DIPLURA AND THVSANURA

70

SMITHSONIAN MISCELLANEOUS

COLLECTIONS VOL. 95

NO. 14 INSECT

ABDOMEN

SNODGRASS 71 each

may

beardistallyastylus (Sty),andin

some

species,

mesad

of the latter, an eversible vesicle (Vs).

The

coxal plates are present alsoon thegenital segments (F,G), but the sternal plate is absent onthe ninth segment of the male

(G)

and on both genitalsegments of the female. Styliarecharacteristically presenton thecoxopodites

Cer

I'lG.24.

—

Hexapoda-Diplura and Thysanura: .structure of the abdominal sternaand externalgenitalia.

A, Ncso)iiachilis inaaricits.ventral view of an abdominal segment, showing sternal surface formedof a small primary sternal plate (Shi) andtwo large coxopodite plates (Cxpd) bearing appendicular organs. B, same, dorsal view of right half of sternal plates, showing muscles of vesicula and stylus. C, Hctcrojapyxgallardi,ventralviewofan abdominalsegment,showingcoxopodites incorporatedinto thesternum. D, same, dorsal surfaceofright half ofsternum, showing musclesof stylus. E, Ncsomachilismaorkus,posterior part of male abdomen, ventralview, showingpenis arisingbetweenbases of coxopoditesof ninth segment. F, MachUis variabilis, ventral plate ofeighth abdominal seg- ment ofmale,with pair ofsmall firstgonapophyses (iGoii), dorsal view. G, same,appendagesandpenisofninthsegment, with secondgonapophyses (2Gon) arisingfrombasesofcoxopoditesatsides of penis.

Apt, sternal apotome; Cer, cercus; cf, caudal filament; Cxpd,coxopodite;

iGon.firstgonapophysis;2Gon,secondgonapophysis;Papt, paraproct;rvs,re- tractormusclesofvesicula;sa,supra-anal lobe;smcls,stylusmuscles; Stn,pri- marysternal plate; Sty,stylus; T,tergum; V, venter; Vs, vesicula; VII^I-XI, abdominalsegments.

of thegenital segmentsof Thysanura,but they

may

be absentonthe other segments; they never occur on the tenth segment, which also has no sclerotic plates in itsventer (E,

X.V). The

abdominal styli of

Thysanura

aremovable by musclesarisingonthecoxopoditeplates (B, F, G, smcls), but the similar processes onthe thoraciccoxae of

MachUis

lack muscles, asdoalsothe abdominal styli of Orthoptera.

y2.

SMITHSONIAN MISCELLANEOUS

COLLECTIONS VOL. 95 Rudiments of appendages have been

shown

by

Heymons

(1897) tobe presentoneach of the firstlosegments of the

abdomen

inthe

young embryo

of

Lcpisma

saccJmrina.

With

the dorsal growth of the embryonic walls, however, the appendage rudiments

become

gradually flattened, until finally they

form

the lateral parts of the definitive abdominal sterna.

The

styli of Lepisma,

Heymons

says, are notdevelopeduntilalong time after hatching, but

when

they do appear theyarise asoutgrowths

from

the parts of the ventral body wallderived

from

theembryonic appendages.

Inthe Diplura the styliare small if present (fig. 24

C)

and are borne bythe sternal plates of the segments; but the stylus-bearing areas

(Cxpd)

of eachsternum

may

be

demarked from

thetrue ster- nal area (Stn),and

upon them

arise the muscles of the styli (D).

From

thiscondition it is then onlyanother steptothat inwhichthe ventral sclerotization of a segment becomes unified in a definitive sternal plate showing no evidence of its coxosternal origin, except forthe possible retention of the ^styli.

On

the genital segments of

some

species of

Thysanura

a slender process arises at or on the inner dorsal angle of the base of each coxal plate (fig. 24F, G, iGon,

2Gon).

These four processes

may

betermed gonapophysesbecause those of the female, which

form

an ovipositor in Thysanura, are without doubt homologous with the ovipositorblades so

named

inotherfemaleinsects.

The

gonapophyses of the ninth segtnent in male

Thysanura

are closely associated with the penis (G,

sGon)

andare often termed parameres because they are supposed to correspond with accessory genital structures called parameres in other male insects (see

Heymons,

_1897).

The

term

"paramere^", however, has been given to

many

different processes of the ^genital complexin pterygoteinsects, anditis not certain that any of

them

is a true gonapophysis.

Heymons

has

shown

that the genital processes of

Lcpisma

are formed as outgrowths

from

the inner marginsof the coxal plates of the eighth andninthabdominal segments. "Gonapophyses"

may

be defined, therefore, as mesal processes of the bases of thegonopods;theywould appeartobe coxal enditesspeciallydeveloped ontheappendagesof thegenital segments.

In the

Thysanura

eachgonapophysis is provided withsmall muscles arising onthe supporting coxal^plate (F).

The

intromittent organ of male

Thysanura

consists of a median tubularpenis,orphallus,arising

from

theventer of the ninthabdomi- nal segment between the bases of the coxal plates of this segment

(fig. 24, E, G, Pen),

where

it is closely embraced by the second gonapophyses ifthese processes are present (G).

The

^{organ appears}

NO. 14 INSECT

ABDOMEN —

^SNODGRASS 73 tobemerelya tubular evagination of thebodywallwiththeopening of the ejaculatory ductonitsextremity. It

may

bedifferentiatedby a circular fold into a proximal part (phallobase) and a distal part (aedeagus).

According to

Heymons

(1897) the embryonic gonoducts of

Lepisma

saccharina extend first to the tenth abdominal segment in the male,andtotheseventhinthefemale. Ineach case the ductsend with ampullar enlargements.

With

the later reduction of the tenth segment during embryonic developmenttheampullae of themaleare transposedtotheninthsegmentand

become

attachedtotheectoderm atthe posteriormarginof thissegment.

Here

an ectodermal ingrowth takes place betweentheampullae,inwhichlater is formed aninvagi- nation that, unitingwiththe ampullae, becomes thedefinitiveductus ejaculatorius. It isthus evidentthatmale

Thysanura must

havehad primarily paired genital openings on the tenth abdominal segment, andthattheseprimitivegonoporessecondarilymigrated forward and approximated each other at the posterior edge of the venter of the ninth segment.

Here

theyacquired a

common

outlet bytheingrowth of a median ectodermal tube.

The

definitive ejaculatory duct, there- fore, is a ductus communis, and not the product of a union of the ends of the primaryducts.

The common

genital duct of the female,

Heymons

says, issimilarly formed byamedianectodermal invagina- tion on the eighth abdominal segment, that is, on the first somite behindtheoneon which thelateral ducts primarilyopened.