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.