auro
p
o
d
), an
d
(D
)
S
cutigere
ll
a immacu
l
at
a
(

s
y
mph
y
lan). [From R. D. Barnes, 19
6
8
,
Inverte
b
rate Zoo
l
og
y
,
2n
d
ed. B
y
permission of the W. B. Saunders Co., Philadelphia.
]
an
d
Pauropo
d
a
d
eve
l
op em

b
ryon
i
c ventra
l
organs t
h
at
b
ecome evers
ibl
eves
i
c
l
es, as we
ll
as contributin
g
to the ventral
g
an
g
lia. Boudreaux’s (1979) overall conclusion was that th
e
P
a
uropoda-Diplopoda and Chilopoda-S
y
mph

y
la are the two sister
g
roups within the taxo
n
(Figure 1.8). An alternative view, based on cladistic analysis of morphological characters of
li
v
i
ng
f
orms (Kraus,
i
n Fortey an
d
T
h
omas, 1998),
i
st
h
at t
h
e myr
i
apo
d
s are parap
h
y

l
et
i
c
:
th
eC
hil
opo
d
a
i
st
h
es
i
ster group to t
h
eot
h
er t
h
ree. Un
f
ortunate
l
y, t
h
oug
h

t
h
ere
i
sar
i
c
h
fossil record of m
y
riapods extendin
g
back to the Upper Silurian, insufficient stud
y
has been
d
one to clarif
y
the monoph
y
letic nature or otherwise of this
g
roup (see Shear, in Forte
y
an
d
T
homas, 1998)
.
S

ome 3000 spec
i
es o
f
c
hil
opo
d
s (cent
i
pe
d
es) (F
i
gure 1.7A)
h
ave
b
een
d
escr
ib
e
d
(Lew
i
s
,
1981). T
h

ey are typ
i
ca
ll
y act
i
ve, nocturna
l
pre
d
ators w
h
ose
b
o
di
es are
fl
attene
dd
orsoven
-
t
ra
lly
.T
h
e
fi
rst pa

i
ro
f
trun
k
appen
d
a
g
es (max
illi
pe
d
s) are mo
difi
e
di
nto po
i
son c
l
aws t
h
at
are used to catch pre
y
. In most centipedes the le
g
s increase in len
g

th from the anterior to th
e
p
osterior of the animal to facilitate ra
p
id movement. The earliest known fossil centi
p
edes
,
f
rom t
h
e Upper S
il
ur
i
an, are remar
k
a
bl
ys
i
m
il
ar to some extant spec
i
es, suggest
i
ng t
h

at t
h
e
group may
b
e cons
id
era
bl
y more anc
i
ent.
I
n contrast to t
h
e cent
i
pe
d
es, t
h
e
di
p
l
opo
d
s(m
illi
pe

d
es) (F
ig
ure 1.7B) are s
l
ow-mov
i
n
g
h
erbivorous animals. The distin
g
uishin
g
feature of the almost 10,000 species in the class
is the presence of diplose
g
ments, each bearin
g
two pairs of le
g
s, formed b
y
fusion of two
or
i
g
i
na
ll

y separate som
i
tes. It
i
s
b
e
li
eve
d
t
h
at t
h
e
di
p
l
osegmenta
l
con
di
t
i
on ena
bl
es t
he
an
i

ma
l
to exert a strong pus
hi
ng
f
orce w
i
t
hi
ts
l
egs w
hil
e reta
i
n
i
ng r
i
g
idi
ty o
f
t
h
e trun
k
re
gi

on. As t
h
e
y
cannot escape
f
rom wou
ld
-
b
e pre
d
ators
by
spee
d
, man
y
m
illi
pe
d
es
h
av
e
e
v
o
lved such protective mechanisms as the abilit

y
to roll into a ball and the secretion o
f
12
CHAPTER
1
F
IGURE 1.8
.
Sc
h
emes
f
or t
h
e poss
ibl
e monop
h
y
l
et
i
cor
i
g
i
no
f
t

h
e art
h
ropo
d
s as propose
db
y Sno
d
grass (1938),
Sharov (19
66
), and Boudreaux (1979). Note also the differin
g
relationships of the Annelida, On
y
chophora, an
d
Arthropoda
.
defensive chemicals (Hopkins and Read, 1992). Fossil millipedes are known from the Lowe
r
D
evonian
.
P
auropoda (500 species) are minute arthropods (0.5–2 mm lon
g
) that live in soil and
l

ea
f
mo
ld
. Super
fi
c
i
a
ll
yt
h
ey resem
bl
e cent
i
pe
d
es,
b
ut
d
eta
il
e
d
exam
i
nat
i

on revea
l
st
h
at
t
h
ey are
lik
e
l
yt
h
es
i
ster group to t
h
em
illi
pe
d
es. T
hi
sa
ffi
n
i
ty
i
s con

fi
rme
db
y suc
h
common
f
eatures as t
h
e pos
i
t
i
on o
f
t
h
e
g
onopore, t
h
e num
b
er o
fh
ea
d
se
g
ments, an

d
t
h
ea
b
sence o
f
appenda
g
es on the first trunk se
g
ment (Sharov, 19
66
). A characteristic feature are the lar
g
e
13
A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
t

er
g
al plates on the trunk, which overlap ad
j
acent se
g
ments (Fi
g
ure 1.7C). It is believed tha
t
t
hese lar
g
e structures prevent lateral undulations durin
g
locomotion.
Sy
mph
y
lans (Fi
g
ure 1.7D) are small arthropods that differ from other m
y
riapods in th
e
possess
i
on o
f
a

l
a
bi
um (t
h
e
f
use
d
secon
d
max
ill
ae) an
d
t
h
e pos
i
t
i
on o
f
t
h
e gonopore (on t
h
e
11th body segment). Although forming only a very small class of arthropods (1
6

0 species),
th
eS
y
mp
hyl
a
h
ave st
i
mu
l
ate
d
spec
i
a
li
nterest amon
g
entomo
l
o
gi
sts
b
ecause o
f
t
h

e severa
l
features the
y
share with insects, leadin
g
to the su
gg
estion that the two
g
roups ma
y
have
h
ad a common ancestr
y
. The s
y
mph
y
lan and insectan heads have an identical number o
f
segments an
d
, accor
di
ng to some zoo
l
og
i

sts, t
h
e mout
h
parts o
f
symp
h
y
l
ans are
i
nsecta
n
i
nc
h
aracter. At t
h
e
b
ase o
f
t
h
e
l
egs o
f
symp

h
y
l
ans are evers
ibl
eves
i
c
l
es an
d
coxa
l
sty
li.
S
i
m
il
ar structures are
f
oun
di
n some apterygote
i
nsects.
H
exapo
d
a.

F
ive
g
roups of six-le
gg
ed arthropods (hexapods) are reco
g
nized: the
wi
ng
l
ess Co
ll
em
b
o
l
a, Protura, D
i
p
l
ura, an
d
t
h
ysanurans, an
d
t
h
ew

i
nge
di
nsects (Pterygota)
.
A
ll
t
h
ew
i
ng
l
ess
f
orms were tra
di
t
i
ona
ll
y
i
nc
l
u
d
e
di
nt

h
esu
b
c
l
ass Apterygota (Ameta
b
o
l
a)
wi
t
hi
nt
h
ec
l
ass Insecta
(
=
H
exapo
d
a). A
l
t
h
ou
gh
most recent stu

di
es
i
n
di
cate t
h
at t
h
e
h
exapods are monoph
y
letic, the nature of the relationships of the constituent
g
roups ha
s
proved controversial, with perhaps onl
y
the th
y
sanurans (now arran
g
ed in two orders Mi
-
crocoryp
hi
aan
d
Zygentoma)

h
av
i
ngac
l
ose a
ffi
n
i
ty w
i
t
h
t
h
e Pterygota
.
T
h
eCo
ll
em
b
o
l
a, Protura, an
d
D
i
p

l
ura are o
f
ten p
l
ace
di
nt
h
e taxon Entognat
h
a(ta)
pr
i
nc
i
pa
lly b
ecause o
f
t
h
eun
i
que arran
g
ement o
f
t
h

