NO. 6 INSECT ABDOMEN SNODGRASS 47 



Bauer terms the lateral muscles " musculi transversales abdom- 

 inis," but, as pointed out by Samtleben (1929), the lateral muscles are 

 dorsoventral and should not be termed " transverse." The largest mus- 

 cles of the Dvtiscus abdomen are the dorsal muscles. These consist of 

 broad bands of fibers forming a wide sheet of muscles against the ter- 

 gal region in each of the first six segments. The ventral muscles are 

 present only in segments III. I\'. and A\ They include large median 

 ventrals and small lateral ventrals. The median ventrals form three 

 pairs of muscle sheets occupying the median sternal region of the seg- 

 ments, the fibers of the opposite groujis in each pair converging pos- 

 teriorly. The lateral ventrals (" musculi ventrales externi " of Bauer) 

 are very small, each arising on the posterior lateral angle of the 

 sternum of its segment, and being inserted on the anterior margin of 

 the sternum following. The lateral muscles ( " musculi transversales '" 

 of Bauer) comj^rise a pair of small, oblique tergo-sternal muscles 

 crossing each other in the form of an X in each side of segments TI 

 to V inclusive, and a single oblique muscle in segment VI. 



Hyincnoptera. — The honeybee furnishes the principal information 

 that we have on the abdominal musculature of Hymenoptera. The 

 muscles of a typical abdominal segment of the honeybee have been 

 described by Carlet (1890). Betts (1923), and Snodgrass (1925); 

 the complete abdominal musculature is given by Morison (1927). 

 The muscles characteristic of the part of the abdomen involved in 

 respiration are well shown in the third and fourth segments (fig. 17). 

 The dorsal muscles consist of three sets of fibers in each half of the 

 segment, two of which are internal and one external. The internals 

 form a broad median band of fibers {diin) slanting "from in front 

 posteriorly and medially, and a slenderer lateral muscle {dil) ex- 

 tending from in front posteriorly and laterally. The external dorsal 

 is a short muscle (del) arising laterally on the posterior margin of the 

 tergum and extending forward to its insertion on the tip of a lateral 

 tergal apodeme of the following segment. The two sets of dorsals 

 are thus antagonistic, the internal fibers being tergal retractors, and 

 the external fibers tergal protractors'. The ventral musculature com- 

 prises internal and external muscles, which are likewise antagonistic. 

 The internal fibers form an oblique median internal ventral (vim) 

 on each side of the sternum, the two converging mesally in the form of 

 a V, and a slenderer lateral muscle (vil) oblique in the opposite direc- 

 tion. The external ventrals consist of a single small, fan-shaped lateral 

 muscle on each side {vel), arising laterally on the posterior part of 

 the sternum and inserted anteriorly on the lateral anterior apodeme 



