APTERA (Greek for "wingless"), a term in zoological classification applied by Linnaeus to various groups of wingless arthropods, including some of the insects, the centipedes, the millipedes, the Arachnida (scorpions, spiders, etc.) and the Crustacea. In modern zoology the term has become restricted to the lowest order of the class Hexapoda or true insects. This order includes the bristle-tails and the springtails.
Many wingless insects - such as lice, fleas and certain earwigs and cockroaches - are placed in various orders together with winged insects to which they show evident relationships. In such cases the absence of wings must be regarded as secondary - due to a parasitic or other special manner of life. But the bristle-tails and springtails, which form the modern order Aptera, are all without any trace of wings, and, on account of several remarkable archaic characters which they exhibit, there is reason for believing that they are primitively wingless - that they represent an early offshoot which sprang from the ancestral stock of the Hexapoda before organs of flight had been acquired by the class.
Characters. - In addition to the complete absence of wings and of metamorphosis, the Aptera are characterized by peculiar elongate mandibles (figs. 1, Mn.; 2, 4), with toothed apex and sub-apical grinding surface, like those of certain Crustacea; by the presence between the mandibles and maxillae of a pair of appendages (superlinguae or maxillulae), fig. 1, Mxl., which are absent or vestigial in all other insects; and, in most genera, by the presence in the adult of abdominal appendages used for locomotion, these latter varying in number from one to nine pairs. Among peculiarities of the internal organs the segmental arrangement of the ovaries in most members of the order is noteworthy. Many Aptera are covered with flattened scales like those of moths.
Classification. - The Aptera are divided into two divergent sub-orders, the Thysanura (q.v.) or bristle-tails, and the Collembola or springtails.
Thysanura. - The bristle-tails have an abdomen of eleven segments, the tenth usually carrying a pair of long many-jointed tail-feelers (cerci, fig. 1, x.); sometimes a median, jointed tail-appendage is also present. To these feelers the popular name is due. There may also be abdominal appendages - in the form of simple unjointed stylets (fig. 1, ii.-ix.), accompanied by paired eversible sacs, probably respiratory in function - on eight (or fewer) other abdominal segments. The head of a bristle-tail carries a pair of compound eyes and a pair of elongate many-jointed feelers.
The air-tube system is developed in varying degree in different bristle-tails, the number of pairs of spiracles being three (Campodea), nine (Machilis), ten (Lepisma), or eleven (Japyx).
Four families of Thysanura are usually recognized. In the Machilidae and Lepismidae (these two families are known as the Ectotrophi) the maxillae are like those of typical biting insects, and there is a median tail-bristle in addition to the paired cerci; while in the Campodeidae and Japygidae (which form the group Entotrophi) the jaws are apparently sunk in the head, through a deep inpushing at the mouth, and there is no median tail-bristle. The cerci in Japyx are not, as usual, jointed feelers, but strong, curved appendages forming a forceps as in earwigs.
Collembola. - In springtails, or Collembola, the jaws are sunk into the head, as in the entotrophous Thysanura; the head carries a pair of feelers with not more than six (usually four) segments, and there are eight (or fewer) distinct simple eyes on each side of the head (fig. 2, 1, 2). These are in some genera like the single elements (ommatidia) of a compound insect eye, in others like simple ocelli. The abdomen consists of six segments only. The first of these usually carries a ventral tube, furnished with paired eversible sacs which assist the insects in walking on smooth surfaces, and perhaps serve also as organs for breathing. From the researches of V. Willem it appears that the viscid fluid which causes the adherence of the ventral tube is secreted by a pair of glands in the head whose ducts open into a superficial groove leading from the second maxillae backward to the tube on the first abdominal segment. The third abdominal segment usually carries a pair of short appendages whose basal segments are fused together; this is the "catch" (fig. 2, 7), whose function is to hold in place the "spring," which is formed by the fourth pair of abdominal appendages - also with fused basal segments. In most Collembola the spring appears to belong to the fifth abdominal somite, but Willem, by study of the muscles, has shown that it really belongs to the fourth. The fused basal segments of the appendages form the "manubrium" of the spring, which carries the two "dentes" (usually elongate and flexible), each with a "mucro" at its tip (fig. 2, 5). The fifth abdominal segment is the genital, and the sixth the anal somite.
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.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)