PHYSIOLOGY OF THE INVERTEBRATA. 393 



L&pidoptera), uses " its legs for running" this being an ex- 

 ceptional habit among butterflies. 



The wings of insects are appendages attached to the meso- 

 thorax and metathorax. They are composed of a double 

 membrane, supported internally by a variable number of 

 nervures. These serve to keep the wings extended. There 

 are never more than two pairs of wings, sometimes only one, 

 and they vary in form. When they really serve for flight 

 they are thin, translucent, and covered with microscopic 

 scales as in the Lepidoptera; but the anterior wings often 

 become hard and opaque, and becoming useless as organs of 

 flight, form elytra — i.e., protecting sheaths, for the posterior 

 pair of wings : such an arrangement occurs in the Goleoptera, 

 Although the wings of insects are usually four in number, the 

 posterior pair is frequently absent, and, in fact, the Diptera 

 is characterised by the possession of only one pair of wings. 

 In these insects a pair of small knobbed filaments, situated 

 on the sides of the thorax behind the wings, and which are 

 called halteres, have been regarded as the representatives of 

 the posterior wings. 



In almost every order of the Insecta there are genera, species 

 and individuals, as certain female aphides, which are apterous 

 or wingless. 



" The movements of the wings are produced by two 

 extensor and several smaller flexor muscles,* which arise 

 from the middle and posterior thoracic segments, and are 

 inserted on the tendinous process at the base of each wing. 

 The size of these muscles is proportionate to the size of the 

 wings and their mode of use in flight. They are, consequently, 

 all equally developed when the four wings participate equally 



* Dr. Allen Thomson has measured the diameters of the muscular fibres 

 in the Insecta with the following results : — 



Greatest diameter ■ tJtt inch. 



Least ,, TTS » 



Average „ .... ■ -ih; » 



Distance of transverse striae irtW » 



