16 THE DURATION OP LIFE. 



so that the time which is spent in the larval state is, as it were, 

 deducted from the life of the imago, and vice versa. That this 

 cannot be the case is shown by the fact already alluded to, that 

 among bees and ants larval life is of the same length in males and 

 females, while there is a difference of some years between the lengths 

 of their lives as imagos. 



The life of the imago is generally very short, and not only ends 

 with the close of the period of reproduction, as was mentioned 

 above, but this latter period is also itself extremely short 1 . 



The larva of the cockchafer devours the roots of plants for a 

 period of four years, but the mature insect with its more complex 

 structure endures for a comparatively short time ; for the beetle itself 

 dies in about a month after completing its metamorphosis. And 

 this is by no means an extreme case. Most butterflies have an 

 even shorter life, and among the moths there are many species (as 

 in the Psychidae) which only live for a few days, while others 

 again, which reproduce by the parthenogenetic method, only live for 

 twenty-four hours. The shortest life is found in the imagos of 

 certain may-flies, which only live four to five hours. They emerge 

 from the pupa-case towards the evening, and as soon as their 

 wings "have hardened, they begin to fly, and pair with one another. 

 Then they hover over the water ; their eggs are extruded all at 

 once, and death follows almost immediately. 



The short life of the imago in insects is easily explained by the 

 principles set forth above. Insects belong to the number of those 

 animals which, even in their mature state, are veiy liable to be 

 destroyed by others which are dependent upon them for food ; but 

 they are at the same time among the most fertile of animals, and 

 often produce an astonishing number of eggs in a very short time. 

 And no better arrangement for the maintenance of the species 

 under such circumstances can be imagined than that supplied by 

 diminishing the duration of life, and simultaneously increasing the 

 rapidity of reproduction. 



This general tendency is developed to very different degrees 

 according to conditions peculiar to each species. The shorten- 

 ing of the period of reproduction, and the duration of life to the 

 greatest extent which is possible, depends upon a number of co- 

 operating circumstances, which it is impossible to enumerate 



1 See Appendix, note 3, p. 38. 



