The most common member of this Order is _Lithóbius forcipátus_ (Pl. X. fig. 4), which is found under stones, in cellars, and among garden-rubbish. It is of a yellowish-brown colour, with long, many-jointed, gradually tapering or setáceous (_séta_, a bristle) antennæ (figs. 4 _a_, 5 _a_), and two large and powerful mandibles (fig. 5 _b_) resembling those of the spiders. It has also a broad, notched, and toothed lower lip, or labium (fig. 5 _d_), above which are two toothed jaws, or maxillæ, and two lip-feelers, or labial palpi (_c_). The eyes consist of a group of ocelli on each side (_e_). The body is protected by alternately larger and smaller dorsal plates, which are fifteen in number; and there are fifteen pairs of legs, which are terminated by a single claw. On the sides of the body will be found some oval dark-looking bodies, fringed with hairs; these are the spir´acles (_spirac´ulum_, a breathing-hole) or breathing-pores. They form the orifices of certain branched and transversely striated tubes, which are distributed throughout the body; the tubes are called _trácheæ_ (_trachea_, the windpipe), and their walls contain an elastic spiral fibre which keeps them open. These parts of the insect can only be distinctly seen when the body has been slit up on the under side; and, after washing away the animal matter with water by the aid of a hair pencil, pressed between two slides with a clip, dried, soaked in turpentine, and mounted in balsam. THYSANÚRA (θὑσανοι, fringe, οὑρἀ, tail).--The insects belonging to the genus _Podúra_, of this Order, are very minute and difficult to examine; but they are specially interesting, on account of the structure of their scales. They are common in gardens and cellars, under flower-pots, &c., and are about one-tenth of an inch long. They are of a brownish or silvery-leaden colour, wingless, with six legs, and when touched they leap like a flea. The leaping motion is produced by the action of the tail, which is forked and bent under the body. The body is usually covered with minute scales (Pl. X. fig. 19, _a_, _c_), and these are used as test-objects. The structure of the scales varies in the different genera and species; those usually used (fig. 19 _a_) are stated to belong to _Podúra plum´bea_; it appears, however, that this is not correct. The scales sold as test-objects under this name are covered with minute and short raised lines (fig. 19 _a_), arranged in irregular but somewhat parallel wavy rows. It requires a good microscope and a high power to show them distinctly, and they should appear perfectly black and separate. The little lines are much coarser in some scales than in others; so that there are easy and difficult scales, as they are called. The _Poduræ_ may be caught by holding a sheet of paper near their haunts and disturbing them; and when they have jumped upon the paper, a slide laid upon them and gently pressed will remove some of the scales for examination.