_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.