18 ON THE ANATOMY OF THE FLY. 



Section V. The Wings and Legs. 



The wings and legs are the most important thoracic append- 

 ages; their homologies have been already discussed.* 



The wings consist of a double layer of the protoderm, which 

 may be demonstrated when the insect first emerges from the 

 pupa, as the upper and under layers are then easily separated. 

 The nervures are folds in one or both layers, which are 

 strengthened by the development of epithelial cells in their in- 

 terior. The larger ones contain the tracheal vessels and nerves, 

 the latter being chiefly distributed to the bulbs of the hairs on 

 the nervures. In the earlier stages of development the wings 

 are represented by mere crumpled sacs attached to the lateral 

 trachea) of the larva; afterwards they become sacs of the proto- 

 derm on the sides of the thorax in the pupa. 



Several complicated folds of integument project into 

 the cavity of the thorax at the insertion of the wing, by 

 which the muscles are attached which regulate its position, but 

 the real muscles of flight are not connected with the wing at all. 

 They consist of a longitudinal mass which fills the greater portion 

 of the back, and the wings are so attached that the flank forms a 

 kind of fulcrum upon which they are elevated or depressed by 

 every alteration in the convexity of the back. The great longi- 

 tudinal dorsal muscles, by shortening the thorax, increase its con- 

 vexity and depress the wings, which are again elevated by the 

 flattening of the back and lengthening of the thorax, due partly 

 to its own elasticity and partly to the action of the lateral thoracic 

 muscles, which are vertical in their direction. So that in point of 

 fact the flight of insects is merely a modification of crawling, both 

 being effected by the alternate approximation and extension of 

 several segments. This may be confirmed by direct experiment ; 



See Page 3, 



