CHARACTERS OF THE GROUP: LARVA. 9 



like over the oral orifice. The maxillit' (PI. Ill, fig. G), which are 

 situated below the mandibles and nearer the center of the mouth, 

 arc useful also in separating the larva? of the different species, though 

 the characters are difficult to see. The labium is heavily chitinized, 

 and presents, in the toothing of its anterior margin, characters which 

 appear to be constant and of a more accessible and a more easily 

 appreciable nature than either the teeth of the mandibles, the sliape 

 and bristling of the palpi, or the structure of the antenna?. This 

 organ (PI. Ill, fig. 3) presents in the two species of Prosimulium 

 {hirtipes and pecuarum), which have been described in the larval 

 stage, a variation in the structure of the teeth which is clearly dis- 

 tinguishable from that of any of the larvae of the genus Simulium 

 with which I am acquainted. In these two species the central tooth 

 is trifid, and the other teeth are irregularly bifid, or at least not 

 simple as in the species of SlmuliuTn. Johannsen, on plate 35 in 

 his work on this group, figures the labium of a species from Leland 

 Stanford, Jr., University campus, which very probably is that of 

 a Prosimulium. It is generally unsafe to accept distinguishing 

 generic characters of this nature without an extended examination 

 of a large amount of material, and I have not used this in my dif- 

 ferentiation of these genera, but it is obvious that it is permissible 

 to indicate its existence and possible significance. 



In addition to the mouthparts, already indicated as of importance 

 in the separation of the species, it will, I believe, be found by careful 

 examination that the other parts with which I have not dealt in this 

 paper supph^ characters of very considerable importance. On the 

 ventral surface of the first segment is an elongated, rounded, and 

 sliglitly conical proleg, the apex of which is furnished with a number 

 of rows of hooks. This proleg is used by the larva in its movements 

 from place to place in much the same manner as the larvae of the 

 Geometridse use the legs on their anterior segments, and their method 

 of progression is somewhat similar to the well-known " loopers " of 

 this family of moths. Allien disturbed, the larva releases its hold 

 on the rock or other surface and floats down stream attached by its 

 proleg to a silk thread which it emits from the mouth, regaining 

 its former position by means of this thread. At the anal end is a 

 sucker-like disk, around the margin of which are arranged concentric 

 circles of minute hooks similar to those at the apex of the proleg. 

 It is by means of this anal process that the larva attaches itself to 

 the silk threads which it spins on the surface of rocks or plants in 

 the bed of the stream. On the dorsal surface of the last abdominal 

 segment there is a slit-like opening from which the blood gills are 

 projected. (PI. VI, fig. 6.) These are retractile and vary in form 

 in the different species. 



