NO. 9 INSECT METAMORPHOSIS — SNODGRASS 31 In various lesser ways the mayfly larva may be characterized by

NO. 9 INSECT

METAMORPHOSIS — ^SNODGRASS

31

32

SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. 122 alimentary tract, however, are fully preserved, but for the purpose of swallowing

and

retaining air.

The stomach

is

shown by

Pickles (1931)

and by Grandi

(1950) to be transformed in the adult into a thin-walled air sack.

The

airprobably serves to

make

the

body more

buoyant

and by

compression to expel the eggs.

Most

mayflies

undergo

a moult after they

have

attained the state of a

winged

imago, the adult stage being thus subdivided into

two winged

stages, distinguished as the

subimago and

the imago.

Con-

cerning the

subimago

^of Cloeon

diptemm, La Baume

₍₁₉₀₉₎ says that it usually issues

from

the larval skin

toward

evening either

on

the surfaceof thewater or

on

the shore.

The

quickness of thechange

is

most

noticeable, particularly the almost instantaneous spreading of the wings.

The

insect

now

^flies to vegetation along the shore,

where

it remains quiet until thenext moult, which, according to the species,

may

occur in a

few

minutes, a

few

hours, or several days.

There

is

probably

no

^specific reason

why

theadultmayfly should

moult

again

it isthe only

winged

insect

known

to moult in the active adult stage,

and

even

some

mayfly species omit a second moult. Evidently the imaginal moultis simply a holdover by a primitive insect

from

wing- less ancestors that shed the cuticle periodically throughout life as

do

the

Thysanura and most

other wingless arthropods. Extraction of the

wings from

the old cuticle is a difficult matter

and

other insects

have

simply discarded auseless

and

dangerous habit.

It is clearthat the mayfly undergoes a ^greater degree of

metamor-

phosisthan doesthe stoneflybecausethe

young

mayflyis

more

exten- sively modified in adaptation to life in the water.

Inasmuch

as the larva in the

two

cases is differently modified for the

same

purpose,

metamorphosis

hasarisenindependently inthe

two

groups.

VI.

ODONATA

The Odonata

present

an example

of

metamorphosis much more

accentuated than that of either the Plecoptera orthe Ephemeroptera,

and

there is

no

relation

between

the special characters of the odonate larva

and

those of the other

two

groups, again

showing

that larval structures in adaptation to aquatic life

have

been independently de- veloped in these three orders. In

common

with other aquatic larvae, the odonate larva has been adapted in its

body form and

its

means

of respiration to life in the water, but in addition it has evolved a very specialmodification of the labium by

which

this organ is greatly enlarged

and

converted into

an

^{efficient device for the} capture of active prey.

no. 9 INSECT

METAMORPHOSIS — SNODGRASS

₃₃

The Odonata

are predaceous both as larvae

and

as adults^; their

mouth

parts are of the biting type of structure.

The

adults entrap their insect preyon the

wing

by

means

of their hairy legs,

and

their

mouth

parts arenot unusuallymodified.

The

short

body

of the adult

Prmt-K

A ^{rPm ^ B}

Pip Lig*

iLb

Fig. 4.

—

Odonata; development and metamorphosis of the labium.

A,

Anax

Junius Drury, labium of 17-day embryo (from Butler, 1904). B, same, 20-day embryo (from Butler, 1904). C, Sympetnimstriolatum (Charp.), labiumofpronymphcontaininglabiumofsecondinstar (fromCorbet, 1951). D, same, free labium of second ^instar, expanding (from Corbet, 1951). E, same, fully expanded labium of second instar (from Corbet, 1951). F,

Anax

sp., labium of mature larva, posterior. G, same, larval labium and early stage of formationofimaginal labium in theprementum. H, same,later stage, theimag-

inal labium retracted into postmentum of larva and takingon the adult struc- ture,posterior. I,same, imaginal labium from

H

^unrolledandspreadout.

iLb, imaginal labium; Lig, ligula; Pip, labial palpus; Plpg, palpiger; Pmt, postmentum; Prmt, prementum.

labium (fig. 4I) consists of adistinct

postmentum and

a

prementum

;

the

prementum

bears a large

median

ligular lobe,

and two

small lateral lobes {Plpg) that support the short, thick palpi {Pip).

The

larval labium is

more

simple in

form

than that of the adult, but both the

postmentum and

the

prementum

are greatly elongated,

and

are articu- lated

on

each other

by

a freely

movable

elbow.

The

larval postmen-

turn is unusualin that, instead of being as in

most

insects a plate

on

the

under

side of the head, it is

produced

into a long, free stalk sup- porting the

prementum on

^{its distal} end.

The prementum

^{is highly}

variable in

form

indifferentgenera; inthe

common

Ana.v Junius

(F)

it is a long flat lobe

somewhat expanded

distally

where

^it bears the relatively small palpi, eachof

which

is

armed

with a long sharp claw.

In the passive position of the labium the

postmentum

is turned pos- teriorly against the

mesosternum

of the thorax (fig. 5

A)

; the pre-

mentum

in

some

species is pressed against the

under

surface of the

head (A),

in others it is applied like a

mask

over the lower part of the face (C). In action the

postmentum

swings

downward and

for-

ward on

^the head, the

prementum

is lowered (B),

and

the entire labium isthenprojected far

beyond

the mandiblesto seize a prospec- tive victim. Associated with the larval labium is a long

T-shaped apodeme

developed

from

the base of the

hypopharynx

that extends posteriorly through the head,

and

the crossbar is

embedded

in the posterior edge of the base of the

postmentum. The

labial muscula- tureis surprisingly simple, butitis probablethat blood pressure

from

the

abdomen

plays an important part inthe projection of the labium.

While

undoubtedlythelarvallabiumis specialized

by

comparison with the adult labium, the labium of the

embryo

develops directly into that of the larva,

and

at

metamorphosis

the adult labium develops within thelarvalorgan.

The

hypopharyngeal

apodeme

is eithergreatly short- enedintheadultorreducedtoa ligamentous band.

The embryonic

labium of

Anax

Junius (fig.

4 A,B),

^as illustrated

by

Butler (1904), has a ^{primitive feature in the} almost complete separation of thestipital lobes of the

prementum (Prmt)

; the unseg-

mented

palpi {Pip) bear fingerlike processes

(A)

that will

become

the apical

hooks

(B). In the

pronymph

of

Sympetrum

(C), accord- ing toCorbet (1951), ^the

prementum

isundivided

and

the palpi arise close together

from

its distal end, but during ecdysis of the second instar

(D)

the

prementum

stretches transversely,

and

^later

(E)

be-

comes more

elongate.

The embryonic

labium thusgoes

from

aprimi- tive labialstructuredirectly intothe specialized structure ofthe larval labium.

The

labium of the adult as described

by Munscheid

₍₁₉₃₃₎

is first

formed

inthe distal part of the larval labium about five days after the larvaceases to feed.

At

first it takes

on

approximately the

form

of thelarvallabium,butlater it

becomes

shorter untilfour days after its formation itoccupies only the basal two-thirds of the larval

postmentum. A

further three days

now

elapses before ecdysis of the imago.

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INSECT METAMORPHOSIS — ^SNODGRASS

35