INSECTS 



possible for a young mosquito, deprived of the power of 

 flight, to live the life of its parents and to feed after the 

 manner of its mother. Hence, the young mosquito has 

 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 even- 

 tually 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 ap- 

 pear 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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