1 8 THE DURATION OF LIFE. [I. 



cases the gradual ripening of the eggs becomes necessary, and 

 involves an increase in the duration of life. In Lepidoptera, 

 we see how the power of flight diminishes step by step as soon 

 as other circumstances permit, and simultaneously how the 

 eggs ripen more and more rapidly, while the length of life 

 becomes shorter, until a minimum is reached. Only two stages 

 in the process of transformation can be mentioned here. 



The strongest flyers — the hawk-moths and butterflies— must 

 be looked upon as the most specialised and highest types 

 among the Lepidoptera. Not only do they possess organs for 

 flight in their most perfect form, but also organs for feeding — 

 the characteristic spiral proboscis or ' tongue.' 



There are certain moths (among the Bombyces) of which the 

 males fly as well as the hawk-moths, while the females are 

 unable 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 

 tail, are unable to distribute 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 provide abundant food for a larger 

 number of larvae than can be produced 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 

 daj^s \ 



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 



^ 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. 



