The Formation and Biological Significance of Aggregations in the Distribution of Collembola

The formation and biological significance of aggregations of 6 species of surface dwelling Collembola of a coniferous forest soil were studied. The species are: Tomocerus minor (Lubbock), Lepidocyrtus cyaneus Tullberg, Orchesella cincta (Linne), Entomobrya nivalis (Linne), Isotomurus palustris (Muller) and Isotoma viridis Bourlet. All species showed a marked aggregation, but to a different degree and in different places, since the degree of aggregation of total Collembola appeared to be less than that of the individual species. The different degree of aggregation could not only in field studies, but also in laboratory experiments be related to the specific humidity preferences of the species. The stenohygric species Lepidocyrtus cyaneus, Isotoma viridis and Tomocerus minor appeared to have a high degree of aggregation, the euryhygric species Orchesella cincta and Entomobrya nivalis a lower one. The observations contribute to the view that aggregation is the result of the coming together of individuals by undirected movements in sites with optimum humidity conditions, the specific movements being stimulated by drought. The biological significance of aggregation is described as a protectional function (survival, moulting) and as a meeting system where reproductive behaviour is not yet developed.