MOUTH-PARTS, WINGS, AND LEGS 51 



show that the smaller the wings are in proportion to the 

 body, the more rapidly they vibrate. Nor is this surpris- 

 ing. W ithout bulk of body, the muscular force necessary to 

 secure a firm grasp upon the air, and to repeat at rapid inter- 

 vals the effective downward thrust, cannot be developed. 

 Thus, we see that as the weight of the insect increases, 

 the relative wing-area usually decreases, a small wing 

 moved rapidly being more effective than a large one 

 moved slowly. The gnat, despite its comparatively feeble 

 flight, has eleven times the wing surface of the swallow, 

 reducing both animals to the same weight. Compare, 

 too, the wings of the small white butterfly with those of 

 the humming-bird hawk-moth, and these again with the 

 wings of the bee or the fly. Further, there is a reason 

 why weight may be regarded as an important asset in 

 flight. We know that within certain limits imposed by 

 our muscular strength, we can throw a large stone to a 

 greater distance than a small one. We say that the 

 former " carries " better than the latter. By this we mean 

 that when once it is set in motion the momentum of a 

 heavy body is far greater than that of a light one. It 

 follows that the body of an insect must not be regarded 

 simply as a dead weight to be upheld by the wings, but 

 as an actual aid to rapid and sustained flight. Once set 

 fairly swinging through the air by the movement of the 

 wings, the body travels forward by its own momentum, 

 just as does a stone when it leaves the hand of the 

 thrower. Moreover, the bodies of rapidly-flying insects 

 are perfectly adapted by their shape and poise to minimise 

 the resistance of the air — to utilise, as it were, the last 

 fraction of the force which the wings develop. If we 

 contrast the swift, fully-controlled flight of a hawk-moth 

 or a bee with the relatively feeble flutterings of a butter- 

 fly, we may say that the former is like a well-ballasted 



