40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



front of the mesothorax, its connection with the latter is for its own 

 security, since the pronotum serves principally for the attachment of 

 prothoracic and head muscles, and takes no part in the pterothoracic 

 mechanism. The pterothoracic segments, on the other hand, are 

 intimately united to form a mechanical unit supporting the wings, 

 the first and second legs, and the abdomen. Between the third ptero- 

 thoracic segment, known as the propodeum, and the abdomen is a 

 deep constriction, which allows the abdomen to be freely movable on 

 the thorax. 



On the dorsum of the pterothorax the segmental plates are well 

 defined, and behind the pronotum (fig. 15 A^i), the mesonotum (A^2), 

 the metanotum (A'^s), and the projwdeal tergum (IT) can readily 

 be identified. On the venter (fig. 16 F) the sternal areas of the 

 mesothorax (S2) and the metathorax (^3) are separated by a distinct 

 groove between the bases of the middle legs, and the propodeal 

 sternum (IS) is an independent sclerite behind the third legs, but 

 laterally there is no demarcation between the sternal and pleural areas 

 in either the mesothorax or the metathorax (fig. 15). The structure 

 of the pleural region of the pterothorax is difficult to interpret, be- 

 cause neither the grooves nor the areas distinctly set off by the latter 

 appear to conform with the pleural pattern of generalized insects. 



The mesonotum. — The back plate of the mesothorax (fig. 15, N2) 

 covers the strongly convex upper and anterior parts of the ptero- 

 thorax, and is completely divided by a transverse fissure (sf) into two 

 plates. For practical description the two plates are commonly called 

 the "scutum" and the "scutellum," but the line of division does not 

 conform with the sulcus that separates the scutellum from the scutum 

 in generalized insects. The true scutoscutellar sulcus in the bee is 

 marked internally by a strong, arched notal ridge (fig. 16 D, NR), 

 which on the middle of the back is coincident with the groove between 

 the two mesonotal plates, but laterally follows a groove (fig. 15, ns) 

 that curves posteriorly and sets off before it on each side an elongate 

 oval or lenticular area (set) which properly belongs to the scutum. 

 The mesonotum of the bee and of related Hymenoptera, therefore, is 

 secondarily divided into two major parts without regard to the original 

 differentiation into scutum and scutellum by the external sulcus (ns) 

 of the notal ridge. The true scutellum forms the prominent swelling 

 on the highest part of the back (Sclz) and tapers laterally to the 

 posterior angles of the mesothoracic wing bases ; the scutum (Sct2 and 

 set) covers the entire anterior rounded surface of the back between 

 the notal sulcus (ns) and the pronotum, and supports the greater 



