NO. 9 INSECT METAMORPHOSIS — SNODGRASS 49 The endopterous condition of the larva and the substitution of

short-sighted simple eyes for long-sighted

compound

eyes

were

con- ceived by

Lameere

(1899) tohave arisen as adaptations ina primary nymphlikejuvenile

form

toboringintoplantstems.

The

theory,

how-

ever,does not take into consideration the facts that

most

present-day larvae of the boring type are specialized

forms

in their

own

orders,

and

that free-living

forms

give

no

evidence of having been recon- structed for life inthe

open from

a primary boring type of larva. It is hard to believe, for example, that the antecedents of the aquatic Corydalas larva or the Dytiscus larva, or even those of terrestrial beetle larvaelived in plant stems.

As

forthechange ofeyes, it

would seem

that aboring larva

would

hardly need any eyes at all.

Though

the

Lameere

theory of larval origin is thus not convincing, it is the only theory that has been proposed to account specifically for the characteristicexternal features of

modern

endopterygotelarvae.

We

can readily imaginethat the suppressionof external

wing

pads duringthenonfunctionalperiod of theirdevelopment

would

be acon- venience to

most any young

insect regardless of its habitat.

Wing-

lesslarvae, by comparison with

winged nymphs,

have certainly

shown

a great superiority in ability toadapt themselves to different environ-

ments and

to different

ways

of living.

A

theory concerning the nature of the endopterygote larva, elabo- rated by Jeschikov (1929), regardsthe larva as a free-livingcontinu- ation of the

embryo;

the larva has even been defined as such

(Hen-

derson, 1949)^{. First,}

we might

ask,

what

animal is not a continuation of the

embryo? The

theory of Jeschikov, however, contends that the larva is

an

embryo,

and

that the

nymphal

stages of its ancestors are all condensed in the pupa.

However,

in

no

^other insects are the wings developed inthe embryo, at

most

they are represented only

by

differentiated groups of cells in the

embryonic

epidermis.

The ame-

tabolous

and

hemimetabolous Pterygota all

show

that

wing

develop-

ment

is a function of postembryonic life. Periodic moulting is

com- mon

to both

nymphs and

larvae, but it

would

be quite exceptional in an embryo. If the larva is

an

embryo, cases of paedogenesis

would

really be embryogenesis,

and

larvalheteromorphosis

would

be

embry-

onic heteromorphosis;

some embryos would

take to the water

on

hatching, others

would burrow

into the ground, still others

would

climbtrees,

and

finally

we

shouldhave

embryos

spinning cocoons

and

transforming into pupae.

These

implications are rather too

much

for the theory.

When

the

embryo comes

out of the egg

and

takes on

allthe functions necessary for a freelife,its embryonicstageisended,

though

of course

what we now

call it is merely a matter of conven- tional definition.

The

endopterous condition of the larva very probably

was

not pro-

duced by

asinglemutation. Inthesimplest typeof

wing

development

among modern

endopterygote insects, as

shown by Tower

(1903) in certain Coleoptera, the

wing

is first

formed

in the early

pupa

beneath the cuticle of the last larval instar,

and

is therefore exposed only at the

moult

to the pupa. If

formed

as a fold of the

body

wall at

any

earlier stage the

wing

rudiment

would

be exposed at the next larval moult.

The

first appearance of

wing

pads

among

exopterygote in- sects

on

different instars

shows

thatthe

wing growth may

be retarded.

In the past history of those beetles in

which

the

wing

is not present as a fold until the early pupa, the external

growth

of the

wing must have

been first retarded

and

then suppressed until the

end

of larval life,

and we may

conclude, therefore, that the first step in attaining the endopterous condition

was

a retardation in the time of develop-

ment

of the

wing

rudiment.

The

formation of a

wing

fold is not the true beginning of the

wing

development; in earlier larval stages the alar rudiment is present in the

form

of a thickening or a differenti- ated

group

of cells in the epidermis,

which

is the

wing

in a state of suppressedgrowth.

