BODY APPENDAGES OF CTIILOPODA

The

centipedes furnish a

good example

of arthropods that have retained a long series of

body appendages

preserving the

form

and

T

Fig. 26.

—

Somewhat diagrainmatic cross section of a body segment of Litlio- bius, showing the relation of the subcoxa (Sex) and coxa (C.v) to the body and tothe telopodite (Tlpd).

function of ambulatory limbs.

Each appendage

of the pregenital re- gion of the

body

is implanted in a

membranous

pleural area of its

supporting

body segment

between distinct tergal

and

sternal |)lates (figs. 26, 27

A). The movable

basal i)iece of a typical chilopod leg is

a smallsegment generally termed the coxa, orcoxopodite (C.r).

The

66 SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. 85

coxa

supportsthe six-segmentedtelopodite (fig. 26, Tlpd), theproxi-

mal

segment of which, or first trochanter (iTr), is articulated to the

coxa by

a typical coxo-trochanteralhinge (fig. 25 _{B, f-g).}

Surround-

ing or partly surrounding the base of the coxa, in

most

of the pre- genital segments but the last, is an area of the

body

wall containing oneor several small sclerites (fig. 27

A,

Sex).

These

sclerites appear to belong to the subcoxal region of the primitive limb basis, since

upon

this region are inserted the tergalmuscles of theappendage, and within ^itarise muscles of thecoxa.

The

coxaturns antero-posteriorly

upon

the subcoxa

by

an approximately dorso-ventral axis (fig. 25 B, c-d).

The

large terminal pair of legs of a chilopodborne

by

the lastpre- genital

segment

(fig. 27

A,

Tlpd) are supported each

upon

a single largeplate inthelateralsegmental wall

(LB). The

basaljointofeach of these legs clearly corresponds w^ith the coxo-trochanteral joints of the preceding appendages,

and

a comparison of the leg-bearing plate of this segment

(LB)

with the coxal

and

subcoxal sclerites of the segments immediatelyanterior to ^it leaves little doubt that the single

"pleural" plate of the last

segment

representsboth the

coxa and

the

subcoxa

of the preceding segments (Cx, Sex). In other words, the largepleural plates supportingthe legs of the last pregenital segment are thelimb bases

(LB)

undividedintocoxal

and

subcoxal parts as in the other segments.

The

condition here, of course,

may

l)e the result of a secondary union of the subcoxal sclerotizations with the coxa, but it gives a convincing demonstration of the potential unity of the coxal

and

subcoxal regions of the limb basis,

and

at least suggests a primitive condition in

which

thelimb basis occupiedthelateralwalls of the

body segment

between the tergal

and

sternal plates (A, C, IT, IStn).

The

levator

and

depressor muscles of the telopodite of the last pair of legs arise

on

the plate of the limb base

and

on the

sternum

(D, O,

Q)

^,

^and

^{have their insertions on the first trochanter}

(fTr).

"

The

basal structure of the last pair of legs in the chilopoda is

paralleled exactly in that of the legs of

more

generalized

Arachnida

as in the Phalangidae (fig.

46 A),

ⁱⁿ

which

the free part of each leg is supported on a large basal plate

(L5)

implanted in thelateral wall of the body.

Borner

₍₁₉₀₄₎ regards the single basal plate of the terminal pair of chilopodlegs as the united

coxa and

subcoxa; but in theArachnida, he concludes (1921) that sulicoxae areabsent

and

that the plates supporting the telopodites are the coxae alone. It is not clear

why

structures so evidently similar should be differently inter- preted.

NO. 6

INSECT ABDOMEN — ^SNODGRASS

67 In the Diplopoda the free basal

segment

of the leg, judging

from

its structure

and

the nature of its articulation with thenext segment,

would

appeartobethe coxa,

and

sincethesternal plates of theDiplop- oda surround the bases of the legs,

we may

conclude with

Borner

(1921) ^{that the definitive sterna include the} subcoxae. Silvestri (1903), however, regards the free basal

segment

of the diplopod leg as the subcoxa,

and

the next

segment

as the coxa,

though

the latter

Fig. 27.

—

Appendages and terminal body structures of Chilopoda.

A, posterior part of body of Scolopocryptops scxspinosa, telopodites removed except fromlastsegment,showingunion of subcoxalsclerites with coxainlarge pleural plate {LB) on last pregenital segment. B, ventral view of genital and pregenital segments ofLithobius. C, lateral viewof same. D, base ofpregenital appendage of Lithobius. E, telopodite of right gonopod of Lithobius, mesal surface.

An, anus; Cx, coxa; C.vpd, coxopodite; GO, gonopore; Gp, gonopod; gS, sternum of genital segment; GScg, genital segment^; 1st, intersternal sclerite

;

LB,limb basis; IStn, sternum of pregenital segment; IT, tergimi of pregenital segment; O, levator of telopodite; Prpt. periproct; Q. depressor of telopodite;

Sex, subcoxa; Stti,segmental sternal plate; Tlpd,telopodite; iTr, firsttrochan- ter; 2Tr, second trochanter (praefcmur).

segment has all the structural features

and

usual relations of a first trochanter.

Following the last leg-bearing

segment

in the Chilopoda

comes

the definitive genital

segment

(fig. 27

A, GSeg), beyond which

is the periproct {Prpt),or anal segment.

According

to

Heymons

₍₁₉₀₁₎ the genital region of the

body

contains

two

small somites in the embryo, parts of bothof

which

are sometimes retained in the adult stage.

The

5

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VOL. 85