THE DURATION OF LIFE. 17 



completely. Even the manner in which the eggs are laid may 

 have an important effect. If the larva of the may-fly lived upon 

 some rare and widely distributed food-plant instead of at the 

 bottom of streams, the imagos would be compelled to live longer, 

 for they would be obliged like many moths and butterflies to 

 lay their eggs singly or in small clusters, over a large area. This 

 would require both time and strength, and they could not retain 

 the rudimentary mouth which they now possess, for they would 

 have to feed in order to acquire sufficient strength for long flights ; 

 and whether they were carnivorous like dragon-flies, or honey- 

 eating like butterflies their feeding would itself cause a further 

 expenditure of both time and strength, which would necessitate a 

 still further increase in the duration of life. And as a matter of 

 fact we find that dragon-flies and swift-flying hawk-moths often 

 live for six or eight weeks and sometimes longer. 



We must also remember that in many species the eggs are not 

 mature immediately after the close of the pupal stage, but that 

 they only gradually ripen during the life of the imago, and 

 frequently, as in many beetles and butterflies, do not ripen simul- 

 taneously, but only a certain number at a time. This depends, 

 first, upon the amount of reserve nutriment accumulated in the body 

 of the insect during larval life ; secondly, upon various but entirely 

 different circumstances, such as the power of flight. Insects which 

 fly swiftly and are continually on the wing, like hawk-moths and 

 dragon-flies, cannot be burdened with a very large number of ripe 

 eggs. In these 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 



c 



