GENERAL PLAN OF THE ABDOMINAL MUSCULATURE

A

review of the literature cited abovegives a fairly comprehensive surveyof theabdominal musculatureofinsects in

most

of the principal orders.

There

are notable blanks, however, since such important orders as

Neuroptera and

Trichoptera are omitted entirely,

and

adult Lepidoptera

and

Diptera

have

been given scant attention.

On

the other

hand

it is gratifying to find that

we

have, as a basis for acomparative study of insect myology, very fullaccounts of the

body

musculature of the Odonata,

Ephemerida, and

Orthoptera. In the Apterygota,

we

are indebted to Berlese for an excellent study of the muscles in Protura, to

Lubbock

for a description of the collembolan musculature,

and

to Grassi for brief descriptions of the characteristic differences in the nuisculature of representative genera of Dicellura

and

Thysanura, to

which

is

added

^{in this} paper

an

account of the abdominal muscles of

Heterojapyx

; but a

more

complete studyof the musculature of Machilidae

and

Lepismatidae,

and

perhaps of

Cam-

podea, is still to be desired.

When we

look to the papers treating ofholometabolouslarvae,

we

find againsatisfactory

and

in

some

cases complete accountsof the

body

musculatureinColeoptera,Lepidoptera,

Hymenoptera, and

Diptera,but notewithregret a lack of information on

Neuroptera and

Trichoptera.

To

present here even a

summary

of the details

known

concern- ing the abdominal muscles of insects

would

occupy an

unwarranted amount

of space.

A

^{careful review of the} facts to be obtained

from

the

works

above listed, however,

shows

that

we may

with confidence

make

certain broad generalizations concerning the fundamental plan of the abdominal musculature of adult pterygote insects.

The

basic plan is

found

to be simple; but, as so often occurs in insect

mor-

phology,

more

difficulties are encountered in finding suitable terms to express the facts than in discovering the facts themselves.

Voss

₍₁₉₀₅₎ classified the abdominal muscles as longitudinal

mus-

cles, transverse muscles,

and

lateral muscles (Flankenmuskeln). This classification is logical

inasmuch

as it probably

conforms

with the primitive

arrangement

of the fibers.

The

muscles of the so-called longitudinal groups, however, do not always preserve a lengthwise arrangement; they are often strongly obliciue,

and some

of

them

frequently take a transverse position.

The

lateral muscles are desig- nated ^"dorsoventral^" muscles

by many

writers, but,

though

their attachments are usually dorsal

and

ventral,

some

of their fibers

com-

monly

run in an oblicjue direction.

The

lateral muscles have also been

termed

"transverse" muscles, but, as Samtleben (1929) ^points

32

SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. 85 out, only the crosswise dorsal

and

ventral muscles are literally trans- verse in position. Again, the

body

muscles are sometimes classed as dorsal, ventral,

and

lateral muscles, the dorsals

and

ventrals

com-

prising longitudinal, oblique,

and

transverse fibers,

and

the laterals including dorsoventral

and

oblique fibers. This classification is evi- dently the

most

nearly consistent one that can be devised,

and

it has the

added

merit of being in conformity with the

embryonic

develop-

ment

of the muscles. Unfortunately, however, in

naming

the secon- dary muscle groups or individual muscles according to it. the plan brings out such termsas ^"

median

longitudinal dorsal muscles," ^" ex- ternal

median

longitudinal dorsalmuscles," or^" secondinternal

median

Fig. 13.

—

Diagrams of the nK)rc simple types of segmental musculature.

A, simple condition of musculature in right half of a segment, with dorsal (d) and ventral (v) fibers attached intersegmentally, external laterals (le) intrasegmentally, and internal laterals (li) on intersegmental folds.

B, upper ends of internal lateral muscles (/;) migrated posteriorly, separating a paratergal muscle band (p) from the rest of the dorsal muscles {d).

