I 



MIGRATION AND DISPERSAL OF INSECTS : LEPIDOPTERA. 255 



without treading on them." Cromptoii records (Ent. Mo. Mag., vii., 

 p. 18) that at the end of Augaist, 1849, he was crossing from Havre by- 

 steamboat, and that "about midday the vessel seemed to phinge into a 

 swarm or snow-shower of common White butterflies, and so continued 

 for nearly an hour. They literally covered us, circling round and 

 playing up and down the vessel, and I was struck with the fact that 

 they seemed to keep up with the vessel's speed — about eight knots an 

 hour — as w^ell as to flutter up and down. Either they flew at our 

 pace easily, or were assisted by the air carried along with us in the 

 calm. Gradually they thinned oft', and a breeze arising, disappeared. 

 At the same time an exhausted pigeon fell on board, and, a thunder- 

 storm on the English coast coming in sight, our pleasant Sunday 

 trip closed with a beauty of a difterent kind. I noticed in the papers 

 a few days afterwards a paragraph about a large flight of white butter- 

 flies having crossed the Channel and landed on the Hampshire coast, 

 and thought I had seen them on their passage." Another observer 

 who w^as on an excursion in a fishing-boat in the North Sea, near the 

 mouth of the Weser, in July, 1872, writes : "So long as the boat was in 

 the river, or at its mouth, only an occasional Cabbage ^¥hite was to be 

 seen crossing the river, and soon disappearing, but, when once out at 

 sea the boat was enveloped in a swarm of these butterflies, so thick as 

 to resemble a snowstorm. The w^eather was hot, and the surface of 

 the sea undisturbed by any wind. Many of the insects were to be 

 seen poising themselves with erect wings on the surface, others were 

 lying flat on it, as if dead, but flew away rapidly if disturbed. They 

 were accompanied by dragonflies {.Eschiia) which evidently preyed 

 upon them, and also by small flies and ichneumons. "■■= It is further 

 recorded by Dr. Schulte, that, in a dead calm oft' Nordeney, in the 

 Baltic Sea, he steamed for three hours and for a distance of thirty 

 miles, through a continuous flock of Fierls rapae ; he was at the time 

 some thirty miles from the mainland, and only five miles less than 

 that from the nearest island. The shore was afterwards found to be 

 strewn with their dead bodies. In July, 1864, on a still, hot day, 

 with hardly a breath of air, Thorncroft records {Entom., ii., pp. 289- 

 290) a large immigration of P. hrasucae and P. rapae, at Shoreham. 

 He was " on the pier about 3 p.m., when the flood-tide set in with a 

 gentle breeze, and then came a host of the above named butterflies, 

 with a few of P. napi. There must have been hundreds arrive within 

 a very short space of time." He expresses his surprise at " their 

 alighting and settling on the sea, with expanded wings, and the ease 

 Avith which they rose again, the same butterfly settling and rising 

 as many as four or five times within a distance of a hundred yards, 

 and with apparently as much ease as on land ; they all came direct in 

 from the sea from a south-westerly direction, and seemed to aim for 

 the entrance of the harbour between the piers, though there were 

 plenty of them came on shore on each side of the piers. The shore 

 Avas covered with a coarse sort of Italian rye-grass, on which they were 

 resting when we returned home, and, in walking through the tall 

 grass, they rose in myriads." Robson gives (Youmj ISatnralut, ii., p. 

 29) an interesting account of an immigration of Pieris brassicae that 



* One wonders much what ichneumons were doing amongst a swarm of the 

 imagines of the Pieris. 



