The bristle tails have an abdomen of eleven segments, the tenth usually carrying a pair of long many-jointed tail-feelers (cerci, fig.
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.
Jaws under Hexapoda); in the presence of a large number of excretory (Malpighian) tubes; in the firm texture of the forewings; in the presence of appendages (cerci) on the tenth abdominal segment; and in the absence of a metamorphosis, the young insect after hatching closely resembling the parent.
The Hemimeridae include the single genus Hemimerus, which contains only two species of curious wingless insects with long, jointed cerci, found among the hair of certain West African rodents.