MIGRATION AND DISPERSAL OF INSECTS : LEPIDOPTERA. 15 



plovers, and sandpipers. Again, on the night of August 12th, 1877, 

 during a Hght east wind, and very light warm rain, myriads of I'liifn'a 

 gamma migrated in company with the shorebirds already mentioned, 

 and among young birds of the wheatear, willow warbler, and other 

 small species. With regard to P. i/amma, the author goes on to say, 

 that never have the migrations of P. gamma reached to such an extent 

 as they did during the middle of August, 1882. On the 15th of that 

 month, the wind was south-east, the weather fine and warm ; during 

 the night the wind was south with a calm and warm rain. Several 

 small birds (the redstart, whitethroat, willowwarbler, &c.), as well as 

 plovers and sandpipers were migrating in numbers, and, intermingled 

 with these, from 11 p.m. to 3 a.m., myriads of P. gamma, like a dense 

 snowstorm, all travelling in a direction from east to west. Early on 

 the morning of the 16th, the wind was west, wath rain, the afternoon 

 was fine, sunny and calm, and in the evening and during the night the 

 wind turned south, when there was again a strong migration of small 

 birds and waders, together with countless numbers of P. gamma. 

 These migration phenomena were repeated during the nights of the 

 17th and 18th, with very light southerly and westerly winds. On the 

 19th, with a south-east wind and fine weather, many warblers, fly- 

 catchers, and like species were seen during the day. During the night 

 the sky was overcast, but large numbers of waders, again accompanied 

 by P. gamma in millions, were still seen, all travelling from east to west. 

 During the night of the 20th, there was a thunderstorm at some distance, 

 which put an end to migration of every kind." Giitke further notes here 

 the connection between migration and electrical disturbances of the 

 atmosphere, and observes that in this instance the migration of P. 

 gami}m and that of the small birds were both interrupted by thunder- 

 storms. He repeats that he has long been convinced that the more 

 extended flights of lepidoptera are sulaject to the same meteorological 

 influences as those of birds, that in the month of July of various years 

 he has captured numerous species of lepidoptera not belonging to the 

 insect fauna of the island, the weather of these nights being invariably 

 such that, if it had occurred a few weeks later, it would have conducted 

 hither numerous wheatears. We have already noticed the great 

 immigration of Hyhernia defoliaria and //. aurantlaria that accompanied 

 a great flight of larks on October 25th, 1872, whilst on October 12th, 

 1883, during an exceedingly strong period of bird-migration, large 

 swarms of the same species of llghernia made their appearance. 



Pierids, PsiY((ra mowrtc/w, and other lepidoptera, have been seen to pass 

 Heligoland in migratory flights of astonishing proportions, though less 

 than those of P. gamm.a, the numbers of which are beyond conception, 

 and the migration of P. gamma already referred to as taking place 

 between August 15th-19th, 1882, was such that, as seen from the 

 lighthouse, they passed from east to west in undiminishing numbers, 

 like the flakes of a dense snowstorm. Giitke states that he was in- 

 formed by Cordeaux that an enormous immigration of these insects 

 actually took place in England at a time corresponding with these 

 observations. 



Gatke's observations lead him to suppose that, in the migrations 

 of Plasia gamma Sbcvoss Heligoland, they travel from Schleswig-Holstein 

 and the adjacent country to Britain, across the North Sea, 400 miles 

 in extent. The insects, he says, always follow an east to west migra- 



