ON A RARE PARASITIC FLY. 31 



2. Note on the Capture (in London) of a rare Parasitic Fly, 

 Hammomyia (Ilylepldla) unilineata Zett. By Lt.-Col. 

 S. Monokton Copeman, F.R.S., M.D., F.R.C.P.,F.Z.S. 



[Received October 14, 1920 : Read February 8, 1921.] 



(Text-figure 1.) 



The exceptional rarity of this fly, of which I have had the 

 pleasure of presenting to the British Museum (Natural History) 

 the only specimens in our national collection, affords reason for a 

 record of its capture in considerable numbers in London (Prim- 

 rose Hill) during the past two years, for some discussion of its 

 nomenclature, and for a brief account of its seasonal prevalence 

 and habits so far as they are at present known. 



In a previous communication I have set out some facts as to a 

 flourishing colony of a burrowing bee (Andrena fulva) on a 

 localized area on the south-western slopes of Primrose Hill, 

 which has been kept under observation over a period of six years, 

 during the latter half of which period the number of individual 

 burrows has, for reasons which were explained in my previous 

 paper, increased to a considerable extent. It was while watching 

 the operations of the bees on an outlying portion of this colony, 

 on May 16, 1919, that, for the first time, I observed and obtained 

 specimens (two in number) of a fly which, from its actions, 

 which I studied carefully for some time, is apparently a parasite 

 of the burrowing bee. 



On the morning (about 10 A.M.) of this day, which was bright, 

 without obvious breeze, I was lying motionless on the grass 

 watching the hees leaving and returning to a collection of burrows 

 on a small patch of bare earth, when my attention was attracted 

 by a homeward-bound bee, which, on approaching its burrow, 

 dived stright down into it, instead of, as usual, hovering around 

 for several seconds before doing so. As it approached, I noticed 

 that it was followed by a couple of flies, of a species unknown to 

 me, which appeared for the moment to be as astonished as I was 

 at its precipitate retreat into its burrow. After what looked like a 

 fight with one another for a few moments over the surface of the 

 ground, one of them followed the bee down into the hole, re- 

 appearing almost immediately, and then again descending into 

 the burrow, this time backwards. Meanwhile, the second fly 

 remained on guard outside. When the first fly at length emerged 

 head foremost, both of the flies were eventually trapped in a 

 match-box, a net not being available at the time. But unfor- 

 tunately one of them managed to escape before they could be 

 taken home and killed. The remaining specimen was despatched 

 at once to Major E. E. Austen, D.S.O., at the British Museum 

 (Natural History), with a request for its identification, as the 

 species was unknown to me. Under the circumstances, it may 



