NO. 8 INSECT ABDOMEN SNODGRASS 11/ 



posterior parts of the oblique metathoracic pleura. The propodial 

 sternum is reduced to a narrow, weakly sclerotic band (S) in the 

 membrane between the hind coxae and the second abdominal sternum. 

 The short petiole of the postabdomen is formed mostly of the con- 

 stricted anterior part of the second abdominal segment, and in- 

 volves both the tergum and the sternum of this segment. The large 

 first abdominal spiracles are located laterally on the anterior part of 

 the propodeum. 



The posterior declivous surface of the dorsal plate of the pro- 

 podeum terminates in a small triangular area above the petiole (fig. 

 38 A, t). The median apical part of the triangle is deeply excavated 

 by a horseshoe-shaped notch, the lateral extremities of which form 

 a pair of points (/, /), which articulate with the anterior margin 

 of the second tergum, and constitute the fulcra on which the post- 

 abdomen moves on the propodeum. The intersegmental membrane 

 between the fulcra is arched upward in a small dome (e), which 

 extends from the propodial notch backward on the dorsal surface of 

 the petiole. There is thus formed upon the anterior part of the 

 petiole beneath the membrane, a small pocket communicating with 

 the cavity of the propodeum through the notch in the dorsal wall of 

 the latter (C). The floor of the pocket is strongly sclerotized and 

 presents a small, median, dorsal ridge (C, r). On the posterior end 

 of this ridge, within the pocket, are inserted the tendons of a pair 

 of large muscles (dim) which take their origins on the lateral walls 

 of the propodeum (A). These several correlated structures and the 

 pair of propodeal muscles constitute the levator apparatus of the 

 postabdomen (F). The insertion of the muscles on the posterior 

 end of the petiole ridge (r) gives the muscles a short leverage on 

 the abdomen distal to the fulcra (/). The mechanism is thus, as 

 noted by Betts (1923), a lever of the third order, and, considering 

 the weight of the postabdomen, it is surprisingly eflfective. 



The levator muscles of the postabdomen are evidently the median 

 internal dorsals of the propodeal segment (fig. 39, dim). They con- 

 verge from the sides of the propodeum into the dorsal pocket of the 

 petiole (fig. 38A, B,C). Each is a broad, fan-shaped muscle (E, 

 dim) with a flat, tapering tendon. A second pair of muscles from the 

 propodeum is inserted on the second abdominal tergum at the 

 lateral extremities of the anterior margin of the petiole (B, dil), 

 and would appear to be the lateral internal dorsals of the first seg- 

 ment (fig. 39, /, dil). Each is distinctly two-branched (fig. 38 E, 

 dil), one branch arising above the propodeal spiracle (Sp), the other 

 below it, the fibers of the two branches converging upon the end 