e
i
r mout
h
parts enc
l
ose
d
w
i
t
hi
nt
h
e ven-
t
rolateral extensions of the head. Other possible s
y
napomorphies of the ento
g
nathans include
protrusible mandibles, reduced Malpi
g
hian tubules, and reduced or absent compound e
y
es.
However, B
i
tsc
h

an
d
B
i
tsc
h
(2000) argue strong
l
yt
h
at most o
f
t
h
ese s
i
m
il
ar
i
t
i
es are
d
ue t
o
convergence; t
h
at
i

s, t
h
e Entognat
h
a
i
s not a monop
h
y
l
et
i
c group. Some c
l
ass
ifi
cat
i
ons un
i
t
e
th
eCo
ll
em
b
o
l
aan

d
Protura as s
i
ster
g
roups w
i
t
hi
nt
h
eE
lli
pura(ta)
b
ase
d
on t
h
e
f
o
ll
ow
i
n
g
s
y
napomorphies: small bod

y
size (8 mm or less), absence of cerci, antennae with four or
fewer se
g
ments, maxillar
y
palps with three or fewer se
g
ments, one-se
g
mented labial palps,
and possibly the coiled immotile sperm and absence of abdominal spiracles (Boudreaux,
1979; Kr
i
stensen, 1991). Desp
i
te t
h
ese s
i
m
il
ar
i
t
i
es, t
h
eCo
ll

em
b
o
l
aan
d
Protura are qu
i
te
di
st
i
nct
b
ot
hf
rom eac
h
ot
h
er an
df
rom ot
h
er
h
exapo
d
s. Co
ll

em
b
o
l
a
h
aveas
i
x-segmente
d
abdomen bearin
g
specialized appenda
g
es (see Chapter
5
, Section 2), total cleava
g
einthe
e
gg
, a lon
g
(composite tibiotarsal?) penultimate se
g
ment in the le
g
s, and spiracles that either
open in the neck region or are absent. Protura lack a tentorium, eyes, and antennae, have 11
a

bd
om
i
na
l
segments (3 o
f
w
hi
c
h
are a
dd
e
db
y anamorp
h
os
i
s, an
dh
ave vest
i
g
i
a
l
appen
d
ages

on t
h
e
fi
rst t
h
ree a
bd
om
i
na
l
segments. In
d
ee
d
,t
h
e extens
i
ve c
l
a
di
st
i
c ana
l
ys
i

so
f
B
i
tsc
h
an
d
B
itsch (2000) re
j
ects the monoph
y
l
y
of the Ellipura. The position of the Diplura is ques
-
t
ionable, and the
g
roup is probabl
y
not monoph
y
letic (Bitsch and Bitsch, 2000). Some earl
y
authors included them in the same order as the thysanurans; Kukalov´
a-Peck (1991) argued
´
th

at t
h
eD
i
p
l
ura are Insecta,
f
orm
i
ng t
h
es
i
ster group to t
h
et
h
ysanuran
s
+
p
terygotes; an
d
many ot
h
er aut
h
ors,
h

ave p
l
ace
d
t
h
em
i
nt
h
e
i
r own c
l
ass.
I
tw
ill b
e rea
dily
apparent t
h
atavar
i
et
y
o
f
sc
h

emes
h
ave
b
een
d
ev
i
se
d
to s
h
ow t
he
possible relationships of the hexapod
g
roups (Fi
g
ure 1.9). The “lumpers” (e.
g
., Boudreaux
)
u
se the terms Hexapoda and Insecta s
y
non
y
mousl
y
, so that the Collembola, Protura,

an
d
D
i
p
l
ura are cons
id
ere
d
or
d
ers o
fi
nsects. T
h
e “sp
li
tters” (e.g., Kr
i
stensen), on t
he
ot
h
er
h
an
d
, ass
i

gn eac
h
o
f
t
h
ese groups t
h
e ran
k
o
f
c
l
ass, on a par t
h
ere
f
ore w
i
t
h
t
he
I
nsecta. As t
h
e
i
r taxonom

i
c status
i
s controvers
i
a
l
,t
h
e Protura, Co
ll
em
b
o
l
a, an
d
D
i
p
l
ura
h
ave been included with the th
y
sanurans in Chapter 5 where details of their biolo
gy
ar
e
p

resented
.
14
CHAPTER
1
F
I
GU
RE 1.9
.
Schemes for the possible relationships of the hexapod groups as envisaged by Boudreaux (1979),
Kr
i
stensen (1991), an
d
Ku
k
a
l
ov´a-Pec
k
(1991).
3. Evolut
i
onar
y
Relat
i
onsh
ip

so
f
Arthro
p
od
s
3
.1. Th
e
Pr
ob
l
e
m
In determinin
g
the evolutionar
y
relationships of animals zoolo
g
ists use evidence from
av
a
riet
y
of sources. The comparative morpholo
gy
, embr
y
olo

gy
,ph
y
siolo
gy
, biochemistr
y
15
A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
and, increasin
g
l
y
, molecular biolo
gy
of livin
g
members of a
g

roup provide clues about the
e
v
o
lutionar
y
trends that have occurred within that
g
roup. It is, however, onl
y
the fossil
record that can
p
rovide the
d
irec
t
evidence for such processes. Unfortunatel
y
, in the case o
f
art
h
ropo
d
st
h
e ear
l
y

f
oss
il
recor
di
s poor. By t
h
et
i
me t
h
e eart
h
’s crust
b
ecame su
i
ta
bl
e
f
or
preservation of dead organisms, in the Cambrian period (about
6
00 million years ago), th
e
art
h
ropo
d

s
h
a
d
a
l
rea
dy
un
d
er
g
oneaw
id
ea
d
apt
i
ve ra
di
at
i
on. Tr
il
o
bi
tes, crustaceans, an
d
eur
y

pterids were abundant at this time. Even after this time the fossil record is incomplete
mainl
y
because conditions were unsuitable for preservin
g
rather delicate or
g
anisms such a
s
myr
i
apo
d
san
di
nsects. T
h
e rema
i
ns o
f
suc
h
organ
i
sms are on
l
y preserve
d
sat

i
s
f
actor
il
y
in
me
di
at
h
at
h
ave a
fi
ne texture,
f
or examp
l
e, mu
d
,vo
l
can
i
cas
h
,
fi
ne

h
umus, an
d
res
i
ns (Ross
,
196
5
). Therefore, arthropod phylogeneticists have had to rely almost entirely on compar
-
ative studies. Their problem then becomes one of determinin
g
the relative importance o
f
similarities and differences that exist between or
g
anisms and whether apparentl
y
identical
,
shared characters are homologous (synapomorphic) or analogous (see Chapter 4, Section 3).
E
vo
l
ut
i
on
i
s a process o

fdi
vergence, an
d
yet, para
d
ox
i
ca
ll
y, organ
i
sms may evo
l
ve towar
d
as
i
m
il
ar way o
f lif
e (an
dh
ence
d
eve
l
op s
i
m

il
ar structures). A
di
st
i
nct
i
on must t
h
ere
f
or
e
b
e made betwee
n
p
arallel an
d
convergen
t
e
v
o
lution. As we shall see below, the difficult
y
in makin
g
this distinction led to the development of ver
y

different theories for the ori
g
in o
f
and relationship between various arthropod groups.
3.2. Theor
i
es o
f
Arthropod Evolut
i
on
As Manton (1973, p. 111) noted, “it has been a zoolo
g
ical pastime for a centur
y
or mor
e
t
o speculate about the ori
g
in, evolution, and relationships of Arthropoda, both livin
g
and
f
oss
il
.” Many zoo
l
og

i
sts
h
ave expoun
d
e
d
t
h
e
i
rv
i
ewsont
hi
ssu
bj
ect. Not surpr
i
s
i
ng
l
y,
f
or t
he
reasons note
d
a

b
ove, t
h
ese v
i
ews
h
ave
b
een w
id
e
l
y
di
vergent. Some aut
h
ors
h
ave suggeste
d
th
at t
h
e art
h
ropo
d
s are monop
hyl

et
i
c, t
h
at
i
s,
h
ave a common ancestor; ot
h
ers
h
ave propose
d
t
hat the
g
roup is diph
y
letic (two ma
j
or sub
g
roups evolved from a common ancestor), and
y
e
t
others believe that each ma
j
or sub

