NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 95 



are evidently muscles of the leg that have been taken over into the 

 service of the wing, for they are attached ventrally on the coxa (fig. 

 49, M', M") ; the other (£) arises on the sternum, or in some insects 

 on the pleuron, and is perhaps a specially developed wing muscle. The 

 two muscles of the basalar sclerites {E, M') are called pronators be- 

 cause they deflect the costal wing margin. The muscle of the subalar 

 sclerite {M") not only deflects the posterior part of the wing, but it 

 acts as a powerful depressor of the entire wing by reason of its con- 

 nection {h) with the ventral plate (c) of the second axillary {2Ax). 

 These muscles probably also enable the insect to alter its course during 

 flight, and, by changing the plane of the wing movements, to hover in 

 the air, or to fly sidewise or backward. 



Finally, most insects have found it advantageous to fold the wings 

 posteriorly over the body. The folding of the wings has involved 

 the development of a mechanism for their flexion and extension. The 

 ability of the wing to be flexed depends upon the mechanism of its 

 axillary region, but the flexing is caused by one or more flexor muscles 

 arising on the pleuron and inserted on the third axillary sclerite (fig. 

 49, D). The extension of the wing is produced by the basalar and 

 subalar muscles {E, M', M"). Considering the other functions of 

 these muscles, the first, therefore, is a pronator-extensor of the wing, 

 the second a depressor-extensor. 



The special features in the mesothorax and the metathorax of the 

 grasshopper that con.tribute to the mechanism of the wings have been 

 described in Section II of this paper. It was there shown that the 

 fusion of the pleurites and sterna of the mesothorax and metathorax 

 converts these segments into a strong trough-like structure covered 

 dorsally by the two wing-bearing terga. The union of the pleural and 

 sternal elements in the pterothorax is probably a direct adaptation to 

 the leaping function of the hind legs, but the resulting structure also 

 gives a strong framework for the support of the wings and the wing 

 muscles. The tergal plates are separated from the edges of the pleuro- 

 sternal trough by the ample membranes of the wing bases, and they are 

 thus free to respond to the downward pull of the tergo-sternal muscles. 

 The close union of the terga (fig. 25) and the great size of the dorsal 

 muscles (fig. 34) give efificiency to the latter as elevators of the wings. 

 When the wings are spread they are pivoted on the pleural wing proc- 

 esses by the second axillary sclerites of their bases, and, being 

 closely hinged to the terga by the first and fourth axillaries, they are 

 sharply thrown upward when the tergal plates are depressed, and 

 are turned downward when the terga are elevated. 



