CHAPTER V 
GALLS CAUSED BY FLIES (DIPTERA) 
HE majority of flies are two-winged; a few aberrant 
JL 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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