NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 93 



present in the abdomen. Being attached ventrally on the sternum, 

 these muscles indirectly impart a strong up-stroke to the wings by 

 flattening the dorsal curvature of the tergum. The down-stroke of 

 the wings produced by the contraction of the longitudinal dorsal 

 muscles has been strengthened in two principal ways : first, by the 

 obliteration of the secondary intersegmental membranes between the 

 terga, thus eliminating lost motion ; and second, by the great enlarge- 

 ment of the dorsal muscles themselves in the wing-bearing segments. 



Fig. 48. — Ventral surface of the base of the left tegmen, and upper part the 

 pleuron of Dissosteira. 



a, thickening of membrane uniting basalar sclerites with humeral angle of 

 wing ; 2Ax, ventral plate of second axillary ; sAx, ventral plate of third axillary ; 

 h, connection between subalar sclerite and ventral plate of third axillary; iBa, 

 first basalare ; 2Ba, second basalare ; Sc, base of subcostal vein ; Sa, subalare ; 

 WP, pleural wing process. 



The suppression of the intertergal membranes has been accomplished 

 by a fusion between the successive tergal plates, or by a forward 

 extension of the precostal lip of the tergum until it meets the pos- 

 terior edge of the preceding tergum. Thus are produced the post- 

 notal plates between the mesothoracic and metathoracic terga, and 

 between the metathoracic and first abdominal terga. The enlargement 

 of the dorsal muscles has been accompanied by the development of 

 supporting plates (phragmata) from the ridges of the muscle attach- 

 ments on the primary intersegmental folds. Furthermore, each tergal 

 plate has been strengthened and better adapted to its function in the 

 flight mechanism by the development of internal ridges, the principal 



