The habits of the frog.—The common grass frog (Fig. 214) is to be found in abundance, from early spring to October, in ditches, marshy land, and other damp places. As winter approaches, frogs generally bury themselves in the mud at the bottom of ponds, etc., and remain there in a state of torpor until the spring, when they emerge and the females lay their eggs. Frogs are said to be “cold-blooded” because their temperature never varies much from that of their surroundings. Birds and mammals, on the other hand, maintain an almost constant temperature,—a healthy man’s blood, for example, being just as warm in the depth of winter as it is on a hot summer day. So long as they are not actually frozen hard, therefore, frogs can endure the winter cold without much inconvenience. Although the frog is essentially a land animal, it is quite at home in the water, and swims gracefully and easily by the help of the webs which connect the long toes of its hind feet. On land, it progresses by long leaps, its limbs and body being [Pg 335] well adapted to this habit. As is usual in leaping animals, the hind legs and feet are markedly longer than the fore limbs; and the shortness of the body enables the two pairs of limbs to be brought together to break the shock of the fall. Fig. 214.—The Frog. (× 1.) Food.—The frog’s method of catching its prey is very interesting, and is graphically described by Dr. Hans Gadow[26] as follows:—“The food, which consists chiefly of insects, snails, and worms, must be moving to excite interest; then the frog, whose favourite position is half squatting, half supported by the arms, erects itself and, facing the insect, turns round upon its haunches, adjusts its position anew by a shifting of the legs, and betrays its mental agitation by a few rapid movements of the throat. All this time [Pg 336] the prey is watched intently until it moves; then there follows a jump, a flap of the tongue, and the insect is seen no more.” This flap of the tongue is well illustrated in Fig. 215. The frog’s tongue is free behind, but is attached, by its anterior end, close to the middle of the lower jaw,—an arrangement which enables it to be flicked out to its full length. Further, it is covered by a glutinous secretion, which sticks tenaciously to the prey. Fig. 215.—Three Stages of the Movement of the Tongue of a Frog. (× 1.)