INSECT METAMORPHOSIS
have been derived from a common ancestor, as seems almost certain, then the original larvae must have been all alike, and they must have had approximately the structure of those larvae of the present time that depart least from the structure of the adult. Therefore it is evident that many larvae of the present time have somehow acquired certain embryonic characters. We may suppose, therefore, either that such larvae have had a retrogressive evolution into the embryonic stage by hatching at successively earlier ages, or that certain embryonic characters representing ancestral characters but ordinarily quickly passed over in the embryonic development, have been retained and carried on into the larval stage. The latter view seems the more probable when we consider that no larva has a purely embryonic structure, and that those larvae which have embryonic features in their anatomy present an incongruous mixture of embryonic and adult characters.
We may, therefore, finally conclude that the larva of insects with complete metamorphosis represents the nymphal stage of insects with incomplete metamorphosis; and that the structure of the larva has resulted from a suppression of the peculiarly adult characters, from an invagination of the wings, a loss of the compound eyes, the retention of certain embryonic characters, and a special development of the body form and the organs suited to the particular mode of life of the larva. By allowing for variations in all these elements that contribute to the larval make-up, except the two constants—the invagination of the wings and the loss of the compound eyes—we may account for all the variety in form and structure that the larva presents.
While, in general, the larva remains the same in structure from the time it is hatched until it transforms to the pupa, there are nearly always minor changes observable that are characteristic of its individual stages. In Chapter I we encountered the case of the little blister beetle that goes through several very different forms dur-
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