MIGRATION AND DISPERSAL OF INSECTS ! DIPTERA. 175 



Kemp Town end of Brighton was caught by a cloud of Bibio marci. 

 These covered the pavements, windows, kc, in great numbers. They 

 were not so numerous as was Syr}>hns purastri in August, 1864, but 

 followed the same course. 



Species of the genus Si/rphus occasionally appear in vast swarms. 

 Kirby and Spence note {Introduction, &c., 7th ed., p. 295) that Curtis 

 informs them that the aphidivorous flies, Scaera ribesii, 8. pyrastri, 

 etc., like the ladybirds, at times, appear in myriads on the seacoast, 

 all flying in one direction, and not even avoiding objects that lie in 

 their path. Curtis himself observes {Brit. Knt., fol. 509) that 

 Syrphiis pyrastri and S. selenetica sometimes appear in myriads on the 

 seacoast, all flying in one direction and not even avoiding objects that 

 lie in their path. He was informed that on July 30th, 181b, a very 

 hot day, they quite covered the fishermen's boats at sea off Broadstairs ; 

 the next day was cloudy with several slight showers, when scarcely 

 any were to be seen, the two days following were warm, and a few 

 were observed flying about, but the next day they had entirely 

 disappeared ; on the 30th they were equally abuudant at Eamsgate 

 and at the Nore. In 1864, three species of Syrphids, Syri)hus pyrastri, 

 S. a7-ciiatus and S. balteatuf, were reported as being exceedingly abundant 

 on various parts of the south coast of England. Gibson records {Ent., 

 ii., p. 84) <S'. pyrastri as swarming in the Isle of Wight during the third 

 week in August, hundreds of dead ones being observed on the 20th 

 lying on the beach between Black-Gang and Atherfield. Seaman also 

 records (loc. cit.) them from Northwood during the same week, as 

 swarming everywhere, being found in great numbers on the seashore, 

 and he notices that when on flight they appeared to travel from south 

 to north. Walker notes (7-7^;^ Mo. May., i., pp. 136-140) that, " on 

 August 13th, 1864, many thousands of Syrplnis arcitatas were settled 

 on Kyde Pier, and most of them were paired. Neither S. pyrastri nor 

 6'. balteatns occurred there, but, on the two following days, these species 

 hovered in hundreds and thousands along the seashore near Cowes, 

 and S. balteatns was more numerous than S. pyrastri. They mostly 

 settled on the wet seaweed, to which they seemed to be very partial. 

 <S. arciiatus was less numerous, but occurred in greater abundance at a 

 short distance from the shore, while the other two species preferred the 

 sea-line. On the 16th, large numbers of SyrpJd were passing through 

 the air between Newport and Freshwater, and some of them became the 

 prey of wasps. On the two following days, N. /^^/r^.s^;/ appeared to be the 

 most common species about Freshwater and Alum Bay. On the 

 19th and 20th the weather was cooler, and the Syrphi were numerous 

 near Black-Gang Chine, in shrubberies and such like sheltered situations. 

 Daring the following week at Ventnor, Shanklin, Sandown and Hyde, 

 .S. jiyrastri and S. balteatns were scarce, but S. arcuatus was still tolerably 

 common, and was accompanied by a few nearly allied species. Two 

 individuals of pyrastri were of the variety nnicolor.'' Walker then says : 

 "It may be asked by what means the usual checks on the increase of the 

 Syrpliiis race were diminished last year, and whether all the three species 

 this year exceeded in numbers the excess of the average annual amount, 

 and on what kinds of Aphides they were dependent. S. pyrastri Tpveiers 

 the so-called sycamore Ajdiis, which is limited to the maple, and does not 

 belong to any of the^ migrating groups of Aphides. The flittings of the 

 migratory Aphides are, I believe, very short, and are only voluntary in 



