The tarsi of all spiders are furnished with terminal claws, usually three in number, though in some families (Drassidae, Thomisidae, etc.) there are only two. The two principal claws are paired and usually dentated, though the number of their teeth may be unequal. The third claw, when present, is always smaller, median, and inferior. [Illustration: FIG. 180.—Spider tarsi. 1, Tarsus of _Epeira_ showing three claws and supplemental serrate hairs (_a_); 2, tarsus of a Thomisid Spider, with two claws; 3, 3_a_, lateral and dorsal view of tarsus of an Attid Spider, showing scopula at _b_. ] In many spiders of climbing habits the place of the third claw is taken by a remarkable tuft of club-like hairs termed a “scopula” (Fig. 180, _b_), by means of which they are able to cling to smooth surfaces where claws would be able to obtain no hold. In some species there is a special false articulation—the “onychium”—at the end of the tarsus to bear the claws. In the Cribellatae the metatarsus is always furnished with a comb-like organ, the “calamistrum,” correlated with an extra spinning apparatus, the “cribellum,” but this will be dealt with when we reach the systematic portion of the subject. The general direction taken by the legs, the comparative length of the different joints, their armature of hairs, bristles, and spines, and the number and conformation of the tarsal claws, are points of great importance in the classification of Spiders. Under considerable magnification the legs of all Spiders exhibit a number of minute organs, arranged with absolute uniformity throughout the Araneae, and known as the “lyriform organs.” They consist of little parallel ridges of thickened chitin, the slit between them being covered by thinner chitin. They are eleven on each leg, and are distributed near the distal extremities of each of the first six joints. Their function is unknown, though some authors consider them to be organs of hearing. [Illustration: FIG. 181.—Spinnerets of _Epeira diademata_. =A=, Ventral view of _Epeira_; =B=, spinnerets magnified; =C=, profile. ] The =Spinnerets= are normally six in number, and, except in rare instances, are placed beneath the abdomen, near its apex and immediately in front of the anal tubercle. Their arrangement varies greatly, but they can generally be recognised as comprising three pairs, a posterior (or superior) pair, a median pair, and an anterior (or inferior) pair.