Insect-Swarms

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Abstract

A WONDERFUL flight of insects has passed over here to day, consisting of the butterfly V. cardui and the moth P. gamma. They all came from the sea from the north-west and passed over the land to the south-east. I first noticed the flight at 7.30 A.M. The morning was bright and sunny with a light wind a little south of east Great numbers of V. cardui were soaring at all heights, up to at least 150 feet, above and between the poplars which surround the house in which I am staying; all were going leisurely to the south-east; lower down P. gamma more erratic in its flight, was going in numbers in the same direction. I went down on to the grassy slope above the shore cliff. The blackberry blossoms were covered with V. cardui and P. gamma, three or four on a flower, the fussy moths much disturbing the more sedate butterflies, but each bent on holding its own. With scarcely an exception they took flight in a south-east direction when disturbed or when satisfied with their often, I fear, vain search for food. I stepped fifty paces from a clump of dark first at right angles to their line of flight and counted the butterflies which passed for two intervals of two minutes; the numbers were 95 and 108, but I probably missed some of the higher ones. On the shore at 10 o'clock I counted 73 in one minute pass a space 50 paces in width; at 11.45 in one minute 50 passed the same space. The numbers of P. gamma were more difficult to ascertain owing to their smaller size and more erratic flight, but as they all flew very low on the shore, not more than a foot or two at most above the water or sand, I stepped 20 paces and tried to count the moths passing within those limits with the result—one minute 32 moths, two consecutive minutes 18 moths, again two minutes 120 at least. In the second interval a strong gust of wind checked the flight altogether, and in the third interval the moths came so fast that I missed many I feel sure. The P. gamma were evidently much exhausted; while bathing I saw several floating on the surface of the water, which took flight when touched or crawled on to a finger presented to them; some settled on me and on others while we were bathing. At 12 o'clock I passed uninterruptedly through the flight while walking from Trouville Harbour for a distance of two kilometres northwards along the shore. There was then an occasional white butterfly (Pieris) in the flight, and I also noticed two dragon-flies coming from, the sea and following the same direction as the other insects; I noticed other dragon-flies with the flight inland, but they abound here. Had those coming from the sea accompanied the flight throughout as hawks are said to follow the flights of birds on which tliey prey? From the shore I climbed up the cliff, the grassy slopes above it were swarming with P. gamma and V. cardui, nearly every flower having one visitor at least. At 1.15 P.M. P. gamma passed over in undiminished numbers, but V. cardui was not so abundant. At 5.30 I rode parallel with the coast line along the Honfleur road to a point rather more than 10 kilometres from Trouville, passing through an uninterrupted flight of P. gamma all the way, but no V. cardui, though the butterfly still abounded on the blackberry and other blossoms by the roadside. Throughout the last two kilometres the moths were much fewer in number, but had not quite disappeared when I turned back. P. gamma generally flew lower than V. cardui, but the force which impelled them in one direction, as if their bodies were magnetised and their north pole was in the south-east, was so strong that when they met an obstruction to the course of their flight they went often over it not round it. While riding I noticed that they rose up and flew over isolated buildings, and I was curious to see whether they would do the same with a church tower. As I passed through Villerville, three came over the top of the church tower, and again at Criquebuf, three fluttered up the wall, and flew over the church tower as I passed, it. At 8 P. M. I went up on to the roof of the house; the moths were then flying up the front of the house and over the roof in. great numbers. The flight of P. gamma continued to pass the house in which I am writing, without interruption, from 7.30 A.M. till dark, and are now at 11.30 P.M., flying in at the open window, so as to be a perfect nuisance. They are still tired moths, for they soon settle; there are certainly many hundreds in the dark corners and along the cornice. 1 My children tell me that numbers of the moths were lying dead on the dry sand above high-water mark. 2 They collected some for a tame young magpie, which has been very happy all day among the flower-beds in the garden catching P. gamma, which, under ordinary conditions, would be far too wide awake for him.

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HAWKSHAW, J. Insect-Swarms. Nature 20, 426–427 (1879) doi:10.1038/020426b0

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