PARASITES WITH A PLANIDIAL STAGE

The

term planidium has a functional rather than a structural sig- nificance, but it is remarkable

how

larvae in different orders have taken on similar characters in adaptation to the requirements of planidial life.

Coleoptera.

—

Larval heteromorphosis

among

the Coleoptera occurs in the Carabidae, Staphylinidae, Meloidae,

and

Rhipiphoridae.

Most

no. 9

INSECT METAMORPHOSIS SNODGRASS

₆₃ familiar are the life histories of the blister beetles, Meloidae,

some

of

which

feed on theeggs of grasshoppers, others infest the nests of bees.

The

transformations of species of the

American

Epicauta are well

known from

the early

work

of Riley (1876)

and

later papers

by Ingram and Douglas

₍₁₉₃₂₎

and

Horsfall (1941).

The European

Mylabris variabilis is described

and

fully illustrated by Paoli (1938).

The

adult females of Epicauta

and

Mylabris deposit their eggs in the

ground where

grasshoppers are likely to be, but not necessarily

Fig. 11.

—

^Threelarval stages ofa meloid beetle,Mylabrisvariabilis Pall, (from Paoli, 1938).

A, first instar, planidium. B, same, more ^enlarged. C, second instar. D, fourthinstar, similar to third andfifth instars.

close toa grasshopper'snest.

The

first-stagelarvais aplanidium hav- ing the

form

of an active generalized coleopterous larva with long slender legs (fig.

nA,B).

It is

commonly

termed a "triungulin,"

though

two

of its pretarsal "claws" are merely strong spines.

The

planidium runs actively about

and burrows

into the

ground

^{until it}

finds the egg nest of an acridid. After feeding

on

a

few

eggs, it

moults

and

transforms into a

more

simple short-legged, soft-bodied second instar (C). This larva resumes feeding, grows,

and

goes through

two more

stagesin

which

it

becomes

a thick scarabaeoid grub

(D). The

next instar,

which

is the fifth, resemblesthe preceding, but

it

comes

out of theeggnest

and burrows downward

a short distance into the ground. This larva in Mylabris, according to Paoli, does not feed,

and

transforms into a sixth larva, in

which

the integument is thick, rigid,

and

dark-colored,

and

the mandibles

and

legs are

much

reduced. This sixth larva, the "ipnotica" of Paoli, is immobile

and

passes the winter in a

dormant

condition. In the spring ^it sheds its tough, protective integument.

The

seventhlarva that

emerges

resem- blesthe fourth

and

fifth instars; it is again active,

though

it does not feed,

and burrows upward

to near the surface of the ground,

where

it transforms intothe pupa,

from which

finally the adult emerges.

The

life history of Epicauta is essentially the

same

as that of Mylabris, though according to Horsfall (1941) ^{the fifth larva of} E. pennsylvanica ^feeds to repletion before it

burrows down

into the ground.

Moreover,

^{this fifth larva in species of Epicauta}

may

trans-

form

directly into the pupa, thus eliminating the hibernating

form and

the seventh instar. Pupation following the fifth instar is said

by Ingram and Douglas

(1932) to take place with individuals of Epi- cauta lemniscata that complete the fifth stage in the spring, while those

maturing

in the fall or

under

unfavorable weather conditions

go

over into the hibernating stage,

which

^is followed

by

the active seventh instar. Horsfall (1941) reports the

same

thing for Epicauta pennsylvanica.

A

^{notable feature in the}

metamorphosis

of these meloid larvae is its reversibility, as

shown by

the transformation of the hibernating larva into an active

burrowing form

^like that

which

preceded^it,thoughgeotropicallyoneispositive

and

the other negative.

Other

species of Meloidae are parasites in the nests of bees inthe families Megachilidae

and

Andrenidae.

The

life history

and

larval stages of one of these, Tricrania sanguinipennis, infesting the under-

ground

nests of the andrenid Colletcs rufithorax is fully described

by

^Parker

and Boving

(1925).

The

female of Tricrania deposits her eggs

under

small objects lying

on

the

ground

in the vicinity of the nesting places of the bees.

The newly

hatched larvae are slender planidia, tapering at each end, with long legs

and

well-sclerotized

body

segments. It

would

appear that they

might

find their

way

^di-

rectly into the nest of a prospective host, but observations

show

that generally they attach themselves to a

male

bee seeking a female

and

are thus carriedintothe nest.

Within

thebroodcell the intruder first

devoursthe eggof thebee, thus

making

sure of

no

competition

from

the bee larva that

might

otherwise hatch.

At

^{the first moult the} planidium transforms into a soft,

smooth

larva having a boat-shaped

form

with the spiracles

on

the back,

an

adaptation that enables the larva to float

on

the food

mass

of

honey and

^{pollen in thecell,}

which

constitutes its food

from now

on.

At

later stages, however, as the larva

grows

larger

and

the food

mass

shrinks, the larva

becomes

a fat scarabaeoid grub. In all, the Tricrania larva goes through six

NO. 9 INSECT

METAMORPHOSIS — ^SNODGRASS

65 instars, but, exceptfora shorteningof thelegs

and

other

minor modi-

fications, it

makes

no radical change of

form

after the second moult.

