Specific glandular complexes in the skin are mostly restricted to a pair of lateral or dorso-lateral folds; they are often absent, but a few species, e.g. R. glandulosa of Borneo, R. temporalis of Ceylon, R. elegans and R. albolabris of West Africa, have a pair of large flat glands at the base or inner side of the arms.
All the species of Rana, except those in the Solomon Islands, spawn in the water, where the development of the tadpoles takes its course. Those of some Indian species, notably R. alticola and R. afghana of the Himalayas, and R. curtipes of Malabar, are very peculiar, being provided on either side of the shoulders with a large oval parotoid-like gland, well defined and crowded with pores; R. alticola possesses in addition an unpaired, sharply marked glandular complex on the top of the root of the tail, or rather upon the future coccyx. These complexes gradually disappear with age.
The genus Rana, with about 140 species and subspecies, is distributed over the whole of Arctogaea so far as this is available for Amphibian life, while there are only a few stragglers in Notogaea, namely, a few species in Ecuador and in the Peruvian or Upper Amazon district. None exist in the rest of the Neotropical region, including the Antilles, and practically none in Australia; but R. arfaki and R. papua inhabit New Guinea and the northern corner of Australia, R. kreffti the Solomon Islands. A few species are restricted to Madagascar, and a few others live there and on the continent of Africa.
So far as number of species is concerned, the home of the genus Rana is the Palaeotropical region; about one dozen (some of them with a very wide range) live in the Palaearctic sub-region, scarcely more in the Nearctic sub-region, and a few in Central America.
R. temporaria (the common European Brown Frog or Grass-frog).–The tympanum is distinct, two-thirds the diameter of the eye in size. The first finger is slightly longer than the second, which is shorter and weaker than the others, whilst the fourth is the longest. All the fingers are quite free. When the hind-limbs are laid forwards along the body, the ankle-joint reaches to a point between the eye and the tip of the snout. The five toes, which are about half webbed, increase in length from the first to the fourth, while the fifth is about equal to the third. The sole of the foot has a small, blunt, inner metatarsal tubercle; the outer one is scarcely visible. The skin is smooth, always moist, owing to the minute mucous glands; but a series of larger glands forms a pair of folds along the upper sides of the back; beginning behind the eyes they converge slightly beyond the shoulders, diverge a little in the sacral region, and converge again towards the vent. Another, much feebler, Λ-shaped ridge lies between the shoulders.
The male has two internal vocal sacs, which, when in use, bulge out the skin of the throat beneath the angles of the mouth like a pair of globes. It is further distinguished from the female by the stronger muscles of the arms and by a pair of swollen pads on the inner side of the first finger. During the pairing season these pads are enlarged into cushions covered with black horny rugosities.
The iris is golden, with dark specks. The coloration is, generally speaking, brown above, with black-brown irregular spots, especially on the sides of the body, and with cross-bands on the legs. The under parts of the male are white or pale yellow, with a bluish tinge on the throat, while the female is more yellow instead of white, inclining to orange. In both sexes the under parts are mostly spotted with darker colours. A large dark-brown patch, extending from behind the eye over the tympanum towards the shoulder, is always present and has given this frog its specific name. Otherwise the coloration varies considerably; more or less according to the locality and nature of the surroundings, and to individual variation and temporary change of colour.
Some specimens are almost spotless above and of a rich brown, or almost yellow colour, the spots being restricted to the sides below the lateral folds. Others have very few spots, but these are then arranged in two interrupted streaks on the back. The under parts, especially the flanks, may be lemon yellow instead of whitish, and the darker markings may be almost absent. Boulenger has figured a beautiful specimen, almost orange red, with red spots and vermiculations on the yellow under surface. I have found similar red specimens of unusually striking appearance between Berlin and Spandau in a forest-glade, through which run little streams with banks of red ferruginous soil. Specimens which live in woods with rich black soil are often very dark, all the brown and reddish tints being absent. The variations are, however, really endless, and it is difficult to find two individuals exactly alike, even amongst a great number collected in the same locality. Moreover, they change colour. Warmth makes them paler, cold causes the chromatophores to expand and the whole frog appears darker. During the breeding season the males assume a delicate bluish hue, especially on the throat, but this film quickly fades away when they are taken out of the water. It is caused by the swelling of the cutaneous lymph-spaces which extend their ramifications into the epidermal layer, and it is not a question of pigmentation or of chromatophores, but a case of interference-colours, blue being frequently the result of the light passing through a cloudy, colourless, but not quite transparent and thin stratum, in this case the turgid epidermis.
The habits of the Grass-frog are essentially terrestrial. It spends most of its time on land, preferably in damp places, but local fashion permits of a great deal of freedom, as these frogs are sometimes found not only in very wet, naturally irrigated places, but also in the water itself. However, the Grass-frog when pursued rarely takes to the water for safety. It trusts to flight, first by a few long and fast jumps, and then to concealment by squatting down between grass, under leaves; it rarely creeps into a hole, even if there be one near. The jumps soon become shorter and shorter after a few dozen repetitions. It swims well, but cannot climb. 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 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. As a rule these frogs do not crawl, they jump or hop, even whilst stalking, and this takes place at any time of the day; in fact they are very diurnal, although they become more active towards the evening. When caught they are at first very wild and, like all true frogs, very impetuous, committing acts of astonishing stupidity without any apparent sense or appreciation of distance or height. The captive will not only jump off the table, whilst a toad stops at the edge and looks carefully down, but without hesitation he jumps out of the window, regardless of the height above the ground. This is due to sheer fright; he loses his head. When at large in his native surroundings, nothing will induce him, although hotly pursued, to commit suicide by jumping down a precipice. But all this wildness calms down wonderfully soon. The captive no longer dashes his head against the glass, he does not struggle or twist when taken up; on the contrary, he makes himself at home, watches your coming with intense expectation, and without hesitation accepts the proffered mealworm, maggot, butterfly or earthworm; in short, he shows what a jolly and intelligent fellow he really is.
The Grass-frog has many more obvious enemies than perhaps any other Amphibian, and it is not even slightly protected by any appreciable poisonous secretion. Nevertheless it is extremely common. A whole host of birds eat it–for instance, buzzards, harriers, and above all storks. Foxes, polecats, and stoats are not averse to it, and the Grass-snake derives its main sustenance from it. In fact the enemies of the little frog are legion, one of the worst being Man. In France, Italy, and other parts of the Continent, the skinned fleshy hind-limbs are turned into a by no means disagreeable ragoût, or into dainty morsels when fried in butter and encrusted with bread-crumbs. This frog, together with its cousin the Water-frog, also suffers from the distinction of being one of the chief martyrs to science. Perhaps the story is true that Galvani was led to his investigations into animal magnetism and electricity by observing that the legs of a number of skinned frogs, strung up by his wife upon the bronze railings of the balcony, jumped whenever the scissors, which cut off the feet, touched the other metal. Frogs have suffered ever since. Easily procured and of a convenient size, they are used in every biological laboratory, and the young student is supposed to be initiated into the mysteries of Vertebrate structure by the careful dissection and study of this, the worst of all the so-called types. Next to Man there is no animal which has been studied so minutely, and has had so many primers and text-books written on it, as this frog. In spite of all this it is very little understood, thanks to its rather aberrant and far from generalised structure.