LIFE-HISTORIES OF FLIES 263 



in perfecting the adaptations of Diptera ; that is the 

 step taken by the next group. Were division of 

 labour adopted by flies as by bees, it is doubt- 

 ful whether domesticated animals would survive 

 or vegetation flourish. But in all the unequalled 

 variety of dipterous habit, there is no indication of 

 the adoption of communal life. The common house- 

 hold flies and the horse-flies are individualists of the 

 highest order. In no other animals are the two diverse 

 ends of life so strongly emphasised. In none are they 

 embodied in more glaring contrasts than in the inactive 

 maggot and the brilliant hovering fly, the one a 

 nutritive centre, the other with only the need of 

 drink to ally it to earth. In no other life-history is 

 the transition so rapid and abrupt. The chief larval 

 tissues are resolved to a cream, and are rebuilt in a 

 week to form a far more complex and active being. 



There is no comparable adaptability elsewhere to 

 ensure as these flies do, the forcing influence of heat 

 and stimulating food, or to utilise borrowed animal 

 heat and nourishment in order to produce, not eggs, 

 nor even young larvae, but a full-grown larva, or even 

 pupa, which needs but a few hours to complete the 

 issue of the fly. And the genius of the Diptera lies 

 in this, that without forsaking their hold on the old 

 inexhaustible vegetarian diet, they have been drawn 

 by the attraction of smell to the new kingdoms of 

 mammals and birds, with all that goes in their train. 



The Hymeuopteya. — The care of the young is most 

 efficiently performed by the Hymeiwptera. 



