: 194. First Larva of Platygaster.]

[Illustration: 195. Second Larva of Platygaster.]

The second larval stage (Fig. 195; _oe_, oesophagus; _ng_,
supra-oesophageal ganglion; _n_, nervous cord; _ga_, and _g_, genital
organs; _ms_, band of muscles) is attained by means of a moult, as usual
in the metamorphoses of insects. With the change of skin the larva
entirely changes its form. So-called hypodermic cells are developed. The
singular tail is dropped, the segments of the body disappear, and the
body grows oval, while within begins a series of remarkable changes,
like the ordinary development of the embryo of most other insects within
the egg. The cells of the hypodermis multiply greatly, and lie one above
the other in numerous layers. They give rise to a special primitive
organ closely resembling the "primitive band" of all insect embryos. The
alimentary canal is made anew, and the nervous and vascular systems now
appear, but the tracheae are not yet formed. It remains in this state for
a much longer period than in the previous stage.

[Illustration: 196. Third Larva of Polynema.]

The third larval form only a few live to reach. This is of the usual
long, oval form of the larvae of the ichneumons, and the body has
thirteen segments exclusive of the head. The muscular system has greatly
developed and the larva is much more lively in its motions than before.
The new organs that develop are the air tubes and fat bodies. The
"imaginal disks" or rudimentary portions destined to develop and form
the skin of the adult, or imago, arise in the pupa state, which
resembles that of other ichneumons. These disks are only engaged, in
Platygaster, in building up the rudimentary appendages, while in the
flies (Muscidae and Corethra) they build up the whole body, according to
the remarkable discovery of Weismann.

Not less interesting is the history of the development of a species of
Polynema, another egg-parasite, which lays its eggs (one, seldom two) in
the eggs of a small dragon fly, Agrion virgo, which oviposits in the
parenchyma of the leaves of waterlilies. The eggs develop as in
Platygaster. The earliest stage of the embryo is very remarkable. It
leaves the egg when very small and immovable, and with scarcely a trace
of organization, being a mere flask-shaped sac of cells.[23] It remains
in this state five or six days.

In the second stage, or Histriobdella-like form, the larva is, in its
general appearance, like the low wo