10 AMEEICAN BLACK FLIES OR BUFFALO GNATS. 



A more extended description of the Itirva, with a consideration of 

 the uses of the various organs, is given by Johannsen (Bui. G8, N. Y. 

 State Museum, 1903, pp. 345-350), but in the present paper it is only 

 considered necessary to indicate the position of the various organs 

 used in the descriptions. 



The larval stage occupies about four to five weeks in summer, but 

 cold weather retards growth, and some species are known to pass the 

 Avinter in this stage. When full grown the larva spins a cocoon, in- 

 side of which it transforms to the i3upa. These cocoons are differ- 

 ently shaped in some of the species. In the case of fictipes the cocoon 

 somewhat resembles a slipper in shape (PI. VI, fig. 8), in venustum 

 it is more like a conical pocket, while in hirtipes it can hardly be 

 called a cocoon, being simply an aggregation of threads, the work of 

 many individuals congregated together on the rock surface, amongst 

 which the larvae transform to pupa?, and by which they are more or 

 less covered. 



PUPA. 



The principal character by means of which the pupse of the differ- 

 ent species may be distinguished is the number of respiratory fila- 

 ments (PL IV, figs. 1-6; PI. VI, figs. 4, 5). These organs are situ- 

 ated on the anterior portion of the thoracic dorsum and consist of 

 from 4 to 60 branches. The number is generally constant in each 

 species, but when there is a very large number of filaments it happens 

 occasionally that there may be a slight variation in numbers in indi- 

 A'idual si^ecimens. In addition to these respiratory organs as a 

 means of identification, there are situated on the abdomen a number of 

 small hooks on some of the dorsal and also on some of the ventral 

 segments, wdiich serve as a means for the retention of the pupa in its 

 cocoon and which vary in number in the different species. 



IMAGO. 



In the male the eyes are very large, coming closely together above 

 and leaving no distinct frontal stripe ; the upper half has the facets 

 very much larger than the lower, from which they are distinctly 

 divided by a horizontal line. The female has the eyes distinctly sepa- 

 rated above, from antennae to vertex, by a more or less posteriorly 

 divergent-sided frontal strijoe, and the eye facets are of almost uni- 

 form size, showing more or less tendency to become larger toward 

 the underside in some species. The ocelli are absent in both sexes. 

 The antennae (PI. VI, fig. 2) are 11-jointed (various authors have 

 given them as 10-jointed), the basal joint generally short, the second 

 elongated and separated by a more distinct constriction from the 

 third than exists between any of the other joints, the third joint gen- 

 erally subequal in length with the second. The palpi (PL V, fig. 9) 



