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.” [Illustration: FIG. 107.—End of tibia and tarsal joints of Anophthalmus; _c_, comb. ] =Tenent hairs.=—Projecting from the lower surface of the empodium are the numerous “tenent hairs,” or holding hairs, which are modified glandular setæ swollen at the end and which give out a minute quantity of a clear adhesive fluid (Figs. 108, 109, 130, 134). In larval insects, and the adults of certain beetles, Coccidæ, Aphidæ, and Collembola, which have no empodium, there are one or more of these tenent hairs present. They enable the insect to adhere to smooth surfaces. [Illustration: FIG. 108.—Transverse section through a tarsal joint of Telephorus, a beetle: _ch_, cuticula of the upper side; _m_, its matrix; _ch′_, the sole; _m′_, its matrix; _h_, adhesive hair; _h′_, tactile hair, supplied with a nerve (_n′_), and arising from a main nerve (_n_); _n″_, ganglion of a tactile hair; _t_, section of main trachea, from which arises a branch (_t′_); _dr_, glands which open into the adhesive hairs, and form the sticky secretion; _e_, chitinous thickening; _s_, sinew; _b_, membrane dividing the hollow space of the tarsal joint into compartments. See p. 111.—After Dewitz. ] Striking sexual secondary characters appear in the fore legs of the male Hydrophilus, the insect, as Tuffen West observes, walking on the end of the tibia alone and dragging the tarsus after it. The last tarsal joint is enlarged into the form of an irregular hollow shield. The most completely suctorial feet of insects are those of the anterior pair of Dyticus (Fig. 132). The under side of the three basal joints is fused together and enlarged into a single broad and nearly circular shield, which is convex above and fringed with fine branching hairs, and covered beneath with suckers, of which two are exceptionally large; by this apparatus of suckers the male is enabled to adhere to the back of its mate during copulation. The line branching hairs around the edge prevent the water from penetrating and thus destroying the vacuum, “while if the female struggle out of the water, by retaining the fluid for some time around the sucker, they will in like manner under these altered conditions equally tend to preserve the effectual contact.” (Tuffen West.) [Illustration: