INSECT METAMORPHOSIS
The degree of departure of the young from the parental form varies much in different insects. In the cicada, for example, the nymph is not essentially different in structure from the adult except in the matter of the wings, the organs of reproduction and egg laying, and the musical
Fig. 132. The life history of a ladybeetle, Adalia bipunctata
A, the adult beetle. B, a group of eggs on under surface of a leaf. C, a young larval beetle covered with white wax. D, the full-grown larva. E, the pupa attached to a leaf by the discarded larval skin
instrument. But the habitats of the two forms are widely separated, and it is unquestionable that, in the case of the cicada, it is the nymph that has made the innovation in adopting an underground life, for with most of the relatives of the cicada the young live practically the same life as the adults.
Animals live for business, not for pleasure; and all their instincts and their useful structures are developed for practical purposes. Therefore, where the young and the adult of any species differ in form or structure, we may be sure that each is modified for some particular purpose of its own. The two principal functions of any animal are the obtaining of food for its own sustenance, and the
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