120 DIPTERA OF NORTH AMERICA. [PART II. 



shorter than the second. Hypopygium entirely disengaged and 

 pedunculated ; its exterior appendages are lamelliform, the inte- 

 rior ones much less developed than those of Hypophyllus. 



The species of Haltericerus are related to the species of the 

 genera Hypophyllus and Hercostomus ; they differ from them 

 sufficiently by the rudimentary second joint of the antennae and 

 also the structure and position of the arista. They have also 

 some resemblance to the species *of the genus Systenus, the first 

 antennal joint of which, however, has no hairs on the upper side 

 and the arista is distinctly apical in both sexes. 



The name of the genus (from dxr^p, the poiser, and xspas the 

 horn) has reference to the remarkable structure of the arista of 

 the male. 



As yet, only the three above named species of Haltericerus, 

 which belong to the Fauna of Southern Europe, are known. 



Gen. XII. mOSTRACUS. 



The North American species, for which I have established this 

 genus, resembles Thinophilus. In the structure of the abdomen it 

 reminds me of Aphrosylus, with which it also agrees in the struc- 

 ture of the hypopygium. It differs from both of these genera by 

 the distinct hairs on the upper side of the first joint of the antenna?. 



The following may be considered as the characters of the pre- 

 sent genus : Face in both sexes broad ; the palpi in the male of 

 extraordinary size, in the female much smaller, and in both sexes 

 loosely recumbent upon the proboscis. Antennae small ; first joint 

 hairy ; second joint transverse ; third joint extremely small, dis- 

 tinctly covered with hairs, and with a dorsal bristle. Thorax, 

 scutellum and feet only with short and very scarce bristles. Ab- 

 domen with short hairs, without any longer bristles before the in- 

 cisures. First joint of the hind tarsi without bristles, considera- 

 bly longer than the second. Wings long and narrow ; the poste- 

 rior transverse vein somewhat close to the margin of the wing ; 

 the last segment of the fourth longitudinal vein without any 

 flexure, but only a little curved, converging somewhat towards 

 the third longitudinal vein and ending beyond the tip of the wing. 

 The fifth segment of the abdomen of the male is rather narrow ; 

 the small sixth segment partially concealed under the former ; the 



