1916.] 19 



flies : on the upper sides of the projecting cross-pieces of wood between the panes 

 the flies rested in masses, literally crawling over each other, while all the part of 

 the ceiling near the Avindow was almost as thickly covered as the window itself. 



Samples of the swarm were submitted to Mr. C. G. Lamb, who reported as 

 follows on its composition : — The two principal constituents were a little yellow 

 Chloropid, Chloropisca ornata Meigen, and a slightly larger, dull greyish 

 Anthomyiid, probably Spilogaster sp.,* but not determinable with exactitude, 

 since all the specimens collected were females. About a week before my visit a 

 large sample of the swarm had been sent to the Cambridge Museum, and the 

 Chloropisca was then found to be a very big factor in its composition. But on 

 October 15th this species seemed to be proportionately less numerous, so that 

 while the Chloropisca, was still much in evidence, the Spilogaster appeared to be 

 the dominant component. Mingled with these small insects several larger 

 species were also present, thoiigh in comparatively small quantity ; Musca 

 corvina, Pollenia rudis, Calliphora, various other Anthomyiids, as well as minute 

 Chalcidids and a few wasps. A remedial measure suggested by Dr. Shipley 

 was carried out with success. The window-panes were painted with a transparent 

 mixture composed of commercial glucose and glycerine. The flies on the windows 

 were soon caught, while those on the ceiling were purposely distiu'bed, and, 

 making straight for the light, immolated themselves in large numbers. 



An interesting part of the matter relates to the reiterated occiu-rence of 

 the swarm in these rooms, and I am much indebted to Mrs. Adeane for supply- 

 ing me with the following particulars. The invasion has taken place for at 

 least five or six years in succession, varying somewhat in intensity, and being 

 especially large this year, but always reaching its maximum, not in the ground- 

 floor, but in the first-floor room. Apart from these immense hosts, which may 

 be definitely called " swarms," Mrs. Adeane tells me that there has been a great 

 quantity of flies in the sunny rooms every year, from at least as far back as 1888 

 onwards, the largest number always being found in this same first-floor room 

 Rooms facing south-west are infested by considerable numbers of flies, but not 

 by anything to be described as a swarm ; while in the rooms facing north-east 

 flies are scarce. The house is surrounded only by lawn and gardens ; rubbish- 

 heaps, etc., ai'e at a distance and on the opposite side to the windows 

 frequented by the fly-myriads. Dr. Sharp remarks {loc. cit.) -. "No clue what- 

 ever can be obtained as to their origin ; and the manner in which these flies are 

 guided to a small area, in numbers that must be seen to be believed, is ijiost 

 mysterious. These swarms always occur in the autumn, and it has been 

 suggested that the individuals are seeking winter quarters." I am not aware 

 that any other explanation has since been put forward. 



It will be noted that among the invading hosts of flies was a small nvimber 

 of examples of Pollenia rudis. Though relatively scarce in the case under 

 review, yet this species does, by itself, produce immense swarms at certain times 

 and places. Its general habit of entering dwelling-houses in numbers in aiitumn 

 was noted long ago by Eobineau-Desvoidy and others : while Dr. D. Keilin, who 



* From my knowledge of the constitueiit.s of similjir swarms, this sjiecies should bo Liitiiio- 

 pluji-n septehitiotoAa Zett. — J. E. Cdllins. 



