104 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



parently, is a case of an advantage derived by a specific organ from 

 a general structure first developed for some other reason. 



In the septum between the two spiracular openings in Dissosteira 

 (fig. 51 A, //) is a strong internal bar (B, h) projecting anteriorly 

 and ventrally from the posterior lip of the spiracle, and terminating 

 in a free process (?) that extends anterior to the spiracular opening. 

 Upon this process is inserted a short muscle (/p) which has its origin 

 ventrally on an inner process (/) of the lower angle of the peritreme, 

 the site of which is marked externally by a pit (A, ni). A second 

 muscle (B, 80) arises from the base of the same process (/) and 

 extends dorsally and posteriorly to its insertion on the base of the 

 posterior lip of the spiracle behind the ventral trachea. The first 

 muscle (79) is the occlusor of the spiracle; the second {80) is evi- 

 dently its antagonist. A downward pull on the head of, the septal arm 

 (i), where the anterior muscle is inserted, closes the spiracle by 

 rotating the movable posterior lip forward on its dorsoventral axis 

 and bringing thus its sharp free edge into the groove of the anterior 

 lip. Conversely, a downward pressure on the base of the posterior 

 lip, at the point where the posterior muscle {80) is inserted, rotates 

 the lip in the reverse direction and opens the spiracle. The dififerential 

 action of the two muscles results from the opposition of their two 

 points of insertion on either side of the long axis of the posterior 

 lip, and is accentuated by the difiference in their points of origin 

 on the ventral process (/) of the peritreme. Vinal (1919), Lee 

 (1925), and other writers have regarded both muscles of the first 

 spiracle in Acrididae as occlusors. 



The second thoracic spiracle. — The second thoracic, or metatho- 

 racic, spiracle of Dissosteira is located in the lower, posterior angle 

 of the mesothoracic epimeron of the adult (fig. 26, Sps), where it 

 is surrounded by a narrow membranous area (fig. 52 A, nib). 

 Externally this spiracle presents two thick, elongate oval, valve-like 

 lips, (fig. 52 A, c, d) separated by a sinuous vertical cleft having a 

 length of about 0.50 mm. in the male insect. Both lips of the second 

 spiracle are movable, though they are united ventrally in a broad lobe 

 (n). The spiracular lips stand out prominently from the body wall 

 (fig. 53, A, c, d), and between them is a shallow atrium (Atr) from 

 which arises a single large trachea (Tra) that soon divides into a 

 dorsal and a ventral branch. The closing mechanism of the second 

 spiracle includes but a single short occlusor muscle (fig. 52 B, iii). 

 The muscle arises ventrally from a small process (0) on the posterior 

 dorsal margin of the mesocoxal cavity, and is inserted on the ventral 