There

is no hibernating larval stage in the life of Tricrania, but the fifth

and

sixth instars remain within the

unbroken

fourth

and

fifth skins,

which

servealso as a covering forthe pupa.

The

adult beetles are

formed

in the fall, but remain within the bee's nest until the followingspring.

The metamorphosis

of the carabid Lebia scapularis,

which

is pre- daceous

on

the larvae of the elm leaf beetle, Galerucclla luteola, is

described

by

Silvestri (1904) as follows.

The young

Lebia is a slen- der, elongate larva of the planidium type, havinglegs adaptedto run- ning, well-developed mandibles,

and

a pair of long, jointed apical processes on the

abdomen.

It attacks a Galerucclla larva

and

^feeds

on

the viscera until its

growing body becomes

so large

and

loaded with fat to such an extent that it can

no

longer

move

actively about.

In this condition it

might

fall an easy prey to other insects, but the Lebia larva

now

encloses itself in a cocoon spun of silk threads

from

the Malpighian tubules,

and

finally includes its prey in the cocoon.

When

finished with feeding,

and

having attained its

maximum

de- velopment, this firstlarva moultsinto a second

form

having a general pale color,

mouth

parts unadapted to feeding, the legs

and

antennae reduced to small conical stumps,

and

the caudal processes suppressed.

From

this second, nonfeeding instar the larva goes into a prepupal stage, in

which

the head takes

on

adult characters,

and wing

rudi-

ments

arepresent

on

boththe

mesothorax and

the metathorax. After another moult the

prepupa becomes

a pupa. Silvestri

makes

no

com- ment on

theunusual occurrence inthe Coleopteraof aprepupa,

which

evidently belongs to the pupal stage of the insect. Heteromorphosis inLebia, therefore,appearsto affectthe

pupa

as well as thelarva.

Heteromorphosis again occurs in several genera of Staphylinidae in

which

the larvae are parasitic

on

the

pupae

of Diptera within the puparium.

Wadsworth

₍₁₉₁₅₎ gives a

good

account of the life his- tory

and

larval stages ofAleochara bilineata, aparasite of thecabbage

fly,Chortophila brassicae.

The newly

hatchedlarvais anactive thysa- nuriform planidium that

must

seek in the

ground

the

puparium

of the cabbage fly.

Having found

a

puparium

^it

gnaws

a hole in the latter

and

feeds

on

the outside of the

pupa from

a puncture in the pupal integument until it

becomes much

swollen.

The

second instar of the larva is quite different

from

the first; the cuticle is soft

and

white, the antennae

and mouth

parts are altered, the legs reduced to vestiges,the clawsarelost,

and

thecaudal spines disappear.

The

third

larva resemblesthe second except for its larger size.

Both

the second

and

the third larvae lie lengthwise on the back of the thorax of the fly pupa,

and

obtain their food by suction. Pupation takes places in the fly puparium,

and

the adult beetle

gnaws

its

way

out.

The

larval heteromorphosisof theRhipiphoridae is too well

known

to need

more

than a brief notice. Species of Rhipidius are internal parasites in all larval stages

on

cockroaches.

Other

species attacking

wasps and

bees are endoparasiticinthefirstinstarbuttake

an

^external position in later instars.

The

first larva of one of these species is an active planidium

which must

find

and

attach itself to a carrier that will transport it to the nest of a prospective host.

Here

it enters the host larva as an internal parasite.

When

fully

grown

in the first in- star, however, it leaves the

body

of the host through a puncture, moults,

and

in the second stage takes a position across the back of the host larva

on

the first or second thoracic segment.

The

external larva loses the features of the first instar

and becomes

a grublike parasite,

some

species being characterized

by

the presence of large tubercles

on

the back.

Neuroptera.

— ^Among

^the

^Neuroptera

^the larvae of nearly ^all spe- cies are predaceous, their prey being mostly other insects

which

they attack in the open.

The

larvae of

some

Mantispidae, however, feed

on

spider eggs within the spider's cocoon.

The

mantispid eggs being laid

on

trees or bushes, the

young

larva

must

actively find ^its food.

The

life history of

Mantispa

styriaca has long been

known from

the

work

of

Brauer

(1869).

The newly

hatched

Mantispa

is a simple, slender neuropteroid larva with relatively small mandibles

and no

distinctive specialization.

Hatched

in the fall, it hibernates through the winter^; in the spring it finds a spider's cocoon

and

cuts its

way

through thesilken wall.

With

feedinginthe first instar, the

abdomen becomes

greatly enlarged, but at the first moult the larva changes to a fat

grub

with a small

head and

greatly reduced legs. This change of the active first-stage mantispid larva to a sedentary

grub

is cer- tainlynot a reversion to

any

ancestral larval

form among

the

Neurop-

tera; it is an individual secondary adaptation to the life of ease

and

plenty the larvaistolead

from now

on inthe protection of the spider's cocoon, within

which

^{it finally}pupates.

Strepsiptera.

— ^The

Strepsipteraare notable for theheteromorphosis of theparasitic larval stages of both sexes,

and

also for the fact that in

most

species the sexually

mature

female retains the larval form, though with a greatly modified reproductive system,

and

remains within the

body

of the host. In the genera

Eoxenos and

Mengenilla, however, the female leaves the host

and

in the adult stage is

found