18 THE DUBATION OF LIFE. 



to use their large wings for flight, because the body is too heavily 

 weighted by a mass of eggs, all of which reach maturity at the same 

 time. Such species, as for instance Aglia tau, are unable to dis- 

 tribute their eggs over a wide area, but are obliged to lay them all 

 in a single spot. They can however do this without harm to the 

 species, because their caterpillars live upon forest trees, which pro- 

 vide abundant food for a larger number of larvae than can be pro- 

 duced by the eggs of a single female. The eggs of Aglia tau are 

 deposited directly after pairing, and shortly afterwards the insect 

 dies at the foot of the tree among the moss-covered roots of which 

 it has passed the winter in the pupal state. The female moth seldom 

 lives for more than three or four days ; but the males which fly 

 swiftly in the forests, seeking for the less abundant females, live 

 for a much longer period, certainly from eight to fourteen days *. 



The females of the Psychidae also deposit all their eggs in one 

 place. The grasses and lichens upon which their caterpillars live 

 grow close at hand upon the surface of the earth and stones, and 

 hence the female moth does not leave the ground, and generally 

 does not even quit the pupa-case, within which it lays its eggs ; 

 as soon as this duty is finished, it dies. In relation to these habits 

 the wings and mouth of the female are rudimentary, while the 

 male possesses perfectly developed wings. 



The causes which have regulated the length of life in these cases 

 are obvious enough, yet still more striking illustrations are to be 

 found among insects which live in colonies. 



The duration of life varies with the sex in bees, wasps, ants, and 

 Termites : the females have a long life, the males a short one ; and 

 there can be no doubt that the explanation of this fact is to be found 

 in adaptation to external conditions of life. 



The queen-bee the only perfect female in the hive lives two 

 to three years, and often as long as five years, while the male bees 

 or drones only live four to five months. Sir John Lubbock has 

 succeeded in keeping female and working ants alive for seven 

 years a great age for insects 2 , while the males only lived a few 

 weeks. 



1 This estimate is derived from observation of the time during which these insects 

 are to be seen upon the wing. Direct observations upon the duration of life in this 

 species are unknown to me. 



[ 2 Sir John Lubbock has now kept a queen ant alive for nearly 15 years. See note 

 7 on p. 51. E. B. P.] 



