NO. 9 INSECT METAMORPHOSIS SNODGRASS 79

Fig. 15.

—

^{Examples of} hymenopterous parasitic larvae without a planidial firstinstar.

A, Platygaster herrickii Packard, first instar (from Kulagin, 1898). B, Platygastcr instricator Kulagin, first instar (from Kulagin, 1898). C, Hclori- morpha sp., first instar (from Kirkpatrick, 1937). D, same,maturelarva (from Kirkpatrick, 1937). E, PlatygastermarchaliKieffer, firstinstar (fromMarchal, 1906). F, Synopcas sp., first instar (from Marchal, 1906). G, same, embryo (from Marchal, 1906). H, Phaenoscrphusviator Hal., first instar (from East- ham, 1929). I, Tricacusremulns (Walker), first instar (outlinefrom Marchal, 1906). J, Hadronotus ajax Girault, first instar (from Schell, 1943). K, same, third instar (from Schell, 1943).

NO.

9

^INSECT

METAMORPHOSIS — ^SNODGRASS

^8l

genetic significance.

The

other appendages

have

simply been sup- pressed as needless.

The same may

be said of the so-called "oligomerous"

and

"poly-

merous

protopod" larvae (fig. 15 A,B,F,I).

They

do not as a

whole

have the structure of

any

one stage in ordinary

embryonic

develop- ment,

and none

^of

them

is suggestive of being a primitive embryo.

An embryo

develops continuously, but these larvae maintain the

form and

structurethey have athatchinguntil thefirstmoult, as does

any

ordinary larva. In short, there is

no

reason for regarding

them

as embryos. Just as a free, active, first-stage larva, or planidium, is

adapted to the predatory life it

must

lead, so these internal parasitic larvae are adapted to

an

endoparasiticlife.

They

are specialized both inthe

forms

they^have,

and

inthedevelopmentalretardation of organs they do not have

and

do not need.

The

principle of

economy

is in-

voked

here just as with the simplified dipterous larvae.

In the first-stage

Hadronotus

larva (fig. 15 J)

we

see again an ex-

ample

of early specialization in

form accompanied by

retardation in the development of organs not immediately needed. If

we

consider the

numerous

^other

forms

of first-instar larva

among

the parasitic Platygasteridae

and

Scelionidae, illustrations of

which

are assembled

by

Clausen (1940, ^{figs. 1}08-111, 113), it is clear there is

no

^evident

logic in picking out

any

one

form

as representing a particular stage of ordinary embryonic development.

The

development of

Synopeus

rhanis within theegg

from

theblastula tothefirst larva (F),as illus- trated

by Marchal

(1906, ^pi. 17),

shows

that the

embryo (G)

de- velops directly

from

the beginning into the platygasteridlarval form, without going through

any

stages suggestive of those of an

embryo

that develops into a typical free-living larva. Evidently the larval

form

is determined in the egg,

and

the embryo, thus relieved

from

phylogenetic influences, develops into a larva of the platygaster type.

The

time ofhatching has nothing ^to do withit.

An example

ofheteromorphosis affecting thefirst larval stage very similar tothat intheparasitic

Hymenoptera

occurs inthepseudoscor- pion (Barrois, 1896;

Vachon,

_1938).

The

eggs at

an

early stage of development are discharged into a brood

pouch

suspended below thegenitalaperture of the female

and

are here nourished

on

a secre- tion

from

the ovaries.

On

hatching, the larva breaks through both the chorion

and

the wall of thebrood pouch, but remains attachedto the outside of the latter

by

its ventral surface

and

the

mouth

region.

It is

now

nourished, as

were

the eggs,

by

the ovarial secretion dis- chargedintothebrood pouch.

At

this stage the

young

pseudoscorpion

isa simple saclikecreature with rudimentary appendages, but without

82 SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. 122

body

segmentation or internal organs.

A

deep musculated invagina- tion

on

its ventral surface

was

regraded

by

Barrois as a sucking or- gan, but

Vachon

has questioned this function.

However,

in

some manner

the larva absorbs the ovarial secretion

from

the brood

pouch and

completes its development in one instar.

At

the next moult it

takes

on

at oncethe adult structure in miniature.

The

so-called larva might be regarded as a second embryo, but clearly it is an adaptive

form

quite unlike

any

early stage in ordinary arachnid development.

The

frequency with

which

larval heteromorphosis occurs

among

unrelatedinsects

shows

that the larval organization is highly unstable

and

thatmutations

make

^it readily responsive to theneed of environ- mental adaptation.

A

case of heteromorphosis

among

the vertebrates

would

be

most

astonishing; with the insects heteromorphosis is

com-

monplace.

The

adaptationalchangesinthe structure ofheteromorphic larvae

from

one instar tothe nextis

good

evidencethat

homomorphic

larvae are themselves merely juvenile adaptations to their various

modes

of living.

The

ease with

which

the insect larva

assumes

a

form

compatible with ^its living conditions is well illustrated

by

the difference

between

a free-livingplanidiumof oneparasitic species

and

the endoparasiticfirstlarva ofanother related species.

The

planidium

is equippedforactivity, for finding

and

attacking its prospective host the endoparasiteis reducedtothebareessentialsneededfor feeding

on an

ambient food supply

and

for

mere

existence otherwise. It

may

be noted here, also, that simplification of structure often occurs in the second or following instars, as with species having a planidial first larva, in

which

case "early hatching" cannot be invoked to account for it.

Whatever form

the early larva

may

^{take on, however, it is}

incumbent on

the larva eventually to return to its parental form,

and

this it does by first reverting in its later stages to the larval

form

typicalof its order or family.