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. [Illustration: LEPISMAS AT WORK. How Books are Destroyed.] Perhaps not more than a half-dozen species of Lepisma are known to exist in this country. Our commonest form is very abundant in the Middle States under stones and leaves in forests, and northward in damp houses, where it has much of the habits of the cockroach, eating clothes, tapestry, silken trimmings of furniture, and doing great mischief to libraries by devouring the paste and mutilating the leaves and covers of books. Our heat-loving form, which is apparently allied to the _Lepisma thermophila_ of Europe, and which may be an imported species, is quite as destructive as its nearest of kin _Lepisma saccharina_. It does not confine its ravages to closets and pantries, and feed upon sugar and cake and pastry, but has latterly taken to bookcases, where it leads an easy, comfortable life, without fear of molestation.