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So fleet of foot are they, and so like a wave of blurred light they cross the vision, that it is vain to try to figure what they are in shape and look. In death they yield their all of earth to prying science. Their body’s form is narrow, flattened; their legs in pairs of threes, each of six joints consisting, the basal joints broad, flat, triangular, the tarsal large, in number two, and armed at end with pair of claws incurved. The three thoracic segments are very like in size, and eight abdominals, of similar length and width. So weak it seems the rather long abdomen is, that two pairs or six of bristles, simple, unjointed, and freely movable, serve as support, and also, as in other groups of insects, as organs locomotive.

The mode of antenna-insertion—and the same prevails in the entire family—is much like that of the Myriopods, the front of the head being flattened and concealing, as in the Centipedes, the base of the antennæ. Indeed, the head of any of the Bristle-tails, as seen from above, bears a general resemblance in some of its features to that of the Centipede and its allies, and so, in a less degree, does the head of the larvæ of certain beetles and neuropters. The eyes are compound, the individual facets constituting a sort of heap. The mouth-parts are readily compared with those of the larva of Perla, the rather large, stout mandibles being hid at their tips by the upper lip, which moves freely up and down when the creature opens its mouth. In length the mandible is three times its breadth, and furnished with three sharp teeth on the outer edge, and with a broad cutting margin within, and still further inwards with a number of straggling small spines. The lower lip is broad and stout, with a distinct medium suture, which indicates a former separation in embryonic life into a pair of appendages. Its palpi are three-jointed, the joints being broad, and directed backwards in life, and not forwards, as in the higher insecta.