It will be convenient to examine the legs next. First, however, it will be well to look at a normal leg of an Insect (the Cockroach), and learn the names of the different parts. First comes the _coxa_ (_a_) or haunch, next the _trochanter_ (_b_), then the _femur_ (_c_) or thigh, the _tibia_ (_d_) or shank, and the _tarsus_ (_e_) or foot, ending in a pair of claws. There are three pairs of legs in perfect Insects, and usually the same number in larval forms, though in some of these legs are entirely wanting. [Illustration: FIG. 23.--Leg of Cockroach.] In the males of the Margined Water Beetle and many of its near relations the first pair of legs deserve special attention. The first three joints of the tarsus have coalesced to form a disk or cup, which in our specimen bears two smaller ones on its inner surface. A power of 20 will show the disk nearly as well as it appears in Fig. 24. The purpose of this disk, or clasper, which is absent in females, is obvious. It was formerly supposed to act as a sucker, but Professor Lowne and Professor Miall[12] have shown that it does not act by atmospheric pressure, but by a viscid secretion discharged from the cup-like hairs with which the inner surface is set. [Illustration: FIG. 24.--Tarsus of Dytiscus (magnified).] The middle pair of legs in the male also bear cup-like hairs on the corresponding joints of the tarsus, and in very much greater number. Professor Miall quotes Simmermacher to the effect that while the large disk on the fore-leg has 170 sucking-hairs, the enlarged joints of the tarsus of the middle leg bear no less than 1590. These hairs are plainly discernible with the half-inch Steinheil, and I have made them out with the inch, and think that I could show them to anybody else with that power. I have not looked for these sucking-hairs on the middle leg of other Beetles of the same family which have disks on their fore-legs, but they do exist in some other genera. If we watch a male Dytiscus in life, in a small aquarium, we shall soon be convinced that Lowne and Miall are correct in their statement that the cup-hairs discharge an adhesive substance. We shall see this all the more plainly if there is much floating vegetation. For, in swimming about, the Beetle will often come in contact with some of this, and it will adhere to the cup-hairs. His struggles to free himself from the encumbrance will show that the attachment is not altogether under his control. The offending weed is rubbed against the spines of one of the other legs till it is removed. [Illustration: FIG. 25.--Female Dytiscus swimming.] The spines with which the legs are set are worthy of a good deal of attention, and, like the adhesive cup-like hairs, though in different fashion, they doubtless assist the animal in holding its prey. The first and middle legs end in strong claws; those of the last pair are not so well developed.