Fig. 2.—Ear of Grasshopper, drum at A, greatly magnified.
Fig. 3.—Drum of Grasshopper's Ear, greatly magnified.
The sense of smell in insects lies mainly in those wonderful organs, the antennæ or 'horns.' Scents of various kinds are perceived either through pits, or through peg- or spike-like teeth filled with fluid. The leaf-like plates of the antennæ of the cockchafer (fig. [5]) have these pits very highly developed. On the outer surface of the first 'antennal' leaf, as also on the edges of the other leaves, only scattered bristles are seen; but on the inner surface of the first and seventh leaves, and on both surfaces of all the other leaves, there are close rows of shallow, irregularly shaped hollows. Their number is enormous—in the males as many as thirty-nine thousand, and, in the female, thirty-five thousand on each antenna. As some of the scent-laden air reaches the surface of these pits, it causes the nerves of smell to be roused, and so guides the beetle to its mate, or to its food, according to the nature of the smell. These pits are so tiny that they cannot be shown on the antennal leaves of the cockchafer shown in fig. [5], but they are there. On fig. [6] a highly magnified section of one of these 'leaves' of the antenna is shown: 'p' is the pit, 'n' is the nerve, and 'S. C.' the sense-bulb of the nerve in which it terminates—the point at which the smell is perceived.
Fig. 4.—Antenna of Gnat, greatly magnified.