Fig. 104.—Divided (ditrochous) trochanter of an ichneumon: cx, coxa; tr, the two divisions of the trochanter; f, femur.—After Sharp.
The fore legs are usually directed forward to drag the body along, while the middle and hind legs are directed outward and backward to push the body onwards. While arachnids walk on the tip ends of their feet, myriopods, Thysanura, and all larval insects walk on the ends of the claws, but insects generally, especially the adults, are, so to speak, plantigrade, since they walk on all the tarsal joints. In the aquatic forms the middle and hind tarsi are more or less flattened, oar-like, and edged with setæ. In leaping insects, as the locusts and grasshoppers, and certain chrysomelids, the hind femora are greatly swollen owing to the development of the muscles within. The tibia, besides bearing large, lateral, external spines, occasionally bears at the end one or more spines or spurs called calcaria. The fore tibia also in ants, etc., bear tactile hairs, and chordotonal organs, as well as other isolated sense-organs (Janet), and, in grasshoppers, ears.
In the Carabidæ the legs are provided with combs for cleaning the antennæ (Fig. 107), and in the bees and ants these cleansing organs are more specialized, the pectinated spine (calcar) being opposed by a tarsal comb (Fig. 106, d; for the wax-pincers of bees, see g). In general the insects use their more or less spiny legs for cleansing the head, antennæ, palpi, wings, etc., and the adaptations for that end are the bristles or spinules on the legs, especially the tibiæ.
Fig. 105.—Foot of honey-bee, with the pulvillus in use: A, under view of foot; t, t, 3d–5th tarsal joints; a n, unguis; f h, tactile hairs; p v, pulvillus; cr, curved rod. B, side view of foot. C, central part of sole; pd, pad; cr, curved rod; pv, pulvillus unopened.—After Cheshire.
Fig. 106.—Modifications of the legs of different bees. A, Apis: a, wax-pincer and outer view of hind leg; b, inner aspect of wax-pincer and leg, with the nine pollen-brushes or rows of hairs; c, compound hairs holding grains of pollen; d, anterior leg, showing antenna-cleaner; e, spur on tibia of middle leg. B, Melipona: f, peculiar group of spines at apex of tibia of hind leg; g, inner aspect of wax-pincer and first tarsal joint. C, Bombus: h, wax-pincer; i, inner view of the same and first tarsal joint, all enlarged.—From Insect Life, U. S. Div. Ent.
Osten Sacken states that among Diptera the aerial forms (Bombylidæ, etc.) with their large eyes or holoptic heads, which carry with them the power of hovering or poising, have weak legs, principally fit for alighting. On the other hand, the pedestrian or walking Diptera (Asilidæ, etc.) “use the legs not for alighting only, but for running, and all kinds of other work, seizing their prey, carrying it, climbing, digging, etc.; their legs are provided not only with spines and bristles, but with still other appendages, which may be useful, or only ornamental, as secondary sexual characters.”
Fig. 107.—End of tibia and tarsal joints of Anophthalmus; c, comb.