INSECTS
it would be greatly handicapped for living its own life, and this would be quite detrimental to the adult, which must be developed from the young. Therefore, nature has devised a scheme for separating the young from the adult, by which the latter is allowed to take full advantage of its wings without imposing a hardship or a disability on its flightless offspring. The device sets aside the ordinary workings of heredity and makes it possible for a structural modification to be developed in the adult and to be suppressed in the young until the time of change from the last immature stage to that of the adult.
Thus we may state as a second principle of metamorphosis that an adult insect may develop structural characters adaptive to habits that depend on the power of flight, which are suppressed in the young, where they would be detrimental by reason of the lack of wings.
When parents, now, assert their independence, what can we expect of the offspring? Certainly only a similar declaration of rights. A young insect, once freed from any obligation to follow in the anatomical footsteps of its progenitors, so long as it finally reverts to the form of the latter, soon adopts habits of its own; and then acquires a form, physical characters, and instincts adapted to such habits. Thus, the young dragonfly (Fig. 134) has departed from the path of its ancestors; it has adopted a life in the water, where it feeds upon living creatures which it pursues by its perfection in the art of swimming and captures by a special grasping organ developed from the under lip (B). Life in the water, too, entails an adaptation for aquatic respiration. All the special acquisitions in the structure of the young insect, however, must be discarded at the time of its change to the adult.
A third principle, then, which follows somewhat as a corollary from the second, shows us that the young of insects may adopt habits advantageous to themselves, and take on adaptive structures that have no regard to the form of the adult and that are discarded at the final transformation.
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