g
roup evolved independentl
y
of the others (a pol
y
ph
y
leti
c
origin). Within the last 50 years, much evidence has been accumulated in the areas of
f
unct
i
ona
l
morp
h
o
l
ogy an
d
comparat
i
ve em
b
ryo
l
ogy
b
ut espec

i
a
ll
y
i
npa
l
eonto
l
ogy an
d
mo
l
ecu
l
ar
bi
o
l
ogy, w
hi
c
hh
as
b
een
b
roug
h
tto

b
ear on t
h
e matter o
f
art
h
ropo
d
p
h
y
l
ogeny
.
T
his does not mean, however, that the problem has been solved! On the contrar
y
,vi
g
orou
s
d
ebate continues, with the proponents of each viewpoint pressin
g
their claims, t
y
picall
y
b

y
u
sing a particular methodology or a specific kind of evidence (for examples, see authors
i
n Gupta, 1979; E
d
gecom
b
e, 1998; Fortey an
d
T
h
omas, 1998; a
l
so Emerson an
d
Sc
h
ram,
1990; Ku
k
a
l
ov´a-Pec
k
, 1992). On
l
y rare
l
y

h
ave aut
h
ors attempte
d
to mars
h
a
ll
all
o
f
t
h
e
evidence in order to arrive at a
n
o
veral
l
c
onclusion. Even then, there ma
y
be no a
g
reement
!
Fo
re
x

ample, the anal
y
ses of Boudreaux (1979) and Wheele
r
et al.
(
1993
)
led them to favor
a
monoph
y
letic ori
g
in whereas Willmer (1990) concluded that, for the present, a pol
y
ph
y
leti
c
or
i
g
i
n
f
or t
h
e art
h

ropo
d
s
i
s more
lik
e
l
y. In out
li
n
i
ng t
h
e pros an
d
cons o
f
t
h
ese t
h
eor
i
es
i
t
i
s use
f

u
l
to separate t
h
e mono- an
ddi
p
h
y
l
et
i
ct
h
eor
i
es
f
rom t
h
epo
l
yp
h
y
l
et
i
ct
h

eory an
d
t
o
present t
h
em
i
na
hi
stor
i
ca
l
context s
h
ow
i
n
g
t
h
e
g
ra
d
ua
ld
eve
l

opment o
f
ev
id
ence
i
n support
of one view or the other
.
3.2.1. Mono- and D
i
phylet
i
c Theor
i
es
I
n a nutshell, proponents of the monoph
y
letic theor
y
simpl
y
point to the abundanc
e
of features common to arthropods (Section 2) and ar
g
ue that so man
y
similarities could

16
CHAPTER
1
F
IGURE 1.10. Ays
h
eaia pe
d
uncu
l
ata
.
n
ot have been achieved other than through a common origin. However, their argument
goes
b
eyon
d
s
i
mp
l
y not
i
ng t
h
e presence o
f
t
h

ese
f
eatures; rat
h
er, as a resu
l
to
fi
mprove
d
tec
h
no
l
ogy an
dk
now
l
e
d
ge, t
h
e monop
h
y
l
et
i
c
i

sts can now po
i
nt to t
h
e
hi
g
hl
y conserve
d
n
ature of ke
y
arthropod structures and the processes b
y
which the
y
are formed, for example,
c
uticle chemistr
y
and moltin
g
, the development and fine structure of compound e
y
es, and
e
mbryonic head development (see Gupta, 1979). To this can be added ever-increasin
g
e

vid
ence
f
rom mo
l
ecu
l
ar
bi
o
l
ogy, most (
b
ut not a
ll
)o
f
w
hi
c
h
supports monop
h
y
l
y. T
his
s
h
ou

ld
not
b
e
i
nterprete
d
to mean t
h
at t
h
ere
i
s agreement among t
h
e monop
h
y
l
et
i
c
i
sts as t
o
a
g
enera
l
sc

h
eme
f
or art
h
ropo
d
evo
l
ut
i
on. On t
h
e contrar
y
,t
h
ere are qu
i
te
di
ver
g
ent v
i
ews
w
ith respect to the relationships of the various arthropod
g
roups (Fi

g
ure 1.8)
.
Space does not permit a detailed account of the earl
y
histor
y
of monoph
y
letic proposal
s
and readers interested in this should consult Tiegs and Manton (1958). Nevertheless, a fe
w
v
ery ear
l
ysc
h
emes s
h
ou
ld b
e note
d
to s
h
ow
h
ow
id

eas c
h
ange
d
as new
i
n
f
ormat
i
on
b
ecame
av
a
il
a
bl
e. T
h
e
fi
rst monop
hyl
et
i
csc
h
eme
f

or art
h
ropo
d
evo
l
ut
i
on was
d
ev
i
se
dby
Haec
k
e
l
(
18
66).
*
Thou
g
h believin
g
that arthropods had evolved from a common ancestor, he divided
them into the Carides (Crustacea, which included Xiphosura, Eur
y
pterida, and Trilobita)

an
d
t
h
e Trac
h
eata (Myr
i
apo
d
a, Insecta, an
d
Arac
h
n
id
a). A
f
ter recogn
i
z
i
ng t
h
at Per
i
patu
s
(
Onyc

h
op
h
ora)
h
a
d
a num
b
er o
f
art
h
ropo
d
an
f
eatures (
i
nc
l
u
di
ng a trac
h
ea
l
system), Mose
l
e

y
(
1894) env
i
sa
g
e
di
tas
b
e
i
n
g
t
h
e ancestor o
f
t
h
e Trac
h
eata, w
i
t
h
t
h
e Crustacea
h

av
i
n
g
evo
l
ve
d
i
ndependentl
y
. Here, then, was the first diph
y
letic theor
y
for the ori
g
in of arthropods
.
At about the same time, after the realization that
L
imulu
s
is an a
q
uatic arachnid, no
t
a crustacean,
i
t was propose

d
t
h
at t
h
e aquat
i
c Eurypter
id
a were t
h
e ancestors o
f
a
ll
ter
-
restr
i
a
l
arac
h
n
id
s. As a resu
l
tt
h
e eurypter

id
-x
i
p
h
osuran-arac
h
n
id
group emerge
d
as an
ev
o
l
ut
i
onar
yli
ne ent
i
re
ly
separate
f
rom t
h
em
y
r

i
apo
d
-
i
nsect
li
ne an
dh
av
i
n
g
per
h
aps on
ly
v
er
y
sli
g
ht affinities with the crustaceans. Thus emer
g
ed the first example of conver
g
ence
i
n the Arthropoda, namel
y

, a twofold ori
g
in of the tracheal s
y
stem
.
Handlirsch (1908, 1925, 1937
)
∗
s
aw t
h
eTr
il
o
bi
ta as t
h
e group
f
rom w
hi
c
h
a
ll
ot
h
e
r

art
h
ropo
d
c
l
asses arose separate
l
y. Peri
p
atu
s
w
a
s
p
l
ace
di
nt
h
e Anne
lid
a,
i
ts severa
l
art
h
ropo

df
eatures presume
d
to
b
et
h
e resu
l
to
f
conver
g
ence. T
h
e
g
reatest
diffi
cu
l
t
y
w
i
t
h
H
andlirsch’s scheme is the idea that the pleura of trilobites became the win
g

s of insects
.
This means that the apter
yg
ote insects must have evolved from win
g
ed forms, which i
s
c
ontrary to all available evidence.
Itwasata
b
out t
hi
st
i
me t
h
at t
h
e Cam
b
r
i
an
l
o
b
opo
df

oss
il
A
y
s
h
eaia pe
d
uncu
l
ata
(
F
i
gure 1.10) was
di
scovere
d
.T
his
Peri
p
atus-
lik
e creature
h
a
d
a num
b

er o
f
pr
i
m
i
t
i
ve
f
ea-
tures (six claws at the tip of each le
g
, a terminal mouth, first appenda
g
es postoral, secon
d
and third appenda
g
es are le
g
s). The associated fauna su
gg
ested that this creature was from
a marine or amphibious habitat. This and other discoveries led Snodgrass (1938) to sug
-
g
est anot
h
er monop

h
y
l
et
i
csc
h
eme o
f
art
h
ropo
d
evo
l
ut
i
on (F
i
gure 1.8). In t
hi
ssc
h
em
e
t
h
e
h
ypot

h
et
i
ca
l
ancestra
l
group were t
h
e
l
o
b
opo
d
s (so-ca
ll
e
db
ecause o
f
t
h
e
l
o
b
e
like
*

