FORMATION OF INSECT'S WING 87 

 case is not really an arm, but a process of 

 chitinous nervures which forms a stiff margin 

 (the costa) ; this gives great rigidity to the 

 wing. These nervures spread out in a net- 

 work of veins which taper gradually to the 

 hinder margin, and being curved act under 

 pressure in the same way as the quills of 

 feathers, and are equally resilient. Thus, 

 when the wing is waved or vibrated, the result 

 is practically the same as in the former case : 

 viz., that the air is " fanned " away in the 

 rear of the wing, which engenders propulsion 

 or lifting power according to the angle. 



The costal margin of an insect's wing is 

 rigid throughout, except in the order Coleop- 

 tera and a few others where a joint shows on 

 the costa ; this has nothing to do with flight, 

 but is merely an adaptation which permits of 

 the wing being folded so that it may pass under 

 cover of the elytron when not in use, the cover 

 being too short to contain the wing in ex- 

 tension. For the greater part, insects are 

 furnished with a dual fl5dng apparatus, a fact 

 which seems to suggest that their ancestors 

 belonged to a period long anterior to birds. 

 The system would appear unnecessarily com- 

 plicated, for though these double-winged 

 insects exhibit high powers of flight, as in- 

 stance the humming-bird moth (Macroglossa 



