Il8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 89 



of a long slender tendon. The tendons of these muscles support 

 the lateral margins of the ventral diaphragm, which latter extends 

 into the propodeum and is attached anteriorly on the endosternum 

 of the pterothorax. The long slender dilator muscle of the spiracle 

 {dls) lies between the two branches of the lateral dorsal muscle, and 

 is attached posteriorly on a small process of the lower margin of 

 the propodeum close to the pleural articular process of the meta- 

 coxa (CxPs). The ventral muscles of the petiole comprise two 

 pairs of muscles arising anteriorly on the endosternum of the ptero- 

 thorax (fig. 39, /, vim, vil). Those of the larger median pair are 

 attached close together on the sternal margin of the aperture of the 

 petiole (fig. 38 B, vim) ; the smaller muscles of the lateral pair {vil) 

 are inserted by slender tendons on the lateral extremities of the sternal 

 margin. The median ventrals are antagonistic to the median dorsals, 

 but both the lateral dorsals and the lateral ventrals serve to move 

 the postabdomen from side to side. 



The postabdomen consists of six exposed segments (fig. 38 D), 

 of which the last, or segment VII, has a conical shape, with a cleft 

 on each side between the tergum and the sternum. Within the sev- 

 enth segment is a large vestibular chamber in which are concealed 

 the genital segments, the proctiger, and the sting (fig. 40). 



The musculature of the postabdomen is rather simple ; it is clearly 

 adapted to produce the movements of respiration and the motions 

 of the segments that accompany the use of the sting. The pattern 

 of the muscle arrangement is repeated in each of the first five seg- 

 ments, with a few modifications in the first segment. We may, 

 therefore, examine the musculature of the third segment of the 

 postabdomen, which has been concisely described by Morison (1927), 

 as an example of the musculature of segments /// to VI. The names 

 here applied to the individual muscles are those proposed by the 

 writer (1931, p. 34) as generally applicable to the muscles of the 

 pregenital region of the insect abdomen. They have a locational 

 significance, and are therefore somewhat briefer than functional 

 terms, and can be used for corresponding muscles where the action 

 is different. The muscles found in each side of the third segment 

 of the postabdomen (fig. 39, IV), omitting the diaphragm muscles, 

 are as follows : 



Musculus dorsalis internus medialis {dim). — A broad band of 

 oblique fibers on median part of dorsum extending from the ante- 

 costa {Ac) of tergum IV posteriorly and medially to antecosta of 

 tergum V. 



