chap, x.] WINGS OF INSECTS. 379 



or less horny body ; this process, when carried to an 

 extreme, ends in the stout wing-covers (Elytra) of the 

 beetle. These tracheal tubes are the " veins " of 

 entomologists, and the finer branches the so-called 

 nervures. 



These wings, when expanded, beat the air by 

 being moved forwards and backwards on their point 

 of articulation to the thorax ; this is effected by special 

 flexor and extensor muscles, the number of which is 

 considerable, and each of which consists only of a few 

 fibres; in considering, however, the causes which give 

 their particular direction to the wings as they move 

 in flight, due attention must be given to the effects of 

 the resistance of the air which is beaten by the wing, 

 which, as a matter of fact, follows a figure of 8 course. 

 In studying the mechanism of the wing we have to 

 bear in mind that the essential points are a rigid 

 anterior nervure, and a flexible membrane behind 

 (Marey). 



Insects vary considerably in the number of move- 

 ments of the wing per second, as may be seen by the 

 following table, which we owe to Marey : 



Common Fly 330 



Drone-fly 240 



Bee 110 



Wasp 130 



Humming-bird Moth 72 



Dragon-fly 28 



Butterfly (Pontia rapes) .... 9 



Among the Apterous insects, or those Hexapods 

 in which wings have never been developed, and which 

 must be carefully distinguished from those that have, 

 owing to parasitic habits and so on, lost wings, which 

 were possessed by their ancestors, the Collembola or 

 Spring-tails are remarkable for the possession of a 

 fork-like appendage to the hinder end of their abdomen, 

 which can be bent backwards, and act like a spring. 



