THE CANADIAN ENTOMOLOGIST 267 



Their movements as usual seemed to lack decision, but the net effect of their 

 apparently purposeless walking and aimless leaping was to spread them out 

 from under the trees in the direction of the light, and they thinned out towards 

 the open meadow into successive bands of approximately 25, 10 and 5 insects 

 to the square foot, until at last in the middle of the marsh only an occasional 

 jumper was to be seen. While the boundary of this edge of the outbreak was 

 indefinite, the other edge, as I had often noticed before in similar cases, ended 

 sharply at the dark evergreens, and not a snow-flea was to be found in the 

 shadows beneath these trees. The insects are evidently possessed of a positive 

 phototropism, although it does not work with the accuracy and directness of 

 the similar tropism of a great many other invertebrates. Another factor in 

 snow-flea distribution is the wind. I once saw the insects blown out on the 

 snow in a long comet tail radiating from the base of an ironwood around which 

 they were emerging during a strong wind, and on this occasion I am sure the 

 fresh southerly breeze aided their spread over the meadow. 



By pacing the distances and carefully estimating the average number 

 of insects to the square foot, I calculated that there were not less than 9,000,000 

 to 10,000,000 A. socinlis disporting themselves on the snow along this narrow 

 half-mile strip. They were of a well-grown generation, most of them reaching 

 a length of 1.5 mm. with here and there a few 2 mm. individuals, which is the 

 maximum length of the species. A. socialis seldom pays any attention to 

 the close approach of the observer, but this day they were more alert than 

 usual, and when I bent to examine a crowded alder stem with my magnifying 

 glass, the occupants all flung themselves off on to the snow, and when, in focusing 

 a wide procession wandering up a tree, I brought my face close to the trunk, 

 a rain of the insects pattered down on my cheek and ear, and the raw turnip 

 smell was very evident. 



It was 11 o'clock in the morning when I reached the beaver meadow, and 

 up to 5 p.m. there was no very apparent change in numbers or distribution of 

 the multitude. They kept ceaselessly crawling and leaping, but without seem- 

 ing to get anywhere in particular. About 5 o'clock, however, a slightly lower 

 temperature set in, and with the first cool breath, the insects began to leave the 

 surface by insinuating themselves between the snow particles, and by 5.30, 

 when I had to leave for home, the numbers visible were noticeably diminished. 

 The temperature was still above freezing, and it was evidently the downward 

 trend and not the absolute degree that drove them to shelter. For although 

 the thermometer registered no lower than 28° F. during the night, and stood 

 at 31° at 9 o'clock next morning — ^temperatures at which the insects often 

 emerge abundantly — when I got back to the marsh about half past nine, not a 

 single Achorutes was to be found on the trees, and 95 per cent, of those on the 

 snow had disappeared, as I ascertained by counting the few individuals remain- 

 ing on areas I had marked out the day before. 



In the course of the next few days I visited the place several times to study 

 the further behaviour of the insects, and my observations may be conveniently 

 summarized as follows: 



When the colder and drier weather drives the snow- fleas to shelter, those 

 on the trees and shrubs reach the soil by walking down the way they came up. 

 The insects on the snow, however, show no tendency whatever to go back to 



