INSECTS 



it would be greatly handicapped for living its own life, and 

 this would be quite as 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 advan- 

 tage of its wings without imposing a hardship or a dis- 

 ability 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 metamor- 

 phosis 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 he 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 de- 

 parted 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 cap- 

 tures 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 dis- 

 carded 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. 



[234] 



