Aggregations of various species of spring-tails on the surface of the snow have been noted and reported upon by several authors. Of these species, the one occurring most commonly is Achorutes socialis Uzel (Podura nivicola Fitch) which Folsom (1916) records as being a common species in most parts of Europe and occurring in several localities in the north-eastern United States. MacNamara (1919b) studied the habits of this species as it appeared in the vicinity of Arnprior, Ontario, reporting that it “may be as thick as 500 to the square foot, while in hollows and depressions in the snow—such as foot-prints—from which they cannot escape, they sometimes accumulate in solid masses that could be ladled out with a spoon.” James (1933) noted an assemblage of this species at Pottageville, Ontario, and reported that “beneath the hemlocks they covered the snow with a frequency of over 400 to the square foot.” A related species, tentatively identified as A. bengtssoni Agren, appeared in a population estimated by Park (1949) as exceeding 4,000,000 from three exit holes in an area of six square feet in Illinois in December, 1946.