_Minute Insects with slender, thread-like, or hair-like antennae; four delicate membranous wings, the front pair of which are the larger; their neuration is not abundant and is irregular, so that the cells are also irregularly arranged; the transverse nervules are only one or two in number.[306] Prothorax very small, in the winged forms quite concealed between the head and the large mesothorax; this latter closely connected with, or fused with, the metathorax. Species quite wingless, or with wings unfitted for flight, exist; in them the prothorax is not so extremely small, while the mesothorax is smaller than in the winged forms. Tarsi of two or three segments. Metamorphosis slight, marked chiefly by the development of wings and ocelli._ [Illustration: FIG. 241.—_Psocus fasciatus_, England. (After M‘Lachlan.)] The Psocidae are without exception small and soft-bodied Insects, and are only known to those who are not entomologists by the wingless forms that run about in uninhabited or quiet apartments, and are called dust-lice or book-lice. They are perhaps more similar to Termitidae than to any other Insects, but the two families differ much in the structure of their wings, and are totally dissimilar in the nature of their lives. {391}[Illustration: FIG. 242.—Transverse horizontal section of head of _Psocus_: _f_, fork or pick; _t_, lingua; _mx_, left maxilla; _c_, cardo; _p_, stipes; _m.m_, muscles; _m.s_, socket of mandible.] [Illustration: FIG. 243.—A, Front of head of _Psocus heteromorphus_; _cl_, post-clypeus; _g_, epicranium: B, transverse horizontal section of post-clypeus of _Psocus_: _cl_, post-clypeus; _c.m_, clypeal muscles; _g_, epicranium; _t_, tendons; _l.m_, labial muscle in section; _oe_, oesophagus; _oe.b_, oesophageal bone. (After Burgess and Bertkau.)]