20 MISC. PUBLICATION 631, U. S. DEPT. OF AGRICULTURE 



The prosternal, propleural, mesopleural, pteropleural, sternopleural, 

 and hypopleural bristles bear the names of the sclerites on which 

 they are situated. In addition to the posterior row of mesopleurals, 

 that sclerite sometimes also bears one or more bristles below the 

 mesothoracic spiracle, the stigmatic bristles (st). The humerus and 

 notopleuron may bear humeral (Kb) and notopleural (npb) bristles 

 respectively. 



The arrangement of the bristles of the mesonotum and scutellum 

 is more complicated. The mesonotum, when its complement of bristles 

 is complete, bears four rows on each side of the median line; these 

 bristles are, from the innermost outward, the acrosticals (acr), dorso- 

 centrals (dc), intra-alars (ial) , and supra-alars (sal). These may 

 be designated, according to their position, as presutural or post- 

 sutural; the acrosticals immediately in front of the scutellum are the 



acr 



msnt 



Figure 2. — Diagrammatic drawing of the thorax of a muscoid fly: A, Side view; 

 B, right half, dorsal view. For explanation of abbreviations, see pages 19^-20. 



prescutellars. The bristles of the postalar callus are the postalars. 

 (pal). The scutellum may bear a row of lateral scutellars (Is), not 

 necessarily all on the same horizontal plane ; a pair, usually small, of 

 apicoscutellars (as) ; and one or more pairs of discoscutellars (ds). 



The Comstock-Needham system is used in the terminology of the 

 wing venation (figs. 3 and 4). There are eight principal longitudinal 

 veins. The first, or costa (<?), forms the anterior margin of the wing 

 but usually disappears near the wing apex. The second, or subcosta 

 (sc), is for practical purposes unbranched, but is usually connected to 

 the costa by the humeral cross vein ( h ) . The third, or radius ( r ) , in the 

 primitive wing branches dichotomously to form the branch r x and 

 the radial sector (rs), the latter branching dichotomously and its 

 branches in turn doing the same, to form veins r 2 , r 3 , r 4 , and r 5 , respec- 

 tively. The fourth, or media (m), is hypothetical^ four-branched, 

 though for practical purposes it may be considered three-branched 

 (m 1; m 2 , and m< 3 ), the branching taking place dichotomously. The 

 fifth vein, or cubitus (cu) , divides into two branches, cu r and cu 2 . The 

 three remaining veins, all unbranched, are the first, second, and third 

 anal veins. The first of these, when present, is usually represented 

 merely by a fold ; so the apparent first in most wings is actually the 

 second. In addition to the humeral, there are several cross veins, 



