AMERICAN DIPTERA. 143 



composed of one or two segments; labella not fleshy. Ab- 

 domen composed of eight distinct segments, only rarely 

 apparently of six; the hypopygium usually prominent. Legs 

 strong, bristly, of moderate length, rarely somewhat elongated, 

 tarsi usually strong; empodia bristle-like, the pulvilli rarely 

 vestigial. Tegulse small. Wing when at rest lying parallel 

 over the abdomen; basal cells long; two or three submarginal, 

 and five, rarely four, posterior cells present; first and fourth 

 posterior cells and the anal cell open or closed. 



EXTERNAL ANATOMY. 



Head. — The hypognathous head is moderately large, usually 

 broader than high, and quite disciform but sometimes as in 

 Stenopogon and Ospriocerus much thicker and about as high 

 as broad. The large compound eyes extend from the top of 

 the head to the oral margin and occupy from two-thirds to 

 nearly all (Leptogaster) the anterior aspect of the head, and 

 extend over the sides to the occiput so that the cheeks are 

 greatly, or quite, reduced. The eyes may be equidistant 

 throughout, or widely divergent above and below (Townsendia 

 and Stichopogon) , more or less contiguous above only in 

 Plesiomma. The faucets may be scarcely or considerably 

 larger in the central region adjoining the facial margin. 



The face in profile may be slightly convex (Habropogon), 

 straight and hardly perceptible (Taracticus) , convex (Micro- 

 stylum), but very frequently more or less gibbose, the gib- 

 bosity extending evenly up in the front (Ceraturgis) , greatest 

 in the middle (Myelaphus), or confined to the oral margin 

 (Psilocurus) ; an antennal protuberance (Dioctria, Myelaphus, 

 and Dicolonus) sometimes occurs to make the profile more 

 complex. The face varies in width from proportionately very 

 broad (Lyphystia and Holcocephala) to very narrow (Lepto- 

 gaster); the facial orbits may be parallel or nearly so (Tarac- 

 ticus) or divergent below (Stichopogon) . The oral margin, the 

 lower edge of the face, is nearly straight or well arched, dome 

 shaped, according to the absence or presence of a facial con- 

 vexity. 



TRANS. AM. ENT. SOC, XXXV. APRIL, 1909. 



