CHAPTER V 

 GALLS CAUSED BY FLIES (DIPTERA) 



THE majority of flies are two-winged; a few aberrant 

 forms, such as fleas and certain ticks, are wingless. 

 The wings are comparatively small ; behind them are a pair 

 of little erect bodies, the halteres or poisers. The maggots 

 or larvae are usually footless, with a small and indistinct 

 head. The pupa may be either exposed and hard, or soft 

 and enclosed in a capsular seed-like body. 



Dr. Sharp* observes : " About 40,000 species of Diptera 

 have been discovered, but these are only a tithe of what are 

 still unknown to science. The order is not a favourite one 

 with entomologists, and by the rest of the world it may be 

 said to be detested. . . . Nevertheless, Diptera have 

 considerable claims to be classed as actually the highest 

 of insects physiologically, for it is certainly in them that 

 the processes of complete life-history are carried on with 

 the greatest rapidity, and that the phenomena of metamor- 

 phosis have been most perfected. A maggot, hatching 

 from an egg, is able to grow with such rapidity that the 

 work of its life in this respect is completed in a few days ; 

 then, forming an impenetrable skin, it dissolves itself almost 

 completely ; solidifying subsequently to a sort of jelly, it, in 

 a few days, reconstructs itself as a being of totally different 

 appearance and habits, in all its structures so profoundly 

 changed from what it was that the resources of science are 



* "Insects," part ii.. vol. vi., pp. 438, 439; "Cambridge Natural 

 History," vol. vi. 



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