NO. 2 HONEY BEE SNODGRASS 6l 



of the scutum has become necessary for carrying out the mechanical 

 function of the notum. The scutal clefts of the Hymenoptera in any 

 case, are simply extensions of the usual lateral emarginations of a 

 wing-bearing notum that are bridged by the first axillary sclerites of 

 the wing bases. 



The opening of the scutal clefts by pressure on the back causes a 

 posterior displacement of the postphragma in the propodeum because 

 of the connection of the phragma with the scutellum (fig. i6 C), and 

 this movement of the phragma must stretch the longitudinal muscles 

 preparatory to their succeeding contraction. The contraction of the 

 longitudinal muscles (//), extending from the anterior notal plate 

 to the phragma, not only closes the lateral clefts of the notum, but 

 causes the edges of the posterior plate to slide beneath those of the 

 anterior plate, thus increasing the curvature of the back to give greater 

 efifectiveness to the downstroke of the wings. The small propodeal 

 muscles (pd) attached on the back of the phragma may be supposed 

 to contribute to the movement of the phragma, though their narrowed 

 and convergent posterior ends (fig. 27 C, q6) would suggest that 

 they pull on the propodeum. The function of the pair of delicate 

 muscles in the mesothoracic scutellum (fig. 16 D, 70) is not clear. 



The metathorax contains no muscles corresponding in function 

 with the dorsal muscles of the mesothorax, since the only interseg- 

 mental dorsal muscles that can be referred to the metathorax are the 

 greatly displaced pair between the postphragma and the propodeum 

 (fig. 16 C, pd). The dorsal muscles of the mesothorax (//), there- 

 fore, must cause the downstroke of both pairs of wings when the 

 wings of each side are connected. Probably in the same way the 

 dorsoventral muscles of the mesothorax are largely responsible for 

 the upstroke of the metathoracic wings, since as long as the wings are 

 connected pressure on the mesonotum alone lifts both pairs of wings, 

 though it has no efifect on the hind wings when the latter are un- 

 hooked. However, the metanotum itself is amply provided with de- 

 pressor muscles, which very probably in the living bee play an im- 

 portant part in the elevation of the wings. 



In each side of the metathorax there are three large muscles at- 

 tached on the notum, which arise ventrally on the metathoracic arm 

 of the pterothoracic endosternum. The first two of these muscles 

 (fig. 21 F, G, P7, p8) arise by broad bases on the endosternal arm 

 and taper upward to their dorsal attachments on the lower edge of 

 the semidetached lateral plate of the notum (E, H3). The third and 

 largest muscle (F, pp) arises from a small lobe (c) of the endosternal 

 arm, and its fibers spread upward to their attachments on the lateral 



