PSILOPUS. 231 



neglect to observe that this picture, at least in many species, 

 varies a great deal not only in intensity, but also in extent ; use- 

 ful and rather reliable are the characters based upon the coloring 

 of the feet, but of course then only, when the sex is stated, as in a 

 good many species the feet of the females are much paler than 

 those of the males. 



The genus derives its name (from 4-aoj, slender, and novs, foot) 

 from the great sleuderuess of the feet, peculiar to all the species. 



The species of Psilopus are numerously represented in all parts 

 of the world. 



Say has described several North American species of Psilopus. 

 Wiedemann has added a few more. His descriptions, even if 

 judged with leniency, will be found very unsatisfactory, as they 

 relate merely to differences in color, and often do not even state 

 to what sex the specimen belonged ; some of them can be applied 

 to whole series of closely allied species. Mr. Macquart's and Mr. 

 Walker's descriptions are not much better ; those especially which 

 the latter published in the Diptera Saundersiana are remarkable 

 for their entire uselessness, so far that one may be sure not to find 

 in them precisely those data which are indispensable for the re- 

 cognition of the species of Psilopus and for their distinction from 

 each other. 



The insufficiency of the existing descriptions renders the deter- 

 mination of the species very difficult ; generally we reach only 

 possibilities, sometimes probabilities, very seldom certainty. . As 

 I am able to identify only a small number of my species with 

 those which have been described before, I deem it necessary to 

 give here the results obtained from the comparison of the species 

 in my possession with the descriptions of the previous authors, 

 and for this purpose I shall enumerate these descriptions one 

 after the other. 



The species heretofore published are the following : 



1. longicornis Fair. Indigenous to the American islands ; it has been 

 described by Fabricius, and afterwards again by Wiedemann, from 

 a specimen in the collection of Fabricius ; the sex was not stated, 

 but Fabricius's expression " cauda uncinata" shows that it was a 

 female ; in the description of Wiedemann the following available cha- 

 racters are found : the face only little dusted with white, the basis 

 of the abdominal segments black, wings without dark picture, hal 

 teres yellow, feet black, fore tibiae luteous. These characters agree 



