A. B. C. Cyclops larvæ of three species of Platygaster.
D. Second larval stage. E. Third larval stage.
mo. mouth; a. antenna; hf. hooked feet; lfg. lateral feet; f. branches of tail; ul. lower lip; slkf. œsophagus; gsae. supra-œsophageal ganglion; bsm. ventral epiblastic plate; lm. lateral muscles (the letters also point in D to the salivary glands); gh. proctodæum; ga. generative organs; md. mandibles; ag. ducts of salivary glands; sp. (in E) salivary glands; mls. stomach; ed. intestine; ew. rectum; ao. anus; tr. tracheæ; fk. fat body.
Towards the close of the second larval period the muscles (lm) become segmentally arranged, and give indications of the segmentation which becomes apparent in the third larval period. The third and last larval stage ([fig. 191] E) of Platygaster, during which it still remains in the tissues of its host, presents no very peculiar features. The passage from the second to the third form is accompanied by an ecdysis.
Remarkable as are the larvæ just described, there can I think be no reason, considering their parasitic habits, for regarding them as ancestral.
Metamorphosis. The majority of Insects are born in a condition in which they obviously differ from their parents. The extent of this difference is subject to very great variations, but as a rule the larvæ pass through a very marked metamorphosis before reaching the adult state. The complete history of this metamorphosis in the different orders of Insects involves a far too considerable amount of zoological detail to be dealt with in this work; and I shall confine myself to a few observations on the general characters and origin of the metamorphosis, and of the histological processes which take place during its occurrence[176].
In the Aptera the larva differs from the adult only in the number of facets in the cornea and joints in the antennæ.
In most Orthoptera and Hemiptera the larvæ differ from the adult in the absence of wings and in other points. The wings, etc., are gradually acquired in the course of a series of successive moultings. In the Ephemeridæ and Libellulidæ, however, the metamorphosis is more complicated, in that the larvæ have provisional tracheal gills which are exuviated before the final moult. In the Ephemeridæ there are usually a great number of moultings; the tracheal gills appear after the second moult, and the rudiments of the wings when the larva is about half grown. Larval life may last for a very long period.
In all the other groups of Insects, viz. the Diptera, Neuroptera, Coleoptera, Lepidoptera, and Hymenoptera, the larva passes—with a few exceptions—through a quiescent stage, in which it is known as a pupa, before it attains the adult stage. These forms are known as the Holometabola.
In the Diptera the larvæ are apodous. In the true flies (Muscidæ) they are without a distinct head and have the jaws replaced by hooks. In the Tipulidæ there is on the other hand a well-developed head with the normal appendages. The pupæ of the Muscidæ are quiescent, and are enclosed in the skin of the larva which shrinks and forms a firm oval case. In the Tipulidæ the larval skin is thrown off at the pupa stage, and in some cases the pupæ continue to move about.