Cited from Tie
g
s and Manton (19
5
8).
17
A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
out
g
rowths of the bod
y
wall that served as le
g
s). After chitinization of the cuticle and
loss of all except one pair of tentacles (which formed the antennae), the lobopods
g
av
e
rise to the Proton

y
chophora. From the proton
y
chophorans developed, on the one hand, th
e
Onyc
h
op
h
ora an
d
,ont
h
eot
h
er, t
h
e Protart
h
ropo
d
a
i
nw
hi
c
h
t
h
e cut

i
c
l
e
b
ecame sc
l
ero-
ti
ze
d
an
d
t
hi
c
k
ene
d
. Suc
h
organ
i
sms
li
ve
di
ns
h
a

ll
ow water near t
h
es
h
ore or
i
nt
h
e
li
ttora
l
z
one. T
h
e Protart
h
ropo
d
a
g
ave r
i
se to t
h
e Protr
il
o
bi

ta (
f
rom w
hi
c
h
t
h
etr
il
o
bi
te—c
h
e
li
cerat
e
line developed) and the Protomandibulata (Crustacea and Protom
y
riapoda). From the pro-
t
om
y
riapods arose the m
y
riapods and hexapods. In other words, two essential features o
f
Sno
d

grass’ sc
h
eme are t
h
at t
h
eOnyc
h
op
h
ora p
l
ay no part
i
n art
h
ropo
d
evo
l
ut
i
on an
d
t
h
at t
h
e
man

dib
u
l
ate art
h
ropo
d
s (Crustacea, Myr
i
apo
d
a, an
d
Hexapo
d
a)
f
orm a natura
l
group, t
he
M
an
dib
u
l
ata
.
Ori
g

inall
y
, the ma
j
or drawback to the scheme was a lack of supportin
g
evidence
,
especiall
y
from the fossil record. Specificall
y
, there were no protomandibulate fossils i
n
t
he Cambrian period. A second difficulty is that all mandibulate arthropods are united on
th
e
b
as
i
so
f
as
i
ng
l
ec
h
aracter. Manton (see Sect

i
on 3.2.2), espec
i
a
ll
y, argue
d
strong
l
y
th
at t
h
e man
dibl
e
h
as evo
l
ve
d
convergent
l
y
i
n Crustacea an
d
t
h
e Myr

i
apo
d
a-Hexapo
d
a
line. The monoph
y
leticists, on the other hand, believe that the mandibles of crustaceans
,
m
y
riapods, and hexapods are homolo
g
ous. Indeed, Kukalov´a-Peck (1992) insisted that the
j
aws of
all
arthropod groups are homologous, being formed from the same five original
segments. A t
hi
r
d diffi
cu
l
ty o
f
t
h
e Sno

d
grass sc
h
eme
i
st
h
e
i
mp
li
e
dh
omo
l
ogy o
f
t
h
e seven-
t
on
i
ne-segmente
dbi
ramous appen
d
age o
f
Crustacea w

i
t
h
t
h
e
fi
ve-segmente
d
,un
i
ramous
appenda
g
e of Insecta. Supporters of the Mandibulata concept, for example, Matsuda (1970)
,
d
erived the insect le
g
from the ancestral crustacean t
y
pe b
y
proposin
g
that the extra se
g
ment
s
w

ere incorporated into the thorax as subcoxal components. This proposal ma
y
be somewhat
c
l
ose to rea
li
ty as t
h
ere
i
snow
f
oss
il
ev
id
ence t
h
at ear
l
y
i
nsects
h
a
d
appen
d
ages w

i
t
h
s
ide
b
ranc
h
es, compara
bl
etot
h
ose crustaceans, an
df
urt
h
er, t
h
e ancestra
li
nsect
l
eg
i
nc
l
u
d
e
d

1
1
se
g
ments (Ku
k
a
l
ov´a-Pec
k
, 1992, an
di
nE
dg
ecom
b
e, 1998).
Over the 75
y
ears since it was proposed, the merits or otherwise of Snod
g
rass’ schem
e
h
ave been debated vi
g
orousl
y
, and there is still no consensus. Broadl
y

speakin
g
, evi-
d
ence
f
rom morp
h
o
l
og
i
ca
l
,
bi
oc
h
em
i
ca
l
,an
d
mo
l
ecu
l
ar
bi

o
l
og
i
ca
l
stu
di
es ten
d
to sup
-
port t
h
esc
h
eme (e.g., Bou
d
reaux, 1979; W¨age
l
e, 1993; W
h
ee
l
er
e
ta
l
.
,

1993
;
W
h
ee
l
er
,
i
nE
d
gecom
b
e, 1998; B
i
tsc
h
, 2001a,
b
). For examp
l
e, W
h
ee
l
er an
d
cowor
k
ers compare

d
more than 100 morpholo
g
ical characters and the 18S rDNA, 28S rDNA and pol
y
ubiquitin
sequences for almost 30 taxa of arthropods, on
y
chophorans, annelids, and a tardi
g
rade
.
T
hey reached the unequivocal conclusion that the arthropods are monophyletic, and tha
t
th
e concept o
f
t
h
eUn
i
ram
i
a (see
b
e
l
ow)
i

sno
l
onger tena
bl
e. In
d
ee
d
,t
h
e
i
r resu
l
ts suppor
t
Sno
d
grass’ (1938) sc
h
eme
i
n every way except t
h
at t
h
e
i
r
d

ata
i
n
di
cate t
h
e monop
h
y
l
et
ic
nature of the M
y
riapoda (Snod
g
rass believed the m
y
riapods to be paraph
y
letic—see Fi
g
ur
e
1.8). B
y
contrast, the Mandibulata concept is re
j
ected b
y

those who examine the fossil
evidence (see authors in Edgecombe, 1998, and Fortey and Thomas, 1998). Rather, these
w
or
k
ers
f
avor a c
l
ose re
l
at
i
ons
hip b
etween Crustacea an
d
C
h
e
li
cerata
.
3.2.2. The Pol
yp
h
y
letic Theor
y
Proponents o

f
apo
l
yp
h
y
l
et
i
cor
i
g
i
no
f
art
h
ropo
d
s, w
h
os
h
are t
h
ev
i
ew t
h
at t

h
e mem
b
er
s
o
f diff
erent groups are s
i
mp
l
y too
diff
erent to
h
ave
h
a
d
a common ancestor, must a
b
ove a
ll b
e
prepare
d
to ma
k
et
h

e case t
h
at t
h
e man
y
s
i
m
il
ar
i
t
i
es
i
n
b
o
dy
p
l
an note
di
nt
h
e open
i
n
g

para-
g
raph of Section 2 are the result of conver
g
ence. Pol
y
ph
y
leticists point out that conver
g
enc
e
18
CHAPTER
1
i
s a fairl
y
common phenomenon in evolution and, on theoretical
g
rounds alone, it could be
e
xpected that two unrelated
g
roups of animals would evolve toward the same hi
g
hl
y
desir
-

able situation and, as a result, develop almost identical structures servin
g
the same purpose.
(
Even t
h
e monop
h
y
l
et
i
c
i
sts
h
ave to accept some
d
egree o
f
convergence,
f
or examp
l
e, amon
g
t
h
e trac
h

eae o
fi
nsects an
d
t
h
ose o
f
some arac
h
n
id
san
d
crustaceans.) Po
l
yp
h
y
l
et
i
c
i
sts argu
e
t
h
at t
h

es
i
m
il
ar
f
eatures o
f
art
h
ropo
d
s are
i
nterre
l
ate
d
an
di
nter
d
epen
d
ent; t
h
at
i
s, t
h

e
y
a
ll
result from the evolution of a ri
g
id exoskeleton. Thus, in order to
g
row, arthropods mus
t
periodicall
y
molt; to move around, the
y
must have articulated limbs and bod
y
;ta
g
mosis is
a
l
og
i
ca
l
consequence o
f
segmentat
i
on an

d
resu
l
ts
i
nc
h
anges to t
h
e nervous an
d
muscu
l
a
r
systems; t
h
e presence o
f
t
h
e cut
i
c
l
e
d
eman
d
sc

