3 io 



ANIMAL MOVEMENT 



tures are developed as outgrowing folds of the body-wall, and are 

 quite independent of the limbs, thus differing from the flying 

 organs of backboned animals. 



When fully developed, the wing may be regarded as an elastic 

 membrane, horny in texture, and strengthened by a more or less 

 elaborate system of thickened "veins" or " nervures ", which have 

 a constant arrangement in the same kind of insect, and are a great 

 aid to classification. They prevent tearing, and the strongest of 

 them run along the front part of the wing, which is thus rendered 



specially resistant, and 

 plays the same part with 

 reference to the more 

 flexible region behind it 

 as does the bone-sup- 

 ported front edge of a 

 bird's wing with regard 

 to the quill-feathers. A 

 very young wing can be 

 separated into upper and 

 lower layers, between 

 which run various struc- 

 tures. Each nervure, 

 for example, is traversed 

 by an air -tube, accom- 

 panied by a blood- 

 channel. The wing is 

 jointed on to the thorax 

 in a complex manner, and its base projects into the body as a 

 sort of fold, to which important muscles are attached. 



The diagrammatic cross-section through a butterfly represented 

 in fig. 839 indicates the position of the chief muscles of flight. 

 The thickened part of the wing to which some of these are at- 

 tached may be regarded as a lever (m h\ of which the fulcrum (g) 

 is situated in the wing-joint. To the short arm of the lever 

 internal to this a muscle is attached (c h\ by which the wing is 

 raised. The effective downward stroke is effected by a more 

 powerful muscle (b f\ which is attached to the wing a little way 

 ,outside the fulcrum. Other muscles, which are not attached 

 directly to the wings, but alter the shape of the thorax, help to 

 bring about the upward and downward movements. This region 



Fig. 839. Diagrammatic Cross-section of a Butterfly 



+ +, Thighs; bu and cd, muscles which pull the legs outwards 

 and inwards. Muscles of flight described in text. The wings have 

 been cut short 



