38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



The dorsal and ventral intersegmental muscles of the prothorax 

 are, respectively, intertergal and intersternal. The dorsal muscles are 

 two large bundles of fibers (fig. 12 A, ^5) arising anteriorly by broad 

 bases on the pronotum (fig. ii F), and tapering somewhat to their 

 posterior attachments on the prephragma of the mesonotum (iPh). 

 The contraction of these muscles would appear to depress the pro- 

 notum in the bee, though in Vespula two pairs of corresponding muscles 

 are regarded by Duncan (1939) as retractors of the mesonotum in 

 opposition to the dorsal muscles of the mesothorax. The ventral in- 

 tersegmental muscles (fig. 12 C, ^2) have their anterior attachments 

 on a pair of small lobes of the' apophyseal components of the pro- 

 thoracic endosternum (fig. ii H, o), and their posterior attachments 

 on two small anterior tonguelike processes of the supraneural bridge 

 of the pterothoracic endosternum (figs. 22 C, 24 A, 52). These 

 muscles are evidently retractors of the propectus. 



The muscles of the propleura comprise tergal and endosternal 

 muscles inserted on the anterior parts of the episterna, and tergal 

 muscles inserted on the extremities of the pleural apophyses. The 

 largest of the propleural muscles (fig. 12 A, 48) arise close together 

 on the posterior inflection of the pronotum and diverge forward above 

 the dorsal muscles of the head to the anterior ends of. the horizontal 

 apodemes of the episterna, A second, slenderer pair (46) arises 

 laterally on the prephragma of the mesonotum and converges an- 

 teriorly to the cervical apodemes (d). Inserted close to the latter 

 muscles on each side is another small muscle {4/), which may be a 

 branch of 48 since its fibers appear to follow those of this muscle 

 from the pronotum. A fourth pair, two large sternopleural muscles 

 (fig. 12 B, 57), arises from the long, straphke apodemes of the 

 supraneural bridge of the endosternum (fig. 11 E, n), and diverges 

 forward to the cervical apodemes of the pleura (fig. 12 B, d). The 

 tergopleural muscles inserted on the pleural apophyses include two 

 fan-shaped muscles on each side (fig. 12 C, 4p, jo) arising anteriorly 

 on the side of the pronotum, and converging posteriorly to the upper 

 extremity of the apophysis. These muscles clearly are protractors 

 of the propectus, their antagonists being the ventral intersegmental 

 muscles (5^). 



The strength of the anterior pleural musculature of the prothorax 

 suggests that movement of the pleural plates has some important 

 purpose. The head, being articulated on a transverse axis between the 

 occipital processes of the episterna (fig. ii B), can have only a tilting 

 movement on the thorax by the action of its own levator and depressor 

 muscles ; but the pleural plates, by reason of their lack of fixity on the 