h
anges
i
nt
h
e gas exc
h
ange, sensory, an
d
e
xcretory systems; an
d
t
h
e open c
i
rcu
l
atory system (
h
emocoe
l
)
i
st
h
e resu
l
to
f

an organ
i
s
m
n
o lon
g
er requirin
g
a bod
y
cavit
y
with h
y
drostatic functions. In a sense, then, all the pol
y-
ph
y
leticists need to demonstrate is the pol
y
ph
y
letic nature of the cuticular exoskeleton. A
s
n
oted in Section 2.1, the onychophorans, tardigrades, and pentastomids have such an oute
r
c
over

i
ng (an
d
s
ome ot
h
er art
h
ropo
df
eatures) yet are genera
ll
y cons
id
ere
ddi
st
i
nct
f
rom true
art
h
ropo
d
s(t
h
e
i
r very ex

i
stence ten
d
stoma
k
e
lif
e “uncom
f
orta
bl
e”
f
or t
h
e mem
b
ers o
f
t
h
e
m
onoph
y
letic camp).
T
he second approach taken b
y
the pol

y
ph
y
letic supporters is to criticize the evidenc
e
o
r the methodology used by those who favor monophyly. For example, they argue that th
e
processes o
f
cut
i
cu
l
ar
h
ar
d
en
i
ng use
db
yt
h
et
h
ree ma
j
or art
h

ropo
d
groups are qu
i
te
di
st
i
nct
;
q
u
i
none-tann
i
ng
i
n
i
nsects,
di
su
lfid
e
b
r
id
ges
i
n arac

h
n
id
s, an
di
mpregnat
i
on w
i
t
h
organ
i
c
salts in crustaceans. Likewise, the
y
claim that there are
g
reat differences in the structure o
f
the compound e
y
es amon
g
the ma
j
or arthropod
g
roups. However, the
y

also point out that the
o
mmatidium (the e
y
e’s functional unit—see Chapter 12, Section 7.1) of some pol
y
chaet
e
w
orms an
dbi
va
l
ve mo
ll
us
k
s
i
s
hi
g
hl
ys
i
m
il
ar to
i
ts counterpart

i
n art
h
ropo
d
s, emp
h
as
i
z
i
ng
t
h
e ease w
i
t
h
w
hi
c
h
convergence occurs. Iron
i
ca
ll
y, even t
h
e monop
h

y
l
et
i
c
i
sts
di
sagree ove
r
t
h
e num
b
er an
dh
omo
l
o
gi
es o
f
t
h
ese
g
ments t
h
at ma
k

eupt
h
e art
h
ropo
dh
ea
d
.
T
he earl
y
pol
y
ph
y
leticists, includin
g
Tie
g
s and Manton (1958), Anderson (1973), an
d
Manton (1973, 1977), also presented direct evidence to support their theor
y
, lar
g
el
y
fro
m

c
omparat
i
ve em
b
ryo
l
ogy an
df
unct
i
ona
l
morp
h
o
l
ogy. More recent
l
y, proponents o
f
po
l
y-
p
h
y
l
y
h

ave a
dd
e
di
n
f
ormat
i
on
f
rom pa
l
eonto
l
ogy (see aut
h
ors
i
n Gupta, 1979, an
d
Forte
y
an
d
T
h
omas, 1998). Accor
di
ng to t
h

ese aut
h
ors, t
h
eev
id
ence we
i
g
h
s
h
eav
il
y
i
n support
o
f the division of the arthropods into at least three natural
g
roups, each with the rank
o
fph
y
lum (Fi
g
ure 1.11). The ph
y
la are the Chelicerata, the Crustacea, and the Unirami
a

F
I
GU
RE 1.11.
A
scheme showin
g
a
p
ossible polyphyletic origin for th
e
m
a
j
or art
h
ropo
dg
roups an
d
re
l
ate
d
ph
y
la. Hatched lines endin
g
in a ques
-

tion mark indicate arthropod fossilsno
t
e
as
ily
ass
ig
ne
d
to ex
i
st
i
n
g
taxa.
19
A
RTHR
O
P
O
D
E
VOLUTION
(On
y
chophora-M
y
riapoda-Hexapoda). In some schemes the Trilobita are included as a sis-

t
er
g
roup of the Chelicerata in the ph
y
lum Arachnomorpha; in others the
y
are ranked as a
n
independent ph
y
lum
.
Manton, espec
i
a
ll
y, argue
d
t
h
at t
h
ere are
f
un
d
amenta
l diff
erences

i
nt
h
e structure o
f
th
e
li
m
b
s
i
n mem
b
ers o
f
eac
h
p
h
y
l
um, re
l
ate
d
to t
h
e manner
i

nw
hi
c
h
t
h
ean
i
ma
l
s move
.
I
n Crustacea t
h
e
li
m
bi
s
bi
ramous,
b
ear
i
n
g
a
b
ranc

h
(exopo
di
te) on
i
ts secon
d
se
g
men
t
(basi
p
odite); in Uniramia there is an unbranched limb; and in almost all Chelicerata ther
e
is a uniramous
(
unbranched
)
limb. However, in Limulus (and, incidentall
y
, trilobites) th
e
li
m
bi
s
bi
ramous, t
h

oug
h
t
h
e
b
ranc
h
or
i
g
i
nates on t
h
e
fi
rst
l
eg segment (t
h
e coxopo
di
te),
suggest
i
ng t
h
at c
h
e

li
cerates may
h
ave
b
een
i
n
i
t
i
a
ll
y
bi
ramous,
l
os
i
ng t
h
e
b
ranc
h
w
h
en t
h
e

group
b
ecame terrestr
i
a
l
.T
hi
sv
i
ew
h
as
b
een strong
l
y
di
spute
db
yKu
k
a
l
ov´a-Pec
k
(1992,
and in Forte
y
and Thomas, 1998) who sees the ancestral le

g
of
all
arthropods as bein
g
b
iramous and points out that man
y
fossil insects have le
g
s with several branches (i.e.
,
t
hey are “polyramous Uniramia”!). She has urged that the term Uniramia be abandoned.
M
anton (1973, 1977) ma
d
e a strong case t
h
at t
h
e man
dibl
es o
f
t
h
et
h
ree ma

j
or groups ar
e
not
h
omo
l
ogous. Base
d
on t
h
e
i
r structure an
d
mec
h
an
i
sm o
f
act
i
on, s
h
e suggeste
d
t
h
at t

h
e
j
aws of crustaceans and chelicerates are formed from the basal se
g
ment of the ancestra
l
appenda
g
e(
g
nathobasic
j
aw), thou
g
h in each
g
roup the mechanism of action is different. In
Uniramia, however, Manton claimed that the mandible was formed from the entire ancestral
appen
d
age, an
d
t
h
at mem
b
ers o
f
t

hi
s group
bi
te w
i
t
h
t
h
et
i
po
f
t
h
e
li
m
b
. Manton po
i
nte
d
ou
t
th
at a segmente
d
man
dibl

e
i
sst
ill
ev
id
ent
i
n some myr
i
apo
d
s, t
h
oug
h
t
h
e man
dibl
eo
fi
nsects
and on
y
chophorans appears unse
g
mented. A
g
ain, this proposal has been severel

y
criticize
d
by
Kukalov´a-Peck (1992, and in Forte
y
and Thomas, 1998) whose paleoentomolo
g
ical
studies su
gg
est the ancestral limb of all arthropods included 11 se
g
ments, 5 of which mak
e
u
pt
h
e
j
aw seen
i
n extant spec
i
es.
An
d
erson (1973, an
di
n Gupta, 1979)

d
rew ev
id
ence
f
rom em
b
ryo
l
ogy
i
n support o
f
th
epo
ly
p
hyl
et
i
ct
h
eor
y
. He compare
df
ate maps (
fig
ures
i

n
di
cat
i
n
g
w
hi
c
h
em
b
r
y
on
i
cce
lls
g
ive rise to which or
g
ans and structures) amon
g
the various
g
roups and concluded tha
t
t
he pattern of development seen in Uniramia bears similarities to that in annelids,
y

et i
s
very
diff
erent
f
rom t
h
at o
f
crustaceans (c
h
e
li
cerates s
h
ow no genera
li
ze
d
pattern,
l
ea
di
n
g
t
o specu
l
at

