34 ON A RARE PARASITIC FLY. 



with mine, should be known as Hammomyia unilineata Zett. 

 rather than H. {Cjhortophila) haccata Fallen. 



Subsequently Mr. Collin wrote to me that he possessed several 

 continental specimens of H. huccata Pin., including a pair so 

 named by Zettsrstedt himself, aiid that he was of opinion that 

 these were abundantly distinct from the British species, which is 

 undoubtedly Hammomyia {Hylejihila) unilineata Zett. Further- 

 more, he kindly directed attention to the fact that in describing 

 ttnilineata Zetterstedt wrote " Feminse ad nidis Andreenaruui 

 inventsB, in quarum larvis larvae liujus speciei Ariciae verisimiliter 

 parasitje vivunt observante D. Professore Wahlberg." Appa- 

 rently, therefore, huccata Fallen requires confirmation so fai' 

 as our British fauna is concerned, as Mr. Collin is convinced that 

 he has never seen a British specimen of the true haccata. 



Seasonal Prevalence. — As stated above, my first specimens of 

 this fly were captured on Primrose Hill on 15th May, 1919. On 

 the same circumscribed area of ground I caught ten more speci- 

 mens on May 19th, ten on May 20th, and seven on May 21st. 

 Then, after an absence from town of several days, a further seven 

 flies were caught on May 27th and three on May 29th — at 

 which date I had again to leave home for about a fortnight. 

 Just before my retui-n the fine weather gave place to rain, 

 accompanied by a considerable fall in temperature, with the 

 result that, subsequently, neither bees nor flies were to be found, 

 notwithstanding careful search on several successive days. 



It may be mentioned that, in 1919, Andrena bees were first 

 seen on May 6th, a warm and sunny day, when a number of 

 males, which always emerge before the females, were flying 

 about; while females were not found, with the exception of one 

 or two solitary specimens, until several days later. So that, as 

 might be expected in view of what is known as to the parasitism 

 of the fly in question, it is obvious that the seasonal prevalence 

 of the female of both fly and bee tallies closely. 



In consequence, doubtless, of the abnormal meteorological con- 

 ditions during the presentyear( 1920), both bees and flies appeared 

 at an earlier date than in 1919, the first flies, three in number, 

 having been obtained on May 11th. Careful search was made 

 for male specimens of the fly, but again without success. By 

 the end of May both bees and flies had disappeared. 



It will be noted that, with some exceptions, the specimens of 

 the fly in Mr. Collin's collection were captured in the month of 

 May — the month during the latter half of which, as my obser- 

 vations on Primrose Hill, extending over a period of sevei'al 

 years, have shown, the female of the bee Andrena falva is 

 more particularly prevalent. 



