The spring serves the Collembola which possess it as an efficient leaping-organ (see [Springtail]). But in some genera it is greatly reduced and in many quite vestigial.
Most springtails are without air-tubes, and breathe through the general cuticle of the body. But in one family (Sminthuridae) a spiracle, opening on either side between the head and the prothorax, leads to a branching system of air-tubes. The Sminthuridae are further characterized by the globular abdomen, which shows but little external trace of segmentation, and by the well-developed spring.
In the Entomobryidae the body is elongate and clearly segmented, but the dorsal region (tergum) of the prothorax is much reduced and the head downwardly directed; the spring is well developed. In the Achorutidae the head is forwardly directed, the tergum of the prothorax conspicuous, and the spring small or vestigial.
In many genera of springtails a curious post-antennal organ, consisting of sensory structures (often complex in form) surrounded by a firm ring, is to be noticed on the cuticle of the head between the eyes and the feelers. It may be of use as an organ of smell. Other sensory organs occur on the third and fourth antennal segments in the Achorutidae and Entomobryidae (fig. 2, 3).
Distribution and Habits.—The Aptera are probably the most widely distributed of all insects. Among the bristle-tails we find the genus Machilis, represented in Europe (including the Faeroe Islands) and in Chile; while Campodea lives high on the mountains and in the deepest caves. The springtails have even a wider distribution. The genus Isotoma, for example, has some of its numerous species in regions so remote as Alaska, Franz Josef Land, the Sandwich Islands, the South Orkneys, Graham Land, Kerguelen and South Victoria Land. As it is unlikely that these delicate insects could be transported across sea-channels, their wide and discontinuous range suggests both their great antiquity and the former existence of continental tracts over which they may have travelled to their present stations.
Springtails and bristle-tails live in damp concealed places—under stones or tree-bark, in moss, and in the decaying vegetable or animal matter which serves as food for most of them. Some species, however, eat fresh plant-tissues. A species of bristle-tail (Machilis maritima) and quite a number of springtails haunt the sea-coast at or below high-water mark. In such localities many thousands of individuals may sometimes be found associated together. The insect fauna of limestone caves both in Europe and North America is largely composed of Aptera, especially Collembola.
Geological History.—A supposed Thysanuran from the Silurian of New Brunswick has been described by G.F. Matthew, and another genus from the French Carboniferous by C. Brongniart. Not till the Tertiary do we find remains of Aptera in any quantity, species both of living and extinct genera being represented in the amber.
Development.—The embryonic development of several genera of Aptera, which has been carefully studied, will be more suitably described in comparison with that of other insects than here (see [Hexapoda]).
Bibliography.—The modern study of the Aptera may be said to date from the classical memoirs of T. Tullberg, “Sveriges Podurider,” in Kongl. Svensk Vetensk. Akad. Handl. x., 1872, and Sir J. Lubbock (Lord Avebury), “Monograph of the Collembola and Thysanura,” Ray Society, 1873. In these, full references to the older literature will be found. Subsequently our knowledge of the Thysanura has been markedly advanced by J.T. Oudemans, Bijdrage tot de Kennis den Thysanura en Collembola (Amsterdam, 1888); B. Grassi, who published between 1885 and 1889 a series of memoirs entitled “I progenitori dei Miriapodi e degli Insetti,” in the Atti Accad. di Scienz. Nat. Catania, and the Memor. R. Accad. dei Lincei; and V. Willem, whose “Recherches sur les Collemboles et les Thysanoures,” in Mem. Cour. Acad. Roy. Belgique, lviii., 1900, are indispensable to the student. In addition to this work of Willem, valuable anatomical papers on Collembola have been published by H.J. Hansen (Zool. Anz. xvi., 1893), J.W. Folsom (Bull. Mus. Comp. Anat. Harv. xxxv., 1899), C. Börner (Zool. Anz. xxiii., 1900), and K. Absolon (Zool. Anz. xxiii. and xxiv., 1900, 1901), the two latter writers having paid especial attention to the peculiar post-antennal and antennal sense-organs of springtails. Absolon has also written on the Collembola of caves. These writers, with H. Schött, C. Schäffer and others, have published many systematic papers on Collembola, as has F. Silvestri on Thysanura. British species are mentioned in Lubbock’s monograph; for recent additions see G.H. Carpenter and W. Evans (Proc. R. Phys. Soc. Edinb. xiv., 1899, and xv., 1903).
(G. H. C.)