1912.] 171 



account of its reddish thorax. From its nearest aUies it may readily 

 be known by this same reddish thorax, red antennae, and yellow legs, 

 but still more certainly by the collection of bristles under the 



hypopygium. 



Longicostalis, sp. n. This also belongs to the same group as the 

 preceding species, but to that part of it which has the halteres yellow. 



9 . Thorax and abdomen black ; frons scarcely shining, about } broader 

 than long, snpra-antennal bristles rather weak, the upper pair very closely 

 approximated and the iinder vei-y much smaller and nearly directly underneath 

 them, palpi narrow, dusky yellow and bristled as usual ; wings rather deep 

 yellowish brown, costa remarkably long — about | the wing length, fringe also 

 very long, 1 rather longer than 2 and distinctly shorter than 2 + 3, angle at 

 fork moderate, 1st thin vein leaves well beyond the fork with a moderate curve ; 

 legs yellowish brown, margins of hind femora blackish, hind tibiae practically 

 bare, though vender a Zeiss' lens numerous small cilia become visible. 



Its singularly long costa, longer than in any other species I am 

 acquainted with in this large genus, sti'ikes the eye at once, and leaves 

 its identification in no doubt. It is the species discovered by 

 Mr. Donisthorpe in a nest of the ant, Lasius fiiliginosns, at Darenth 

 Wood (The Entomologists' Record, Vol. LI, Nos. 10, 11, and 12). 

 Another female has also been obtained by the same observer, which I 

 have seen, and which bore the label Whitsand Bay, April, 1907. And 

 still another female I have taken myself, March 23rd, 1910, from under 

 a dead mole in Stoke Wood. 



Section D. 



In the four species commencing with tarsalis and coming under 

 the numbers 46 — 51, no reference was made to the halteres. In all of 

 them the colour of these organs is yellow. 



The male hypopygium. We shall, I think, gain a better under- 

 standing of the organ if we look upon it as consisting of two parts, an 

 internal and an external one, the former carrying the outlet of the 

 digestive system and the latter that of the generative, and each as 

 representing a separate segment. The external portion or shell — the 

 part that chiefly concerns us here — is made up of two pieces, a large 

 (as a rule) dorsal plate, which may roughly be compared to a saddle in 

 which the dorsum is the seat, and the sides, the panels, and a small (as 

 a rule) under or ventral plate. Between them they commonly form a 

 chamber within which in ordinary circumstances lie concealed the 

 generative organ and its complex appendages or, as I have called them 

 in these notes, the subanal body. 



