28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 89 basis for all subsequent discussions in this paper on the morphology

of theovipositor.

The

assumptionseemstohaveample support

from

embryology; but, if the plates in question should be

shown

to be somethingelsethanthe basal elements ofappendages,itwillbeneces- sary onlytochangetheir

names

;their relationsto structures evolved

from them

inthePterygota willnotbealtered.

The

abdominal styli of the

Thysanura

support the

abdomen

andplayanactivepartinthe locomotion oftheinsect.

The

writerhas elsewhere (1931) discussed the evidence bearing on ^the morphologyof the styli, showing that, while thereis

much

reason for believing that thestyliandtheir

homo-

logues invarious pterygote larvae are therudimentarytelopodites of the abdominal segments, the questioncannot beregardedas settled;

the ^styli

may

be basal epipodites of the appendages,

from

whichthe telopoditeshaveentirelydisappeared.

The

writerhas notbeenableto

make

anexactstudy of the

abdom-

inal musculature in the Thysanura, but even with poorly preserved specimensitcaneasilybe seenthateach stylus-bearingplateof theab-

domen

is

amply

provided withlateralmusclesthattaketheiroriginon theabdominalterga. Infact,itappearsthatallthe dorsoventral

mus-

cles of the

abdomen

in the Machilidae are attached ventrally on the stylus-bearing plates, there being no muscles of any kind con- nected with the small median sternal plates. It

would

thus

seem

that inthe

Thysanura

thelateral

body

musculature of the

abdomen

is

formed

entirely of the tergal musculature of the limb bases. In studying the

abdomen

of pterygote insectsthewriter (1931)

was

led to

make

the statement that no evidence

was

found suggesting the derivation of the lateral abdominal musculature

from

the muscles of the limbbases;andthis istrue, since in exopterygote larvaethat preserve distinct limb base lobes ontheabdomen, none of the

body

muscles is attached directly on the limb bases.

The

diversification of the muscle attachmentsin adult Pterygota, inwhich

some

of the lateral muscles

may

be tergopleural and others tergosternal, cannot be taken as evidence of a primitive condition, since it is evidently an adaptation to the

mechanism

of respiration

and

of other

body

movements. Likewise,inendopterygotelarvaeasecondaryspecializa- tion in the musculature

may

obscure the primitive muscle pattern.

The

conditioninthe Machilidae,however,isperhaps

more

significant it mightbe interpretedas indicatinga derivation of thelateral body musculature

from

the primitiveleg muscles,though theextreme re- duction of the sternamustbe takenintoaccountasa factor bringing about a secondarytransposition of

some

ofthe body muscles to the expandedlimbbases.

NO. 8 INSECT

ABDOMEN —

SNODGRASS 29

The

gonopods, or appendages of the eighth and ninth abdominal segments, differ in'female

Thysanura from

the appendages ^{of the} pregenital segmentsinthatthey lack eversible vesicles,andthateach is providedwith a long genital process, or gonapophysis (fig.6B, C, iGon, 2Gon).

The

basalplates of each segmental pair of gonopods alsoare not united with eachother.

The

gonapophysis is a slender hollowextension

from

the dorsal aspect of themedian proximalangle of thebasis of the

gonopod

^(fig. 7). Its scleroticwall iscontinuous

VUI IX

X

XI

Fig.^6.

—

Theovipositorandterminal structuresoffemaleThysanura.

A,abdomenofMachiUs,showingthestyli {Sty) in position in^life.

B,C, genitalsegmentsofMachiUs,showing gonapophyses (Gon) arisingfrom limb baseplates (LB).

D, basesofcaudal filament (cf) andcercus (Cer) ofMachiUs,with muscles oftenthsegment.

E,genitalsegmentsandbaseofovipositorofThcnnohia,ventral view.

F,diagrammaticfigureofthe principalmusclesoffirstgonopodofThcnnohia.

with that of the ventral plate of the basis, and the genital process

is thus not articulated with the latter, as in the stylus; but, being flexible at its base, the gonapophysis is movable,

and

it is provided with short muscles (A, gmcl) arising on the basal ^plate

mesad

of thelonger musclesof the stylus (smcl). In

some

of the Thysanura, asin

N

esomachilis (fig.7), the fourgonapophysesof the ovipositor are individually free structures, except that those of the ninth seg-

ment (B)

are united at theirbases; in others,however, as in Ther- mobia (fig.6E),thegonapophysesof the secondpairare fusedwith each otherthroughoutthe length of the ovipositor,

and

only those of

30

the first pair are free processes.

The

length and shape ^of the ovi- positorvary

somewhat

in different

Thysanura

;inMachilis (fig.6

A)

thedistalpart of theorganisabruptly bent upv^ard.

The

morphological origin of the gonapophyses cannot be deter- mined

from

the evidenceat hand.

The

structures are treatedin this paperas being processes of the

gonopod

bases havingthe nature of enditelobes.

Some

writers prefertoregard

them

as the true telopo- ditesof the genitalappendages,but their limitation tothegonopods, theirorigins

from

theproximalparts ofthe^latter,their special func-

Mb

smcl

^m

VIllLB cl ermcL

Mb

Sty smcl

r~iVl(lGor>)

Fig.7.

—

Female gonopods ofNcsomachilismaoriciis, dorsal view.

