234 ANIMAL LIFE 



many districts the whole local population of one or 

 two year old may-fly larvae undergoes its transforma- 

 tion in two or three successive afternoons. 



The More Complex Life-histories Metamorphosis 



A distinctive feature of the life-histories we have 

 described is the gradual evolution of the fly. The 

 assumption of the last phase coincides with the 

 completed formation of wings and the display of 

 flight. Only in its last phase is the organism some- 

 times transformed in outward appearance for aerial 

 life. This gradational life-history is one of great 

 antiquity, and the insects that exhibit it come of an 

 old stock. Young and old are in close touch with the 

 primitive life of earth and water, feeding on moulds 

 and herbage, devouring the flesh of simple organisms 

 or the mingled decay of organic life. The most 

 primitive of all modern insects remain groundlings ; 

 but the necessity for disseminating their eggs and 

 colonising new districts has acted as a spur which has 

 driven the adult insect of other orders to become 

 a fly. In its simpler forms this power does not involve 

 any great change of habit, but for its perfection flight 

 requires the nicest adaptation. The changes needful 

 to this end are therefore worked out towards the close 

 of the earlier and more sedentary phase of life, and 

 involve a less or more complete transformation of the 

 organism less as the time is long and the preparation 

 gradual, more complete as the life is shortened and the 



