96 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 52 



The mechanism for extending and flexing the wing is highly com- 

 plex. The muscles that produce the movements of extension and 

 flexion depend for their eiifect on the details of shape and inter-rela- 

 tionships in the axillary sclerites, on the articulation of the sclerites 

 with the tergum and pleuron, on their connections with the bases of 

 the wing veins, and on the structure of the wings themselves. 



Fig. 49- 



PR' 



-The pleural elements of the wing mechanism in the mesothorax 

 of Dissosteira. 



a, thickening of cuticular membrane uniting basalar sclerites with humeral 

 angle of wing (see fig. 48) ; sAx, second axillary; sAx, third axillary (first and 

 fourth axillaries removed) ; b, thickening of cuticular membrane uniting subalar 

 sclerite (Sa) with ventral plate (c) of second axillary; Ba, first basalare; c, 

 ventral plate of second axillary resting on pleural wing process (see fig. 48) ; 

 D, flexor muscle of wing, inserted on third axillary ; E, pleuro-sternal muscle, 

 or first pronator-extensor of the wing, inserted on basalar sclerite ; M', episternal 

 pleuro-coxal muscle, or second pronator-extensor of the wing, inserted on 

 basalar sclerite ; M", epimeral pleuro-coxal muscle, or depressor-extensor of the 

 wing, inserted on subalar sclerite ; PIR, pleural ridge ; Sa, subalare ; tg, tegular 

 rudiment ; [^2, base of tegmen, showing dorsal surface. 



During extension and flexion the wings do not simply turn forward 

 and backward on the pleural wing processes, since each wing is at- 

 tached to the tergum by its entire basal width. The horizontal move- 

 ments of the wings are made possible mainly by the flexible lines in 

 the wing bases and by the articulations of the axillary sclerites on one 

 another. The working of the parts involved may be easily observed in 

 a freshly killed specimen if the extended wing is slowly flexed. 



