146 ERNEST A. BACK. 



of the wings running downward is a more or less indistinct 

 suture, bending to the rear on the lower portion of the pleura, 

 the mesopleural suture, dividing the mesopleura and the 

 sternopleura before, from the pteropleura and the metaster- 

 num behind. About midway between the dorsopleural suture 

 and the coxae, the mesopleural suture gives off a branch suture 

 running forward, the sternopleural suture, separating the meso- 

 pleura above, from the sternopleura below. Immediately 

 above the halteres and below the scutellum is a rounded area, 

 the metanotum, usually pruinose; before the halters there is 

 an ill defined area, somewhat swollen, and often with a fan- 

 like fringe of hair or bristles, the hypopleura. Between the 

 hypopleura and the mesopleura, and between the insertion of 

 the wings and an indistinct backward directed branch of 

 the mesopleural suture is the pteropleura. The metasternum 

 and the metapleura are ill defined and occupy the remainder 

 of the lower posterior portion of the thorax, and are not re- 

 ferred to in the descriptions. 



The clothing of the thorax presents too numerous variation 

 to receive full attention here. In species of Holcocephala (PI. 

 XI, figs. 5 and 6) it is without bristles or noticeable pile, 

 in some species of Cyrtopogon (PI. VIII, fig. 1) it is rather 

 densely pilose, but in a larger percentage of species there will 

 be found an irregular line of either well-developed bristles or 

 hairs extending from the dorsopleural suture before the in- 

 sertion of the wings backward and upward to above the 

 posterior callosities, on the posterior callosities, posterior mar- 

 gin of the scutellum, and sometimes on the humeral angle as 

 well. Usually when these bristles reach their fullest devel- 

 opment, as in species of Ospriocerus, Microstylum, and Dero- 

 myia, the mesonotum is clothed with very short, more or less 

 microscopic, stiff hair (PI. VII, fig. 5) and the prothorax is 

 strongly armed with bristles. This is not always the case. 

 In species of Stenopogon (PI. XII, figs. 1 to 3) there may be 

 a great development of bristles and hairs upon the lateral 

 and posterior portions; in Lestomyia the bristles are arranged 

 in four rows running longitudinally along the mesonotum from 

 the pronotum to the scutellum; again, the pile may be almost 



