202 DIPTERA OP NORTH AMERICA. [PART II. 



sixth almost entirely obliterated or existing only as but a short 

 rudiment. 



In the genus Liancalus we have already mentioned the charac- 

 ters which this genus has in common with Scellus and Hydro- 

 phorus, as also those which distinguish Scellus from Liancalus ; 

 to the latter may be added the presence of the two appendages, 

 peculiar to the males of Scellus. The presence of these append- 

 ages also distinguishes the species of Scellus from Hydrophorus, 

 where they are entirely wanting. Moreover the under side of the 

 fore femora and of the fore tibiae in the species of Scellus is beset 

 with long spines, catching into each other when the knee is bent, 

 while in the species of Hydrophorus there are at the utmost some 

 spine-like bristles on the under side of the fore femora near the 

 basis, otherwise the under side of the fore femora and of the fore 

 tibiae is only beset with very short thorn-like bristles. 



The name of the genus (from oxfk-kos, with crooked feet) has 

 reference to the peculiar structure of the fore feet. 



I know as yet only six species of Scellus, of which three are 

 peculiar to North America, one is common to Europe and North 

 America, and two are exclusively European ; one of the latter 

 species, occurring in Sweden, is as yet undescribed. 



Table for the determination of the Species. 



, ( Wings entirely blackened. 1 exustus Walk. 



( Wings not entirely blackened. 2 



o ( Apical half of the wings black. 2 spinimanus Ztt. 



\ Apical half of the wings not black. 3 



o ( Wings entirely tinged with blackish gray. 3 avidus, n. sp. 



I Wings scarcely a little tinged with gray. 4 filifer, n. sp. 



Description of the Species. 



1. S. extistus Walk. % and 9 • — Thoracis dorso aeneo-nigro opaco, 

 abdomine cnpreo, latera versus viridi, nitidissimo, halteribus nigris, 

 alis nigricantibus adversus costam nigris, lamellis analibus maris albis, 

 in basi nigris, apicem versus flavis, in summo apice puncto nigro notatis. 



The upper side of the thorax bronze-black, opaque ; the abdomen copper- 

 colored, laterally green, very bright ; halteres black ; wings blaokish, 

 towards the fore margin entirely black ; the anal appendayvs of the male 

 are white, near the root black, towards the tip yellow, at the extreme tip 

 with a black spot. Long. corp. 0.22. Long. al. 0.26. 



