NO. 2 HONEY BEE — SNODGRASS 43 



be described later. The metanotum is separated 'from the propodeum 

 by a prominent groove (fig. 15, I), which forms a low ridge internally, 

 but there is no metathoracic postphragma. 



The propodeum. — The dorsal region of the propodeum (fig. 15, 

 IT) is a large, strongly convex plate covering the posterior part of the 

 pterothorax, containing the spiracles of its segment laterally, and 

 posteriorly descending steeply to the petiole of the abdomen. This 

 plate, commonly termed the "propodeum," is really the propodeal 

 tergum, the sternum of the propodeal segment being a narrow ventral 

 plate behind the bases of the third legs (fig. 16 F, Z^") . 



The pterothoracic pleura. — The pleuron of a wing-bearing segment 

 in most insects is differentiated into an anterior epistermtm and a 

 posterior epimeron by a groove, the pleural sulcus, which extends 

 upward from the coxal articulation to the wing base, and forms in- 

 ternally a strong pleural ridge, which bears an apodemal process, or 

 pleural apophysis, associated with the extremity of the endosternal 

 arm of the corresponding sternum. In the bee this structural pattern 

 of the pleuron is obscured in both the m.esothorax and the metathorax. 



The mesopleural area of the honey bee is well defined anteriorly 

 and posteriorly by deep grooves separating it from the prothoracic 

 notum in front and the metathoracic pleuron behind (fig. 15), its 

 "free" upper margin borders the subalar membrane, ventrally, how- 

 ever, as already noted, the lateral sclerotization of the segment is 

 continuous to the midline of the venter (fig. 16 F), and there is noth- 

 ing to mark the transition from pleuron to sternum, except the 

 ventral attachment of the huge dorsoventral muscle of the meso- 

 thorax (fig. 16 C, 72), which is assumed to arise on the sternum. 



The upper part of the mesopleuron is crossed by an obliqiie groove 

 (fig. 15, PIS2), which appears to be the pleural sulcus, separating a 

 small upper epimeral area (Epm) from a large lower episternal 

 area (Eps). The groove contains near its posterior end a conspicuous 

 pit (fig. 17 A, pla) that forms internally a small pleural apophysis 

 (B, PIA) intimately associated with the mesothoracic arm of the 

 pterothoracic endosternum (G, PIA, SAo). Furthermore, the line 

 of the posterior part of the groove is marked internally by a strong 

 ridge (B, PIR) bearing the apophysis. These several features almost 

 certainly identify the oblique groove (A, pis) as the true pleural 

 sulcus of the mesothorax. Beyond the apophysis the sulcus merges 

 with the intersegmental groove (fig. 15, i) that runs downward to the 

 pleural articulation of the middle coxa. A second groove of the 

 pleuron (fig. 17 A, e) arises near the middle of the pleural sulcus, 

 runs upward and posteriorly through the epimeral region, cutting the 



