Page:Jardine Naturalist's library Entomology.djvu/119

Rh hairs assume a lanceolate shape, and spring like small leaflets from each side of the longitudinal nervures. In assuming this form, they make an approach to the scales of the Lepidoptera. These hairs, doubtless, serve the purpose of preserving the wings from the lacerations to which they would be exposed from the nature of the places which the insects frequent for food; and, particularly, in the case of many of them, when constructing their nests. In addition to this use, M. Chabrier is of opinion that they may likewise contribute to fix the atmospheric fluid on the wings during flight, and thereby increase their buoyancy. The ribs alluded to, ramifying over the surface of the wing, are properly called nervures. They are horny tubes, whose office it is to support and give tension to the membrane. A transverse section shews that they contain a soft parenchyma, and a large spiral vessel, accompanied by a fine nerve. This spiral vessel is a genuine trachea, emanating from the cavity of the thorax, and therefore adapted for conveying air throughout the wing. The nervures are usually uninterrupted, but in some instances, particularly among the Hymenoptera, (especially such as have a petiolated abdomen,) their continuity is frequently broken. This takes place chiefly at the points where they anastomose with each other, and the place where it occurs presents the appearance of a small transparent spot, resembling an air bubble, which led Jurine, who first noticed it, to call it a bulla. The nervures there lose their tubular form, and become scattered in the shape of very fine