instrument of motion. From this circumstance, a gradation can be traced, as to the share the upper wings take in flight from the Coleoptera to the Hemiptera, and from these to the Orthoptera. The insects possessing the greatest powers of flight, must, therefore, be sought among those in which the upper wings are wholly membranous; and we accordingly find them among the Neuroptera, Hymenoptera, and Lepidoptera. Every one who has paid the slightest attention to the subject, must have noticed how much more vigorous and sustained is the flight of a dragon-fly, a bee, or a butterfly, than that of a beetle, a grasshopper, or a plant-bug. The under wings are in every instance membranous, and are exclusively organs of flight.
A wing of the latter sort, whether it belong to the superior or inferior pair, has the external appearance of a firm, dry, membrane, usually transparent, and traversed by numerous salient horny ribs. Although the membrane appears simple, it consists, in reality, of two membranous leaves, closely applied to each other, and enveloping the ribs just spoken of. This can be made to appear very distinctly when the insect has just emerged from the pupa and immersed in spirits of wine, as the fluid can be introduced between the still flaccid membranes, and thus distends them like a bag. Even this membrane, which appears of the finest and most glossy surface to the naked eye, is found under the microscope to be clothed more or less densely with hairs; and in some gnats[1] these
- ↑ See the immensely magnified figure of a gnat in Swammerdam's Book of Nature, Plate XXXVI.
