Art. 17.—A Monograph of the Psocoptera, or Copeognatha, of New Zealand. By R. J. Tillyard, M.A., Sc.D. (Cantab.), D.Sc. (Sydney), C.M.Z.S., F.L.S., F.E.S.; Entomologist and Chief of the Biological Department, Cawthron Institute, Nelson, N.Z. [Read before the Nelson Institute, 16th June, 1921; received by Editor, 20th June, 1921; issued separately. 17th February, 1923.] Plate 18. Introduction. The Psocoptera, or Copeognatha, are a somewhat small but remarkably distinct and well-defined order of insects, having no very close affinities with any other group. With the exception of a single family of comparatively large forms, the Thyrsophoridae, confined to South America, the order contains only small insects, ranging from little more than 1 mm. long (wingless forms) to forms having an expanse of wing up to about ½ in. They are for the most part beneficial insects, feeding upon fungi, lichens, and algae, though some gnaw the outer bark of trees, and two genera at least have become world-wide pests of houses—the notorious “book-lice” (Troctes and Atropos). Morphologically, the order is distinguished by some remarkable characters in the form of the head, mouth-parts, thorax, and wings. These may be briefly discussed here. In the head, the epicranium (or vertex) is divided into two parts by a longitudinal suture—a primitive character not to be found in any other order of winged insects. A wide V-shaped suture divides the epicranium from the frons. The three ocelli, when present (as they usually are), lie close in at the three angles formed by the meeting of this frontal suture with the median vertical suture. The median ocellus lies on the frons, in the angle of the V, while the two lateral ocelli lie on the epicranium, one on each side of the median vertical suture. The clypeus is high and very convex, almost bulbous, and generally striated in a complicated manner; below it there is a smaller piece, lying above the labrum, which is called the “clypeolus.” It will be clear that the clypeus really corresponds with the postclypeus of the Odonata, while the clypeolus corresponds with the anteclypeus of that order. In the mouth-parts, the mandibles are interesting because of their distinct asymmetry, the teeth of one fitting into the clefts between those of the other. Each mandible carries, besides teeth, a corrugated grinding or chewing plate; and these also are of different shape in right and left mandibles. The maxillae are divided into two very distinct parts by means of the separation of a long maxillary chisel, supposed to represent the lacinia. This remarkable organ, which is without parallel in the Insecta, and from which the name Copeognatha (“chisel-jawed”) is taken, is a long, hard rod, with a sharp and peculiarly-formed end, reminding one of some of the more complex of a set of carving-chisels. These chisels can be extruded from the mouth to a considerable distance, and are used