The Edible Frog (Rana Esculenta.)
Two species of Rana are common in America and Europe, viz., Rana esculenta and Rana temporaria. The latter alone is indigenous to Great Britain, and varieties of it extend throughout temperate Europe and Asia to Japan, and one variety (pretiosa) exists in the United States. The edible Frog (Rana esculenta), however, has been introduced into England. An Indian species (Rana breviceps) and several South African species burrow in the ground.
Ecology and Habits. The skin of Frogs is usually smooth and free from warts or horny excrescences. It is invested with a colorless epidermis, which is shed from time to time as the creature grows; this splits along the back and thighs, is worked over the head like the taking off of a shirt, and usually eaten by the wearer. The deeper layers contain much pigment, in cells which are more or less under muscular control, enabling Frogs to change their hue to conform to the background.
Frogs are carnivorous, and in the season of activity are likely to be very voracious. The terrestrial and arboreal forms feed mainly on insects, worms, etc. The aquatic kinds also catch insects, but subsist more on aquatic animals--#8212;worms, tadpoles, small fishes, and other Frogs. These are seized and slowly swallowed, often, where before the remainder, perhaps still alive, has been got within the mouth.
Extremes of cold or drought in climate must be avoided by Frogs. Moisture of the skin is necessary to their health, and in very dry places or seasons they survive only by going deeply under ground. Thus some tropical species get through the "dry season." The frogs of northern climates endure the winter by clustering about spring-holes and other places where the water is comparatively warm and free of ice; or else by hibernating in the mud. Terrestrial species bury themselves for the winter in the loam, or burrow into the dry dust of rotting logs and stumps. Their vitality is strong, and their power of regeneration from partial congelation is very great.
Though most species live always in or near water, many spend the greater part of their time away from it, and often in bushes or trees. These, however, go to the water to breed; and as this function is likely to demand attention early in the spring, it is then that these animals make themselves most conspicuous by the incessantly uttered croaking or rattling calls of the males, which are almost as varied as the songs of the birds, and more ventriloquistic. These are wholly the cries of the male Frogs, and cease when the mates have been found and have spawned; and to assist in producing them many species have gular air-sacs, which are connected with the vocal organs and furnish the power required for the loud and insistent utterances. The great ear-drums correlated with this vocal power are conspicuous in many species.
The reproductive habits of Frogs are various. All of our common species lay their eggs in water, the eggs being fertilized as they are laid. As the eggs are laid they are inclosed in a gelatinous envelope secreted by the female. This swells and protects the eggs from injury, from being fed upon, from the direct rays of the sun, and in some species it serves to float the eggs at the surface of the water, where oxygen is most abundant; finally, the envelope serves as food for the young frogs. The mouth of the tadpole is small and provided with a horny beak, which takes the place of the teeth which are not yet developed. The tadpole feeds on algæ that cover stones, and on the flesh of dead animals. The long, spirally coiled intestine, which can be seen on the under side of the animal, is an adaptation to its prevailingly herbivorous diet, which requires a prolonged digestion.
The tadpole usually lives in the water for two or three months before it takes to land. In the Bullfrog, however, the transformation (see Toad) does not take place until the second summer.
In many tropical Frogs the reproductive habits are much modified. One species (Phyllobates trinitatis) of Venezuela and Trinidad carries its tadpoles on its back, to which the young attach themselves by means of their suckers. A frog of the Seychelles Islands lives in the tree-ferns far from water, and carries its young about on its back, to which they are attached by their bellies. In the Kameruns lives a Frog that lays its eggs in a foamy mass on the leaves of a tree. When the larvæ are developed the mass becomes slimy and the tadpoles swim about it, and when a heavy rain falls they are washed into pools of water lying at the bases of the trees. The foam is probably produced as it is in culinary operations, by air being entangled in it by a beating that the Frog gives the jelly with its feet. The inclosed air may well serve in respiration. Compare Toad.