l6 THE DURATION OF LIFE. [1. 



rare intervals and by the expenditure of considerable energy. 

 Thus among the dragon-flies larval life lasts for a year, and 

 among many may- flies even two or three j'ears. 



All these results can be easily understood from well-known 

 physiological principles, and they indicate that the length of 

 larval life is very elastic, and can be extended as circumstances 

 demand ; for otherwise carnivorous and wood-eating larvae 

 could not have survived in the phyletic development of insects. 

 Now it would be a great mistake to suppose that there is any 

 reciprocal relation between duration of life in the larva and in 

 the mature insect, or imago ; or, to put it differently, to suppose 

 that the total duration of life is the same in insects of the same 

 size and activity, 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 onl}' 

 ends with the close of the period of reproduction, as was men- 

 tioned above, but this latter period is also itself extremely' 

 short ^ 



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 montli 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 onl}' 

 live for a few days, while others again, which reproduce by the 

 parthenogcnctic 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 b}' 

 the principles set forth above. Insects belong to the number 



^ See Appendix, note 3, p. 38. 