i
on t
h
at t
h
ey may t
h
emse
l
ves
b
epo
l
yp
h
y
l
et
i
c). It s
h
ou
ld b
e note
d
t
h
at not a
ll
em

b
ryo
l
og
i
sts agree w
i
t
h
An
d
erson’s met
h
o
d
so
f
ana
l
ys
i
san
d
,t
h
ere
f
ore,
hi
s conc

l
us
i
ons
(e.
g
., We
yg
oldt, in Gupta, 1979).
W
hen the Mantonian viewpoint was initiall
y
presented, there was little supportin
g
ev-
idence from the arthro
p
od fossil record. Within the last three decades, however, there has
b
een cons
id
era
bl
e act
i
v
i
ty
b
ot

hi
n ana
l
yz
i
ng new spec
i
es an
di
nre
i
nterpret
i
ng some spec
i-
mens
d
escr
ib
e
d
ear
li
er. Many o
f
t
h
ese
f
oss

il
s cannot
b
ep
l
ace
di
n extant art
h
ropo
d
groups
or even alon
g
evolutionar
y
lines leadin
g
to these
g
roups (Whittin
g
ton, 198
5
), indicatin
g
t
hat arthropodization was experimented with man
y
times and impl

y
in
g
that arthropods ha
d
multiple origins. Most of the groups to which these Cambrian fossils belong rapidly be-
came ext
i
nct. T
h
e art
h
ropo
d
groups seen to
d
ay represent “success
f
u
l
attempts
i
n app
l
y
i
ng
a cont
i
nuous, part

i
a
ll
yst
iff
ene
d
cut
i
c
l
etoaso
f
t-
b
o
di
e
d
worm” (W
ill
mer, 1990, p. 290).
W
illmer (1990) drew attention to the very different methodology used by polyphyletic
W
W
and monoph
y
letic schools, b
y

which the
y
reach opposite conclusions re
g
ardin
g
arthropo
d
ev
o
lution. The approach taken b
y
Manton and her supporters has been to search for differ
-
ences among groups
i
nt
h
e
b
e
li
e
f
t
h
at t
h
ey prov
id

e
d
ev
id
ence
f
or po
l
yp
h
y
l
y. On t
h
eot
h
e
r
h
an
d
,t
h
emo
d
ern monop
h
y
l
et

i
c
i
sts, nota
bl
y Bou
d
reaux an
d
W
h
ee
l
e
r
et a
l
.
,h
a
ve
attempte
d
t
o
d
eterm
i
ne s
i

m
il
ar
i
t
i
es an
d
use t
h
ese as proo
f
o
f
a common or
igi
n
f
or a
ll
art
h
ropo
d
s.
20
CHAPTER
1
T
he debate as to arthropod relationships and evolution continues to be vi

g
orous (and
polarized!) (see Ed
g
ecombe, 1998; Forte
y
and Thomas, 1998). Overall, the balance currentl
y
rests in favor of monoph
y
l
y
, with the ma
j
or
g
roups havin
g
had a common ori
g
in from
a
pr
i
m
i
t
i
ve segmente
d

worm
lik
ean
i
ma
l
.
3
.3. The
U
n
i
ram
i
an
s
In a
g
reement with the 19th centur
y
zoolo
g
ists Haeckel and Mosele
y
,Tie
g
s and Manto
n
(
1958) and Manton (1973) made a forceful case for uniting the Onychophora, Myriapoda,

an
d
Hexapo
d
a
i
nt
h
e art
h
ropo
d
group Un
i
ram
i
a. In t
h
e
i
rv
i
ew t
h
e many structura
l
s
i
m
il

ar-
i
t
i
es
b
etween on
y
c
h
op
h
orans an
d
m
y
r
i
apo
d
s (see Sect
i
on 2.1)
i
n
di
cate
d
true a
ffi

n
i
t
y
an
d
w
ere not the result of conver
g
ence. This view received support from the fate map anal
y
ses
m
ade b
y
Anderson (1973, and in Gupta, 1979) showin
g
the similarit
y
of embr
y
onic devel-
o
pment in the three groups. These authors envisaged the evolution of myriapods and insects
f
rom onyc
h
op
h
oran

lik
e ancestors as a process o
f
progress
i
ve cep
h
a
li
zat
i
on. To t
h
eor
i
g
i
na
l
t
h
ree-segmente
dh
ea
d
(seen
i
nmo
d
ern Onyc

h
op
h
ora) were a
dd
e
d
progress
i
ve
l
y man
dib
u-
l
ar, first maxillar
y
, and second maxillar
y
(labial) se
g
ments,
g
ivin
g
rise to the so-called
m
ono
g
nathous, di

g
nathous, and tri
g
nathous conditions, respectivel
y
. Of the mono
g
nathous
c
ondition there has been found no trace. The dignathous condition occurs in the Pauropod
a
an
d
D
i
p
l
opo
d
a, an
d
t
h
etr
i
gnat
h
ous con
di
t

i
on
i
s seen
i
nt
h
eC
hil
opo
d
a(
i
nw
hi
c
h
t
h
e secon
d
m
ax
ill
ae rema
i
n
l
eg
lik

e) an
d
t
h
e Symp
h
y
l
aan
d
Hexapo
d
a(
i
nw
hi
c
h
t
h
e secon
d
max
ill
a
e
f
use to form the labium
)
.

F
ew modern authors would support the idea of the on
y
chophorans havin
g
common
ancestry with the myriapods and insects, preferring to believe that the similarities are due
to convergence. In
d
ee
d
, some aut
h
ors
d
o not accept t
h
at t
h
e myr
i
apo
d
san
dh
exapo
d
sar
e
sister groups. For example, Friedrich and Tautz (199

5
) concluded from their comparison of
r
ib
osoma
l
nuc
l
ear
g
enes t
h
at t
h
em
y
r
i
apo
d
s were t
h
es
i
ster
g
roup to t
h
ec
h

e
li
cerates, w
hil
e
the crustaceans were the sister
g
roup to the hexapods. Unfortunatel
y
, the term Unirami
a
i
s still used in some texts (e.
g
., Barnes
et al.
,
1993
; Barne
s
,
1
994) to include onl
y
th
e
Myr
i
apo
d

aan
d
Hexapo
d
a(
i
.e., as a synonym o
f
t
h
e Ate
l
ocerata). As note
d
ear
li
er, Ku
k
a
l
ov´a-
P
ec
k
(1992, an
di
n Fortey an
d
T
h

omas, 1998)
h
as recommen
d
e
d
t
h
at use o
f
t
hi
swor
db
e
di
scont
i
nue
d
as t
h
e
g
roup
i
nc
l
u
d

es or
g
an
i
sms w
i
t
h
po
ly
ramous
l
e
g
s
.
3
.3.1. M
y
ria
p
oda-Hexa
p
oda Relationshi
ps
Th
es
h
ar
i

ng o
ff
eatures suc
h
as one pa
i
ro
f
antennae, Ma
l
p
i
g
hi
an tu
b
u
l
es (t
h
oug
h
these ma
y
be secondaril
y
reduced or lost in both
g
roups), anterior tentorial arms, and
a

tracheal s
y
stem
g
ave rise to the traditional view that the M
y
riapoda and Hexapoda ar
e
sister groups, collectively forming the Atelocerata (Tracheata), with a common multilegged
ancestor (Sharov,1966; Boudreaux, 1979). Indeed, the existence of several shared features in
S
ymp
h
y
l
aan
d
Hexapo
d
a (Sect
i
on 2.4)
l
e
di
nt
h
e 1930s to t
h
e

d
eve
l
opment o
f
t
h
e Symp
h
y
l
a
n
T
h
eor
yf
or t
h
eor
igi
no
f
t
h
e
h
exapo
d
s. W

i
t
hi
nt
h
e
l
ast
d
eca
d
e,
h
owever, a ma
j
or c
h
an
g
e
i
n
o
pinion has occurred with respect to the relationship between m
y
riapods and hexapods
.
It is now believed that their common features are the result of conver
g
ence or, at best