Note in each appendage musclesfrom basal plate to stylus (smcl) and to gonapophysis (gmcl).

tion,andthefactthattheynever

show

aleglike

form

seemtobe sug- gestive, rather, that the gonapophyses aresecondary structures de- veloped on the appendages of the eighth and ninth abdominal seg- ments in adaptation to the special function these appendages have assumed.

The

bases of thethysanuran gonopods,inconformity withthe bases of the pregenitalappendages,are wellequipped with musclesarising onthe terga of their respective segments (fig.

6F). Some

of these musclesofeachappendageare insertedontheplate formingthe ven- tral wall of the limb basis, and others at the inner margin of its

membranous

dorsal wall. Smaller groups of median, longitudinal

NO. 8 INSECT

ABDOMEN

SNODGRASS 3I fibers, apparently intersegmental muscles, are attached nearthe base of the gonapophysis and evidently act as protractors and retractors of thelatter.

An

exact studyof the

gonopod

musculature could not be

made

on the material at hand, but there is

no

doubt that each gonopod, as indicated

somewhat

diagrammatically at

F

^{of figure 6.}

ismovable^asawhole bythetergalmusclesinsertedonitsbase

(LB).

The

gonapophysis, onthe otherhand, ismovable individually onthe basis

by

thegroup of short fibers

(gmd)

^arising in the latter, and inserted within its proximal end.

The

stylus (.S"^v), being set in a

membranous

socketonthedistalmarginof thebasis,is freelymovable by its long muscles (smcl) arising on the proximal margin of the basalplate. In

Thermobia

the gonapophysisof the ninth segmentis attacheddirectlytoa smallscleritelyinginanotchofthebase of the large stylus-bearingplate,but the small scleriteis evidently a secon- darysubdivison of the larger one.

The

postgenital segments of the

Thysanura

are

more

generalized in

some members

ofthegroup thanthey areinanyof the Pterygota.

The

tenth segmentis a complete annulus (fig.6 A,

X),

but it lacks appendagesinpostembryonicstages,as itdoes inallthe

more

gener- alizedPterygota.

The

small eleventh segment (A,

XI)

isnormally concealed within the tenthsegment,but it bears thecerci (Cer)

and

thelongmedian caudal filament (cf). In Ncsomachilisthe

body

of the eleventh segment,

when removed from

the tenth (D), is seen toconsistof a dorsal arch

(IXT)

produced intothe caudal filament (cf), of

two

lateral lobes

(LB),

evidently the bases of the cereal appendages (Cer) whichthey support,andofanarrow,

membranous

venter bearing the

two

paraproctial lobes.

The

cerci appearto have no muscles arisingin their

own

bases,but the tenthsegmentisfilled with a

mass

of intersegmental fibers (D,xmcls) attached posteriorly onthe anteriormarginof the eleventh segment.

Some

^ofthesefibers, insertedatthebases of thecerci,serveto

move

thecerci.

The

twelfth segment, or periproct, is practically obliterated,but in

some

species of

Thysanura

thereis asmall, fleshy dorsal lobebeneaththebase of the caudal filament, possibly a remnant of the lamina supra-analis of the telson.

Inorder betterto

compare

thestructureof the pterygota ovipositor withthat of the ovipositor ofThysanura,

we may

briefly

summarize

the principal features of the simplethysanuranovipositor as follows

:

I.

The

thysanuranovipositor consists of

two

pairsof processes (the firstand second gonapophyses), and of

two

pairs of basal platesbe- longingtothe eighthandninthabdominalsegments,respectively.

32

2.

The

gonapophysesare continuouswiththemesal proximalangles of the basalplates, and each^isprovidedwith a short muscle arising onthe supportingplate.

3.

The

basal platesof the ovipositor are evidently the bases of the appendages (gonopods) of the eighthandninthabdominalsegments.

Each

bears agonapophysis proximally

and

a stylus distally,

and

is

equipped with dorsal muscles arising onthe tergum of its segment.

4.

The

styliof thethysanuran gonopods donot

form

a part of the ovippsitof,but servetosupport theendof theabdomen, andareactive in thelocomotion of theinsect.

Each

ismovableby muscles arising proximally inthe basalplateon whichit isborne.

IV.

GENERAL STRUCTURE OF THE PTERYGOTE OVIPOSITOR AND ASSOCIATED ORGANS

There appearstobelittle doubtthat the pterygote ovipositoris an organ

formed

of the appendages of the eighth andninth abdominal segments, and that it is, therefore, homologous with the ovipositor of the Thysanura. If, in the embryonic development, the blades of the pterygote ovipositor appear to arise directly

from

the ventral walls of the^genitalsegments, ^itis probable that the apparent venter ofeach segmentincludes the areas of the limb basesandthe sternum.

In thedefinitivecondition the limb bases arealwaysdistinctly dififer- entiated

from

thesterna. Associatedwith thebase of the ovipositor are typically three internal organs

formed

by median invaginations of the ectoderm.

The

^first is the

median

oviduct, the second isthe spermatheca, the third is a pair of accessory glands. ^Finally, there

may

be

formed

cavities at the base of the ovipositor, covered by a posterior extension of the eighth sternum, and of the seventh sternum. These cavities are the genital cliamher above the eighth sternum, andthevestihulum abovethe seventh sternum.