52 DIPTERA OF NORTH AMERICA. 



segments. The latter, moreover, have some peculiarities in the 

 structure and neuration of their wings, which, however, allow of 

 no very sharp limitation. Most frequently a dilatation of the 

 second basal cell and of the space between the third and fourth 

 longitudinal veins (in consequence of which the second longitudinal 

 vein is poshed towards the costal margin) and the posterior angle 

 of the anal cell extended in a long point, are the most striking 

 peculiarities in the structure of the wings. But a sharp limitation 

 of the two groups is perhaps not to be urged too much, as transi- 

 tions from the one to the other are certainly not wanting, and 

 another division quite as useful seems to be possible. In the mean 

 time the groups Trypetina and Ducina, as we have defined them, 

 are characterized as well as our purpose requires. We have now 

 to fix the relation of the family Trypetidce to the nearest families, 

 particularly to that of Ortalidce, since almost all authors have mixed 

 the species of these two families. For this purpose we want only 

 to find out the natural character of the family, which cannot be 

 obtained but from a close examination of the greatest possible 

 number of species and from a careful appreciation of the system- 

 atic value to be assigned to the observed peculiarities aud differ- 

 ences. 



3. Nalt/ral character. 



After examining nearly 300 species from different parts of the 

 world, I believe I may speak as follows about the organization of 

 the Trypetidce. 



The bare eyes, in both sexes, are separated by the front, which 

 is of equal breadth or only a little narrowed anteriorly. The mid- 

 dle of the front is not sharply separated from the lateral lists, but 

 has often a different color. The front is even, usually with an 

 almost microscopic, rarely with a longer pubescence, sometimes it 

 is totally bare. On its vertical border it always bears two very 

 strong bristles, rather distant from each other. Two short callo- 

 sities, usually little perceptible, run from them, converging bat 

 faintly anteriorly, and bearing one or two bristles directed upwards. 

 On the vertical border itself there are two bristles, each near the 

 upper angle of the eye, and in the middle behind the ocelli there 

 is another pair of bristles, sometimes very stunted. This is also 

 the case with a pair of bristles directed anteriorly, and inserted be- 

 tween the ocelli. More anteriorly on the front there arc. at each side 



