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.
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.
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.
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.)
The antennae consist of eleven to twenty-five joints, or even more, about thirteen being the usual number; the basal two are thicker than the others, and are destitute of setae or pubescence such as the others possess. The maxillae and labium are remarkable. The former possesses a peculiar hard pick or elongate rod; this is considered by many naturalists to be the inner lobe, but Burgess thinks it more probably an independent organ,[[307]] as it has no articulation of any kind with the outer lobe. The latter is remarkably thick and fleshy; the palpus is 5-jointed. Other authorities consider the pick to be certainly the inner lobe; if it be not, the latter is quite wanting. Hagen agrees with Burgess in stating that the pick slides in the outer lobe as in a sheath. The labium has a large mentum and a ligula divided anteriorly into two lobes; at each outer angle in front there is a globular projection, which is doubtless the labial palpus; reposing on the labium there is a large free lingua. The labrum is large, attached to a distinct clypeus, behind which there is a remarkable post-clypeus, which is usually prominent as if inflated; to its inner face are attached several muscles which converge to be inserted on a plate placed below the anterior part of the oesophagus, and called by Burgess the oesophageal bone; under or within the lingua there is a pair of lingual glands. Judging from Grosse's study of the mouth of Mallophaga, we may conclude that the oesophageal bone will prove to be a sclerite of the hypopharynx. The eyes of the winged forms are frequently remarkably convex, and there are also three ocelli, triangularly placed on the vertex. The head is free and very mobile. The coxae are rather small, exserted, contiguous; the sterna small. The abdomen has usually ten segments, though sometimes only nine can be detected.
The thorax in Psocidae usually looks as if it consisted of only two segments. This is due to two opposite conditions: (1) that in the winged forms the prothorax is reduced to a plate concealed in the fissure between the head and the mesothorax bearing the first pair of wings; (2) that in the wingless forms (Fig. 247), though the prothorax is distinct, the meso- and metathorax are fused into one segment.