rsiLorus. 



229 



pale-yellow ; tarsi towards the tip only very little darker, but their 

 small last joint more or less distinctly infuscated, especially that of 

 the fore tarsi. Cilia of the tegulae whitish. Halteres white-yel- 

 lowish ; their knob infuscated on its lower half. Wings hyaline, 

 only a little tinged with grayish ; the last segment of the fourth 

 longitudinal vein is only very gently inflected forward from its 

 middle, is in general somewhat distant from the third longitudinal 

 vein, and approaches it, especially at its end, less closely than is 

 the case in the preceding species. 

 Hob. Middle States. 



Gen. XLIII. PSILOPUS. 



The genus Psilopus can be easily distinguished from the re- 

 lated genera by its peculiar slender structure, the slenderness of 

 its feet, the broad and excavated vertex and the peculiar neuration 

 of its wings. The peculiarity of the neuration consists, besides the 

 great proximity of the posterior transverse vein to the margin of , 

 the wing, especially in the structure of the fourth longitudinal 

 vein ; this vein either does not reach the margin of the wing at 

 all, or becomes extremely thin before it reaches it ; at the same 

 time it emits anteriorly a robust branch, which bends forward 

 either in a smooth or in an angular curve and ends in the neigh- 

 borhood of the third longitudinal vein into the margin of the 

 wing ; strictly speaking, this anterior branch is the real continua- 

 tion of the fourth longitudinal vein ; its apparent continuation 

 beyond the origin of this branch is an adventitious appendage ; 

 and that such is really the case, is proved by those exotic species, 

 where this appendage is entirely wanting. 



The species of Psilopus exhibit in the structure of the head 

 and of its parts, especially in the antenna?, as also in the structure 

 of the feet, of the wings, and of the male organs of copulation, 

 numerous plastic differences, which may easily mislead to the for- 

 mation of smaller genera. The greater part of these characters are 

 merely ornaments of the males and exclusively specific distinctions, 

 so as to be of little use for the definition of smaller genera ; this 

 applies even to the most striking among the other characters, as, 

 for instance, to the either dorsal or apical position of the arista, 

 the either very short or very long pubescence of the second joint 

 of the antennae &c. ; these also afford no sharp limits, and there 