C, cross section of segment, showing relations of principal groups of muscles.

dl, lateral dorsal muscles; dm, median dorsals; le, external laterals; //, internal laterals; p, paratergal muscle; td, dorsal transverse muscles; tv, ventral trans- verse muscles; ?', ventrals.

longitudinal dorsal muscle." Still

more

unwieldy do such terms 1)e-

come when

put into Latin form.

It is evidentthat strictanatomical

and

nomenclatural consistency in dealing with the

body

musculature leads to impractical results.

The

writer, therefore, has adopted a classification

and

nomenclature that recognizes the anatomical

arrangement

of the muscles, but which,

inorder toshorten thenames, errs

somewhat on

the side of specificity in terminology. Five principal groups of muscles are distinguished

and

designated as follows: I.

dorsal muscles

(fig. 13A, d), the fibers of

which

aretypically longitudinal

and

primarily intersegmental in their attachments. II.

ventral muscles

(v), resembling the dorsals in that their fibers are typically longitudinal

and

attached primarily on the intersegmental lines. III.

lateral muscles

(/),

NO. 6 INSECT

ABDOMEN — ^SNODCRASS

33 typically dorso-ventral, their fibers both intersegmental

and

intraseg- niental. I\'.

transverse muscles

(C, t), lying internal tothe longi- tudinals, including dorsal transverse inuselcs (td),

and

ventral trans- versemuscles (tv). \.

spiracular muscles

{s). generally not

more

tha.n

two

connected with each spiracle,one an ocelusor (os), the other a dilator (dls).

All the

body

muscles are bilateral in their origin,

and

all of

them

except the ventral transverse muscles remain so in the definitive state.

The

fibers of the ventral transverse muscles,

which

primarily Are intersegmental,

Heymons

(1895) ^{says are at first}attached mesally on a foldof the l)odywall betweenthe nervecords. Later the foldis sup- pressed

and

the nl)ers

from

opposite sides

become

continuous across the sternal region.

The

fibers of the longitudinal dorsal

and

ventral muscles are always separated into symmetrical lateral groups along the midline of the

dorsum and

venter, but the ^{lateral sets} of dorsal transverse muscles

come

togetheron the ventral wall of theheart.

Each

i)rimar\- grou]^of muscles

may undergo

an endless diversifica- tion resulting both

from

a multiplication of fibers in the group,

and from

a rearrangement of the fibers brought about by changes in the points of attachment.

With

respect to the dorsal

and

\entral muscles, the

most

general departure

from

the sim])le plan, in

which

the fibers all lie in a single i:)lane, consists of a difi'erentiation of the fibers in each grou]) into internal inuselcs

and

esternal muscles.

Thus

it is

found in nearly all insects that the dorsal

and

ventral muscles

com-

prise each

two

layers of fibers, namely, internal dorsals (di)

and

external dorsals (de),

and

internalventrals {vi)

and extenml

z'cntrals (ve).

A

second

form

of diversification afifecting the dorsal

and

^ven-

tral muscles is a

more

or less distinct grouping of the fibers into

median and

^{lateral muscles. In}

most

insects,therefore,

we may

distin- guish four sets of dorsal fibers,

and

correspondingly four sets of ventralfibers.

The

several resultingmuscles

may

thenbe distinguished asmedian

and

lateral internaldorsals (fig. 14A, B, dim. dil).

median

and lateral external dorsals {dem. del),

median and

^lateral internal ventrals {vim, vii),

and median and

^lateral external ventrals (z>em, vel).

The

lateral muscles are

more

sul)ject to irregularities than are the dorsal

and

ventral muscles, but they likewise are often divided into internal laterals (fig. 13 B, C, ^//')

and

external laterals [le).

Associated withthe dorsoventral lateral muscles there is sometimes present a longitudinal muscle or

group

of longitudinal fibers lying external to the upper ends of the internal laterals (fig. 13, p). This muscle is called the ^"epipleural" muscle by

Ford

(1923), ^{but since}