PARASITES WITHOUT A PLANIDIAL STAGE

NO. 9

INSECT METAMORPHOSIS — SNODGRASS

₇₅

spiracles,

and

a

few

longitudinal muscles

have

been developed.

The

caudal diverticula of the first instar have lengthened into a pair of tails nearly half the length of the body,

and

fine tracheal branches laterpenetrateinto their

open

basalparts. In the third stage the

body

preserves the general

form and

structure of the second stage, but thetails have increased greatly in length, being one

and

a half times or

more

the length of the body. In its fourth stage

(C)

the larva

Fig. 14.

—

^Threelarval stages ofanagromyziddipteron,Cryptochaetumiceryae (Will.), parasitic in the coccid Icerya purchasi Maskell (from Thorpe, 1931).

A, firstinstar. B, second instar. C, fourthinstar.

becomes

an ovoid, yellowish-white

maggot composed

of a

head and

10

body

segments; the tailsare greatly lengthened, slender filaments, but

have become

brittle

and

are easily broken.

Each body segment

has abelt of

minute

spines

around

its anterior end, anterior

and

pos- terior spiracles are

now

present, but the hooklike posterior spiracles are still closed, the alimentary canal is open, the muscular system is fullydeveloped.

In

Cryptochaetum

striatum (Thorpe, 1941) ^{the larval stages are} said to be

much

the

same

as in C. iceryae, but in the third stage the respiratory tails are 10 times the length of the

body and

^are filled for at leasttwo-thirds oftheir lengthwith fine tracheal branches.

In contemplating a larva of such incomplete structure as that of

NO. 9

INSECT METAMORPHOSIS — SNODGRASS

_Jj thefirst instar of Cryptochaetum, thequestion

comes up

as to

how

it

got that way.

The

usual

answer

to the question is that the

embryo

hatched at an early

immature

stage. Concerning the "early hatching"

idea,

Thorpe

(1931) ^says:

"The

theory obviously cannot be

pushed

too far, for there are

many

truly adaptive characters

which

arise de

novo

ⁱⁿ insect larvae,

and

cannot in

any way

be described as

embry-

onic."

There

is

no

question that "adaptive characters"

may

include the suppression of structures that are temporarily useless, as well as the development of

new

structures that are only temporarily useful.

Nature

is always economical

where

there is

no

need of prodigality.

A

^larvaliving inthemidst ofliquid food

which

it absorbsthrough its

skin has

no

use for a mouth, feeding organs, or a functional alimen- tary canal,

and no

need of a locomotor muscular system. If also it

can get sufficient

oxygen

by absorption

from

the

medium

in

which

it lives, there is

no

immediate need of a tracheal system. All these negative conditions might be supposed to have been acquired by the simple expedient of early hatching, but the larva, if so produced, is

not a

normal

early-stage embryo.

The

retarded state of development very probably

was

early determined in the egg,

and

the larva

must

then be

what

it is regardless of

when

^{it hatches.}

The

principal

new

structures of the

Cryptochaetum

larva,

Thorpe

points out, are the respiratory tails. Otherwise the larva simply develops the other or- gans

when

they are needed.

The

^delay in development is a

mere

economy,

and numerous

examples of various degrees of

economy

might becited

from

other species.

Hymenoptera. —

^Inthe

Hymenoptera

endoparasiticfirst-stagelarvae often have such strange

forms

that they

would

hardly be

known

for

young

insects if their development

had

not been followed. Clausen (1940) distinguishes, describes,

and

illustrates 14 different types of first-stage parasitic larvae in the

Hymenoptera,

nearly all of

which

but the planidium are endoparasitic.

The

eggs of

some

species are deposited

on

the outside of the host, of others in the

body

cavity of the host, and of still others in the host egg.

The

so-called

"egg

para- sites," however, Clausen observes, are truly larval parasites, since they feed

on

the larva

and

"their development is primarily at the expense of that stage." In the present discussion

we

are concerned entirelywith the

forms

of these first-instar larvae,

which

later take

on

the

more

conservative structure of typical

hymenopterous

grubs.

The

species are therefore heteromorphic,

though

their

heteromorphism

affects principally the first instar.

The

change tothe final

form may

take place at the first moult, but often the second instar is inter- mediate in

form

between the first

and

the followinginstars.

As

with

the parasitic larvae of Diptera, these aberrant

hymenopterous

para- sitespresent various special developments in combination with differ- ent degrees of

undevelopment

of usual organs.

Whatever

their

form

or structure

may

be, however,

we must assume

that in

some way

it is fitting tothe life these larvae live.

As

an

example

of greatly simplified

and

specialized first-stage larvalstructure inthe

Hymenoptera we may

take thebraconid Helori-

morpha

antestiae,

an

internal parasite of thepentatomid Antestia, de- scribed

by

Kirkpatrick (1937),or the similar larva of the

ichneumonid Limnerium

validum, endoparasitic inthe fall

webworm,

described

by Timberlake

_(1912). In each of these species the first-instar larva (fig. 15

C)

has an

enormous "head" on

a relatively small, simple, un-

segmented body

ending in a long tapering tail.

The

only appendages present are a pair of slender, incurved, sharp-pointed mandibles.

An

even simpler larva of the

same

type ^is that of Platygaster marchali (E).

Inthe secondstage the

Limnerium

larva takes

on

a

vermiform

type of structure witha small

head and

12

body

segments, the tail of the first instar being greatly shortened.

The

third instar, as also that of

Helorimorpha

antestiae (fig. 15

D),

is a typical

hymenopterous

larva.

The

heteromorphosis of these species, therefore, results

from

the

extreme

modification of thefirst instar^; in its subsequent changes the larva merely returns to the usual form.

A somewhat more

specialized type of first-instar larva occurs

among

the Platygasteridae,

examples

of

which

are here illustrated at

A,B,F, and

I of figure 15.

The

largeanteriorpart of the

body

carries the mandibles, antennal rudiments,

and

a pair of simple posterior appendages. This headlike part of the larva has been

shown

by

Marchal

(1906) ^to be a cephalothorax bearing the antennae,

mouth

parts,

and

the prothoracic legs.

The body

region behind the cepha- lothorax is partly or entirely segmented,

and may end

with tail ap- pendagesof variouspatterns. In their developmentthese larvae even- tually attain the

form and

structure of an ordinary

hymenopterous

grub.

A

curious type of first instar larva is characteristic of the Sceli- onida; it is classed

by

^Clausen (1940) as the "teleaform" type of larva, but in

form

it suggests the

embryo

of a

mouse

(fig. 15 J).

Hadronotus

ajax, an egg parasite of the squash bug,

Anasa

tristis,

furnishes a

good

example.

The newly

hatched larva (J) as described

by

Schell (1943) ^is a slender creature with a sharp, tail-like caudal horn curved anteriorly.

The body

is constricted between a large an- terior part, probably a cephalothorax,

and

an elongate posterior part,