MIGRATION AND DISPERSAL OF INSECTS: LEPIDOPTERA. 15 
plovers, and sandpipers. Again, on the night of August 12th, 1877, 
during a light east wind, and very light warm rain, myriads of Plusia 
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. yamma, 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 8 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, with 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. gammain 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. 
gamma 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 subject 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 Hybernia defoliaria and H. aurantiaria 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 Hybernia made their appearance. 
Pierids, Pstlura monacha, and other lepidoptera, have been seen to pass 
Heligoland in migratory flights of astonishing proportions, though less 
than those of P. gamma, the numbers of which are beyond conception, 
and the migration of P. yamma 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. Gitke states that he was in- 
formed by Cordeaux that an enormous immigration of these insects 
actually took place in England ata time corresponding with these 
observations. 
Gitke’s observations lead him to suppose that, in the migrations 
of Plusia gamma across 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- 