On

the other hand, in

most

of the endopterygote insects the de- velopment of the

wings

has been expedited

by

the early recession of the

growing wing

rudiments into pockets of the epidermis beneath thecuticle,

which become

closed

and

are thusnotaffectedby thelarval moults.

Within

these pockets the

wings

can

grow

without being ex- posed until they are everted at the moult to the pupa.

According

to

Tower

(1903) ^the

wings

develop in this

manner among

the Coleop- terainScarabaeidae, Coccinellidae,

and

Chrysomelidae; Patay (1939) says the

wings

of Leptinotarsa develop in closed pockets

toward

the

end

of the third instar.

A

familiar

example

of the usual recessedtype of

wing

development beginningin thesecond larval instaristhatgiven

by Mercer

(1900) for Pieris rapae.

The

endopterous condition in itsevolution, therefore, has probably

gone

through

two

phases, both existing

among modern

insects. In the first phase the

growth

of the

wings presumably was

suppressed until the

end

of the juvenile period; in the second phase the

wing

rudiments developed again at

an

earlylarval stage, but

now

sank into the epidermis beneath the cuticle, thus still preservingthe "wingless"

state of the

young

insect. It

must

be evident, then, that there is

no

truly wingless larva of

any winged

insect; the

wings

exist in

some

NO.

9 INSECT METAMORPHOSIS — SNODGRASS

51 retarded stage of growth.

The

endopterygote larva, therefore, does not represent

an

apterous stage of ontogeny,

and much

less does it

recapitulatean apterous stageof phylogeny.

Similarly, legless larvae are not truly apodous; the leg rudiments are present in

some

form, though they

may

be greatly reduced. In the

honey

bee, for example, Nelson (1915) has

shown

that external leg rudimentsare present on the embryo, butat the time of hatching are reducedto discs inthe epidermis,

which

later redevelop internally inthelarva.

The

leg tissue,therefore, is continuously present,

though

it

may

not take the

form

ofaleg

bud

until late in larval^life.

The

suppression of

compound

eyes during larval life, unlike the suppression of

wing

pads,

would

not

seem

to confer

any

advantage

on

afree-living

young

insect.

The

typical larval eyes are simple single eyes, usually only a

few

in a

group on

each side of the head.

They

are developed

on

the site of the future

compound

eyes

and

are con- nected with the

same

part of the brain; but generally at the

end

of larval lifethe larval eyes degenerate,

and

they never take

any

part in the formation of the definitive

compound

eyes. In the Culicidae

and

related Diptera it is

shown by

Constantineanu (1930) ^{that the}

com- pound

eyes begin their development in an early stage of the larva,

and

thatthelarvaleyes,

which

are

formed

intheembryo, are retained in the adult.

Yet

the

two

remain as entirely distinct organs. In the larva of

Panorpa

there are

30

^to 35 ^{single eyes in a}

group on

each side of the head, and, as described

by

Bierbrodt (1942), ^these pan- orpid larval eyeshave attained the structure of ommatidia,

and

prob- ably function as appositional

compound

eyes.

However,

the larval eyes

and

their nerves degenerate during thepupal

metamorphosis and do

not

become

the

compound

eyes of theadult.

Here

is a case,there- fore, in

which

a larva has succeeded in reacquiring functional

com- pound

eyes, but^theselarval eyes, as those of other insects, give place to adult

compound

eyes

newly

developedinthe pupa.

In discussing the origin

and

^{evolution of} endopterygote larvae,

Chen

(1946) contends ^{that the}

primary

larva, derived

from

an exop- terous

nymph, was

aquatic,

and

he cites the megalopterous larvae, particularlythelarvaof Corydalus,as beingthe closest

modern

repre- sentative of the primary larva.

Though

^it

may

be conceded that the megalopterous larvae are relatively generalized

modern

forms, they are nevertheless superficially modified for aquatic life,

and

life in the water doesnot accountfor their

more

fundamental characters,

which

are those of endopterygote larvae in general.

The

^{stonefly, mayfly,}

and

dragonfly larvae areallaquatic,

and

yet they have

compound

eyes

and

external

wing

pads,

and

they transform without a pupal stage.

52

SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. 122