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THE SWARMING OF COLLEMBOLA. 



JAMES MEIKLE BROWN, B.SC, F.L.S., F.E.S., F.C.S. 



The peculiar phenomenon, caused by the presence of small 

 Collembola of the genus Achorutes [A. socialis, A. cocklei, 

 etc.), which occur at times in such enormous numbers as to 

 colour the surface of the snow over considerable areas in 

 •Canada and Northern Europe, is well-known from the writings 

 of various authors (Linnaniemi, Folsom, Macnamara, etc.). 

 The extraordinary numbers of these small insects, whose 

 length may reach 2 mm., which must be associated together 

 to produce the effect is very remarkable. Mr. Macnamara 

 states that ' the vast swarms of A . socialis literally blacken 

 square yards of the snow around the principal foci from 

 which they emerge,' and that ' in depressions in the snow, 

 such as foot -prints, they accumulate in solid masses.' He 

 further describes the emergence of swarms of the same species 

 from a decaying trunk ' from which they were issuing in 

 solid blue masses.' {Canadian Naturalist, LI., November 

 and December, 1919.) 



In our own country ' snow- fleas ' do not occur, and we 

 never have the opportunity of witnessing such vast hordes of 

 Collembola ; yet, to a more limited extent, a somewhat 

 similar swarming may be noticed among some of the more 

 common species. 



Podura aquatica L. thrives on the surface of water in small 

 pools — cattle-pools seem to be a favourite situation in Derby- 

 shire — and during the summer months they become so numer- 

 ous that they form a bluish-black marginal belt of varying 

 width of actively-moving creatures, especially on the lee-side 

 of the pool. A similar appearance may sometimes be noticed 

 produced by other aquatic species. In July, 1918, the margin 

 of a small pool, partly shaded by trees, in the Hack-Fall 

 Wood, near Mash am, Yorkshire, was simply alive with the 

 very small, pale yellow-green Sminthurides malmgreni var. 

 elegantula Reut., in assocation with Isotomurus palustris 

 Mull. There must have been enormous numbers of individuals 

 present, both mature and immature. Similar swarms of 

 Amirida maritima (Guer.) Laboulb. may also be seen in rock- 

 pools at low tide at various places round the coast, e.g., 

 Filey, Robin Hood's Bay, etc. In cases such as these, the 

 rapid multiplication of individuals during early summer, 

 associated with the limited extent of their habitat, and their 

 very restricted powers of migration, appear sufficient to account 

 for the formation of the swarms. 



Occasionally the swarming is undoubtedly to be associated 

 with a plentiful food-supply of limited extent. Such a case 



1921ApriH 



