INSECTS
possible for a young mosquito, deprived of the power of flight, no live the life of its parents and to feed after the manner of its mother. Hence, the young mosquito bas adopted its own way of living and of feeding, and this has allowed the adult mosquitoes to perfect their specialties without inflicting a hereditary handicap on their offspring. Thus again we see the great advantage which the species as a whole derives from the double life of its individuals.
The fly will only give another example of the same thing. The specialized form of the young fly, the maggot (Fig. 171), which is adapted to the requirements of quite a different kind of life from that of the adult fly, relieves the latter from all responsibility to its offspring. As a consequence, the adult fly has been able to adapt its structure, during the course of evolution, to a way of living best suited to its own purposes, unhampered as it would be if its characters were to be inherited by the young, to whom they would become a great impediment, and probably a fatal handicap.
A fourth principle of metamorphosis, then, we may say, is that the species as a whole has acquired an advantage by a double mode of existence, which allows it to take advantage of two environments during its lifetime, one suited to the functions of the young, the other to the functions of the adult.
We noted, in passing, that the young insect is free to live its own life and to develop structures suited to its own purposes under one proviso, which is that it must eventually revert to the form of the adult of its species. At the period of transformation, the particular characters of the young must be discarded, and those of the adult must be developed.
Insects such as the grasshoppers, the katydids, the roaches, the dragonflies, the aphids, and the cicadas appear in the adult form when the young sheds its skin for the last time. The change that has produced the adult,
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