THE DURATION OF LIFE. 19 



These last examples become readily intelligible when we remember 

 that the males neither collect food nor help in building- the hive. 

 Their value to the colony ceases with the nuptial flight, and from 

 the point of view of utility it is easy to understand why their lives 

 should be so short 1 . But the case is very different with the female. 

 The longest period of reproduction possible, when accompanied by 

 very great fertility, is, as a rule, advantageous for the mainten- 

 ance of the species. It cannot however be attained in most 

 insects, for the capability of living long would be injurious if all 

 individuals fell a prey to their enemies before they had completed 

 the full period of life. Here it is otherwise : when the queen-bee 

 returns from her nuptial flight, she remains within the hive until 

 her death, and never leaves it. There she is almost completely 

 secure from enemies and from dangers of all kinds ; thousands of 

 workers armed with stings protect, feed, and warm her ; and in 

 short there is every chance of her living through the full period of 

 a life of normal length. And the case is entirely similar with the 

 female ant. In neither of these insects is there any reason why 

 the advantages which follow from a lengthened period of repro- 

 ductive activity should be abandoned 2 . 



That an increase in the length of life has actually taken place in 

 such cases seems to be indicated by the fact that both sexes of the 

 saw-flies the probable ancestors of bees and ants have but a 

 short life. On the other hand, the may-flies afford an undoubted 

 instance of the shortening of life. Only in certain species is life as 

 short as I have indicated above ; in the majority it lasts for one or 

 more days. The extreme cases, with a life of only a few hours, 

 form the end of a line of development tending in the direction of a 

 shortened life. This is made clear by the fact that one of these 

 may-flies (Palingenia) does not even leave its pupa-skin, but repro- 

 duces in the so-called sub-imago stage. 



It is therefore obvious that the duration of life is extremely 

 variable, and not only depends upon physiological considerations, 

 but also upon the external conditions of life. With every change 

 in the structure of a species, and with the acquisition of new 

 habits, the length of its life mayj and in most cases must, be 

 altered. 



1 See Appendix, notes 7 and 9, pp. 59 and 63. 



2 See Appendix, note 6, p. 58. 



C 2 



