THE FLIGHT OF INSECTS 



313 



The subordination of the hind-wings to the fore-wings has led 

 in many cases, e.g. Bees, to a diminution in size of the former. 

 And in ordinary Two-winged FHes (Diptera) this reduction has 

 gone so far that the hind-wings have dwindled to a pair of club- 

 shaped " balancers " (halteres) which appear to be chiefly useful as 

 sense organs, though they also play a subordinate part in flight. 



But, on the other hand, there ^ 



are many insects in which the 

 hind-wings are the chief agents 



Fig. 841.— Wings of Insects 



A, A Dragon-Fly. B, Left fore- and hind-wings of a Moth; note frenulum projecting from base of latter. 

 c, left fore- and hind-wings of a Bee, showing row of hooks on latter, d, Two- Winged Fly; b, one of reduced 

 hind-wings. E, Male StyIops;y?to.j reduced fore-wing. The figures have been drawn to various scales. 



in flight, the fore-wings having become more or less hard and 

 thick to constitute protective wing-covers. The most familiar 

 case of the kind is afforded by the Beetles (Coleoptera), although 

 it is supposed that in these insects the firm wing-covers {elytra) 

 play a passive part in flight by acting as "wind-catchers", or kites. 

 The membranous hind-wings of a Beetle, when not in use, are 

 neatly folded up under the wing-covers, and are so long that 

 they do not merely shut up like fans, but also have to be bent 

 up along a transverse crease. In some Beetles which possess a 



Vol. III. 



83 



