1920] Forbes: General Discussion 199 



differentiation was a fortunate one for the future of insect life 

 in the world is shown by the present great predominance of 

 holometabola over ametabola and hemimetabola, and by the 

 obvious advantages which they have in the struggle for 

 existence. By their premature hatching from the egg before 

 the characters of the adult have been laid down in the embryo, 

 their larvae are much more capable of adaptive modification 

 for their life as larvae than are young Orthoptera or Hemiptera, 

 already virtually adult at birth, except as to size, sexual organs, 

 and organs of flight. Hence we see, in the vast majority of 

 cases, the holometabolous larva taking its own course in its 

 own interest, quite regardless of the coming necessities of the 

 adult, with the result that larva and adult have widely different 

 ecological relations, belonging, indeed, to different ecological 

 associations, and do not compete with each other in any way. 

 As the closest competitors of an ametabolous insect are the 

 members of its own species, the division of any species into 

 two non-competing groups diminishes by a certain considerable 

 fraction the dangers of this interspecific competition. On the 

 other hand, the fact that the holometabolous insect must alter- 

 nate between two quite different environments, one for the 

 larva and the other for the adult, makes its failure to find 

 either one a fatal catastrophe, requires the coincidence of two 

 favorable environments, instead of the occurrence of only 

 one, for its survival; but this danger is largely overcorrie by the 

 remarkable development of instinct which leads the female 

 adult to deposit her eggs at a place and time as favorable as 

 possible to the success of the larva; and in the social insects it is 

 of course much more than compensated by the solicitous care 

 which the young of all stages receive from the mature. 



I think we may also count the holometabolous insect as 

 relatively fortunate in respect to its exposure to predaceous 

 enemies, parasites, and contagious diseases. A grasshopper is 

 endangered during its entire life cycle by the same kinds of 

 destructive agencies — the same species of predators (except as 

 it outgrows some of them), the same kinds of insect and other 

 animal parasites, and the same bacterial and other fungous 

 diseases — but white-grubs and May-beetles differ one from the 

 other almost completely in these respects. The attacks of 

 parasites and contagious diseases commonly increase in intensity 

 with the length of time during which their victims are exposed 