,
p
ara
ll
e
l
evo
l
ut
i
on
f
rom a
di
stant common ancestor. Muc
h
recent researc
h
,
i
nmo
l
ecu
l
ar
bi
o
l
ogy, neuro
bi

o
l
ogy, an
d
comparat
i
ve morp
h
o
l
ogy, o
f
ten com
bi
ne
di
n extens
i
ve c
l
a
di
st
ic
ana
ly
ses, supports t
h
e
hy

pot
h
es
i
st
h
at
h
exapo
d
s are more c
l
ose
ly
re
l
ate
d
to crustacean
s
than to m
y
riapods. Equall
y
, the data su
gg
est that m
y
riapods are allied with the chelicerates.
21

A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
Comparisons of mitochondrial and nuclear
g
ene sequences and lar
g
e hemol
y
mph proteins,
examination of e
y
e and brain structure, and studies of nerve development have come ou
t
stron
g
l
y
in favor of insects and modern crustaceans as sister
g
roups (Dohle, in Forte

y
an
d
Th
omas, 1998; S
h
u
l
tz an
d
Reg
i
er, 2000; G
i
r
ib
et
e
ta
l
.
,
2001; Hwang et a
l
., 2001; Coo
k
e
ta
l.
,

2001; Burmester, 2002). Some
d
ata even suggest t
h
at
i
nsects arose
f
rom t
h
e same
crustacean linea
g
e as the Malacostraca (crabs, lobsters, etc.), an idea for which Sharov
(19
66
) had been criticized almost 40
y
ears a
g
o
.
4.
S
ummar
y
T
h
e art
h

ropo
d
s are a very
di
verse group o
f
organ
i
sms w
h
ose evo
l
ut
i
on an
di
nterre
l
at
i
on
-
s
hi
ps
h
ave
b
een v
ig

orous
ly d
e
b
ate
df
or more t
h
an a centur
y
. Supporters o
f
a monop
hyl
et
ic
ori
g
in for the
g
roup rel
y
heavil
y
on the existence of numerous common features in the
arthropod bod
y
plan. Their opponents, who must account for the extraordinar
y
de

g
ree o
f
convergent evo
l
ut
i
on
i
n
h
erent
i
nanypo
l
yp
h
y
l
et
i
ct
h
eory, argue t
h
at a
ll
o
f
t

h
ese
f
eature
s
are essent
i
a
ll
yt
h
e resu
l
to
f
as
i
ng
l
ep
h
enomenon, t
h
eevo
l
ut
i
on o
f
a

h
ar
d
exos
k
e
l
eton
,
an
d
t
h
at art
h
ropo
di
zat
i
on cou
ld
eas
ily h
ave
b
een repeate
d
severa
l
t

i
mes amon
g
t
h
evar
i
ou
s
ancestral
g
roups. In the pol
y
ph
y
letic theor
y
, therefore, the four dominant
g
roups of arthro
-
pods (Trilobita, Crustacea, Chelicerata, and Insecta), as well as several smaller
g
roups bot
h
f
oss
il
an
d

extant, or
i
g
i
nate
df
rom
di
st
i
nct, unre
l
ate
d
ancestors. T
h
e proponents o
f
po
l
y-
p
h
y
l
y use ev
id
ence
f
rom comparat

i
ve morp
h
o
l
ogy (nota
bl
y stu
di
es o
fli
m
b
an
d
man
dible
structure), comparat
i
ve em
b
r
y
o
l
o
gy
(
f
ate maps), an

d
more recent
ly
t
h
e
f
oss
il
recor
d
(w
hi
c
h
shows an abundance of arthropod t
y
pes not easil
y
assi
g
nable to alread
y
known
g
roups)
.
T
he monoph
y

leticists claim, in turn, that these comparative embr
y
olo
g
ical and morpho
-
l
og
i
ca
l
stu
di
es are o
fd
ou
b
t
f
u
l
va
l
ue
b
ecause o
f
t
h
e met

h
o
d
o
l
ogy emp
l
oye
d
an
d
assump
-
ti
ons ma
d
e. Overa
ll
,t
h
e current
b
a
l
ance seems
i
n
f
avor o
f

a monop
h
y
l
et
i
cor
i
g
i
n
f
or t
he
art
h
ropo
d
s
.
The unitin
g
of On
y
chophora, M
y
riapoda, and Hexapoda as the clade Uniramia is hi
g
hl
y

questionable. Most modern authors a
g
ree that apparent similarities between on
y
chophorans
an
d
mem
b
ers o
f
t
h
eot
h
er two groups are
d
ue to convergent evo
l
ut
i
on. T
h
e Myr
i
apo
d
a, a
l
-

th
oug
hi
nc
l
u
di
ng
f
our rat
h
er
di
st
i
nct groups (D
i
p
l
opo
d
a, C
hil
opo
d
a, Pauropo
d
a, an
d
Sym

-
p
h
y
l
a), are w
id
e
l
yt
h
oug
h
tto
b
e monop
h
y
l
et
i
c. For many years, myr
i
apo
d
s were cons
id
ere
d
t

he sister
g
roup to the Hexapoda. However, recent research indicates that m
y
riapods ma
y
b
e
allied more closel
y
to the chelicerates, and hexapods to crustaceans. Five distinct
g
roups o
f
h
exapods occur: collembolans, proturans, diplurans, thysanurans, and winged insects. O
n
th
e
b
as
i
so
f
t
h
e
i
r entognat
h

ous mout
h
parts an
d
ot
h
er synapomorp
hi
es t
h
e
fi
rst t
h
ree group
s
are p
l
ace
di
nt
h
e Entognat
h
aan
d
are
di
st
i

nct
f
rom t
h
et
h
ysanurans an
d
pterygotes w
hi
c
h
form the true Insecta
.
5
. Literatur
e
Numerous genera
l
text
b
oo
k
son
i
nverte
b
rates, as we
ll
as spec

i
a
li
ze
d
treat
i
ses prov
id
e
information on the biology of arthropods. Tiegs and Manton (19
5
8) give a detailed historical
account o
f
sc
h
emes
f
or t
h
eevo
l
ut
i
onar
y
re
l
at

i
ons
hi
ps o
f
art
h
ropo
d
s. Ot
h
er ma
j
or contr
ib-
u
tors to this fascinatin
g
debate include Manton (1973, 1977), Sharov (19
66
), Anderson
22
CHAPTER
1
(
1973
)
, Boudreaux
(
1979

)
, Willmer
(
1990
)
, Wheele
r
et al.
(
1993
)
, Bitsch
(
2001a,b
)
, and
authors in Gupta (1979), Ed
g
ecombe (1998), and Forte
y
and Thomas (1998).
An
d
erson
,
D. T.
,
1973
,
Em

b
ryo
l
ogy an
d
P
h
y
l
ogeny of Anne
l
i
d
san
d
Art
h
ropo
ds
,
Per
g
amon Press, E
l
ms
f
or
d
,NY
.

B
arnes
,
R. D.
,
1994
,
I
nvertebrate Zoolo
gy
,
6th ed., Saunders, Philadel
p
hia.
B
arnes, R. S. K., Calow, P., and Olive, P. J. W., 1993
,
T
he Invertebrates: A New S
y
nthesi
s
, 2nd ed., Blackwell,
O
x
f
or
d.
B
itsch, J., 2001a, The hexapod appenda

g
e: Basic structure, development and ori
g
in,
A
nn. Soc. Entomol. Fr. (N.S.
)
3
7
:
175–193
.
Bi
tsc
h
, J., 2001
b
,T
h
e art
h
ropo
d
man
dibl
e: Morp
h
o
l
o

gy
an
d
evo
l
ut
i
on. P
hyl
o
g
enet
i
c
i
mp
li
cat
i
ons
,
Ann. Soc.
E
ntomol. Fr. (N.S.
)
37
:305
–
3
21

.
Bi
tsc
h
, C., an
d
B
i
tsc
h
, J., 2000, T
h
ep
h
y
l
ogenet
i
c
i
nterre
l
at
i
ons
hi
ps o
f
t
h

e
hi
g
h
er taxa o
f
apterygote art
h
ropo
d
s
,
Z
oo
l
. Scrip
.
29
:
131–1
5
6
.
B
oudreaux, H. B., 1979, Arthropod Phylogeny, with Special Re
f
erence to Insect
s
,
W

iley, New York.
W
W
B
urmester, T., 2002, Or
i
g
i
nan
d
evo
l
ut
i
on o
f
art
h
ropo
dh
emocyan
i
ns an
d
re
l
ate
d
prote
i

ns, J. Comp. P
h
ysio
l
.
B
1
7
2
:95
–107
.
C
isne, J. L., 1974, Trilobites and the origin of arthropods,
S
cienc
e
1
86
:1
3
–1
8
.
C
oo
k
, C. E., Sm
i
t

h
,M.L.,Te
lf
or
d
, M. J., Bast
i
ane
ll
o, A., an
d
A
k
am, L., 2001, Hox genes an
d
t
h
ep
h
y
l
ogeny o
f
a
rt
h
ro
p
o
d

s,
C
urr. Bio
l
.
11
:
7
59
–763
.
Edgecombe, G. D. (ed.), 1998
,
A
rthropod Fossils and Ph
y
lo
g
en
y
,
Columbia University Press, New York.
Emerson, M. J., an
d
Sc
h
ram, F. R., 1990, T
h
eor
i

g
i
no
f
crustacean
bi
ramous appen
d
ages an
d
t
h
eevo
l
ut
i
on o
f
Art
h
ro
p
o
d
a
,
S
cienc
e
2

5
0
:
66
7–
669
.
Foelix, R. F., 1997, Biology o
f
Spider
s
, 2nd ed., Harvard University Press, Cambridge, Massachusetts.
Fortey, R. A., an
d
T
h
omas, R. H., 1998, Art
h
ropo
d
Re
l
ations
h
ips
,
C
h
apman an
d

Ha
ll
, Lon
d
on.
Friedrich, M., and Tautz, D., 199
5
, Ribosomal DNA ph
y
lo
g
en
y
of the ma
j
or extant arthropod classes and th
e
ev
o
lution of myriapods
,
N
atur
e
3
76:1
65
–1
6
7.

Gi
r
ib
et, G., E
d
gecom
b
e, G. D., an
d
W
h
ee
l
er, W. C., 2001, Art
h
ropo
d
p
h
y
l
ogeny
b
ase
d
on e
i
g
h
tmo

l
ecu
l
ar
l
oc
i
a
n
d
morp
h
o
l
o
gy
,
N
ature
413
:
1
5
7–161
.
G
upta, A. P. (ed.), 1979
,
A
rthropod Ph

y
lo
g
en
y
,V
an Nostrand-Reinhold, New York.
V
V
H
op
ki
ns, S. P., an
d
Rea
d
, H. J., 1992, T
h
e Bio
l
ogyofMi
ll
ipe
d
es
,
Ox
f
or
d

Un
i
vers
i
ty Press, Lon
d
on.
H
wan
g
,U.W.,Fr
i
e
d
r
i
c
h
, M., Tautz, D., Par
k
, C. J., an
d
K
i
m, W., 2001, M
i
toc
h
on
d

r
i
a
l
prote
i
np
hyl
o
g
en
yj
o
i
n
s
myriapods with chelicerates
,
N
atur
e
4
1
3
:1
5
4–1
5
7.
Ki

nc
hi
n, I. M., 1994
,
Th
e Bio
l
ogyoft
h
eTar
d
igra
d
a
,
Port
l
an
d
Press, Lon
d
on
.
Ki
n
g
, P. E., 1973
,
Pycnogoni
d

s
,
St. Mart
i
n’s Press
,
New Yor
k.
K
ristensen, N. P., 1991, Phylogeny of extant hexapods, in
:
The Insects o
f
Australia
,
2nd ed., Vol. I (CSIRO, ed.),
Me
lb
ourne Un
i
vers
i
ty Press, Car
l
ton, V
i
ctor
i
a.
Ku

k
a
l
ov´a-Pec
k
, J., 1991, Foss
il hi
stor
y
an
d
t
h
eevo
l
ut
i
on o
fh
exapo
d
structures,
i
n
:
T
h
e Insects o
f
Austra

l
ia, 2n
d
e
d., Vol. 1 (CSIRO, ed.), Melbourne University Press, Carlton, Victoria.
Ku
k
a
l
ov´a-Pec
k
, J., 1992, T
h
e “Un
i
ram
i
a”
d
o not ex
i
st: T
h
e groun
d
p
l
an o
f
t

h
e Pterygota as revea
l
e
db
y Perm
i
a
n
Diaphanopterodea from Russia (Insecta: Paleodict
y
opteroidea),
C
an.
J
. Zool
.
7
0
:
2
36
–2
55
.
L
ewis, J. G. E., 1981
,
T
he Biology o

f
Centipede
s
, Cambridge University Press, Cambridge
.
M
anton, S. M., 1973, Art
h
ropo
d
p
hyl
o
g
en
y
—A mo
d
ern s
y
nt
h
es
i
s
,
J
.
Z
oo

l
.
(
Lon
d
on)
171
:
111
–
130
.
M
anton
,
S. M.
,
1977
,
T
he Arthropoda: Habits, Functional Morpholo
gy
and Evolution
.
Oxford Universit
y
Press,
London
.
M

anton, S. M., 1978, Ha
bi
ts,
f
unct
i
ona
l
morp
h
o
l
o
gy
an
d
t
h
eevo
l
ut
i
on o
f
p
y
cno
g
on
id

s, Zoo
l
. J. Linn. Soc.
63
:
1
–
21
.
M
atsuda, R., 1970. Morpholo
gy
and evolution of the insect thorax
.
M
em. Entomol.
S
oc.
C
an. 7
6
:
431
pp
.
N
elson, D. R., 2001, Tardigrada, in
:
Ecology and Classi
fi

cation o
f
North American Freshwater Invertebrates, 2nd
ed
., Aca
d
em
i
c Press, San D
i
e
g
o.
P
olis, G. A.
(
ed.
)
, 1990
,
T
he Biology o
f
Scorpions
,
Stanford Universit
y
Press, Stanford.
R
oss, H. H., 1965

,
A
T
e
xtbook of Entomology
e
e
, 3rd ed., Wiley, New York.
Sharov
,
A. G.
,
19
66,
B
asic Art
h
ropo
d
an Stoc
k
,
P
er
g
amon Press, E
l
ms
f
or

d
,NY
.
Shultz, J. W., and Re
g
ier, J. C., 2000, Ph
y
lo
g
enetic anal
y
sis of arthropods usin
g
two nuclear protein-encodin
g
g
enes supports a crustacea
n
+
hexapod clade
,
Proc. R. Soc. Lond. Ser.
B
267
:
1011
–
1019.
Sno
dg

rass, R. E., 1938, Evo
l
ut
i
on o
f
t
h
e anne
lid
a, on
y
c
h
op
h
ora, an
d
art
h
ropo
d
a,
S
mit
h
son. Misc. Co
ll
ect.
97

:1
5
9
pp
.
23
A
RTHR
O
P
O
D
E
V
O
LUTI
O
N
T
ait, N. N., 2001, The On
y
chophora and Tardi
g
rada, in
:
I
nvertebrate Zoolo
gy
, 2nd ed.
(

D. T. Anderson, ed.
)
,
Oxford University Press, South Melbourne.
Ti
e
g
s, O. W., and Manton, S. M., 19
5
8, The evolution of the Arthropoda, Bio
l
.Rev
.
33
:
2
55
–337
.
W
agele, J. W., 1993, Rejection of the “Uniramia” hypothesis and implications of the Mandibulata concept,
¨
Zool.
Ja
h
r
b
.A
b
t. Syst.

k
o
l
. Geog. Tier
e
120:253–288.
Wh
ee
l
er, W. C., Cartwr
igh
t. P., an
d
Ha
y
as
hi
, C. Y., 1993, Art
h
ropo
d
p
hyl
o
g
en
y
: A com
bi
ne

d
approac
h,
Cl
a
d
istic
s
9
:1–3
9
.
W
hittington, H. B., 1985,
Th
e Burgess S
h
a
l
e,
Y
ale University Press, New Haven.
YY
Whi
tt
i
n
g
ton, H. B., 1992,
T

ri
l
o
b
ite
s
,T
h
eBo
yd
e
ll
Press, Roc
h
ester, NY.
W
illmer, P., 1990, Invertebrate Relationshi
p
s: Patterns in Animal Evolutio
n
, Cambridge University Press, London.