HE stroller along brooksides, is likely to be surprised some day, at seeing a bit of moss and earth suddenly make a high leap and a far one, without apparent provocation. An investigation resolves the clump of moss into a brilliantly green and yellow, striped frog, and then the stroller wonders how he could have overlooked such an obvious creature. But the leopard frog is only obvious when it is out of its environment. The common green frog is quite as well protected since its color is exactly that of green pools. Most frogs spend their lives in or about water, and if caught on land, they make great leaps to reach their native element; the leopard frog and a few other species sometimes wander far afield.
In form, the frog is more slim than the toad, and is not covered with great warts; it is cold and slippery to the touch. The frog's only chance of escaping its enemies, is through the slipperiness of its body and by making long, rapid leaps. As a jumper, the frog is much more powerful than the toad because its hind legs are so much larger and more muscular, in comparison with its size. The first toe in the front feet of the leopard frog is much swollen, making a fat thumb; the mechanics of the hind legs make it possible for the frog to feather the webbed feet as it swims. On the bottom of the toes are hardened places at the joints, and sometimes others besides, which give the foot a strong hold when pushing for the jump. The toe tips, when they are pressed against the glass, resemble slightly the tree-toads' discs. The hind foot is very long, while on the front foot the toes radiate almost in a circle. The foot and leg are colored like the back of the body above, and on the under side resemble the under parts.
The frog is likely to be much more brightly colored than the toad, and usually has much of green and yellow in its dress. But the frog lives among green things, while it is to the toad's advantage to be the color of the soil. Frogs also have the chameleon power of changing color, to harmonize with their environment. I have seen a very green leopard frog change to a slate-gray when placed upon slate-colored rock. The change took place in the green portions. The common green frog will likewise change to slate-color, in a similar situation. A leopard frog changed quickly from dark green to pale olive, when it was placed in the water after having been on the soil.
The eyes of frogs are very prominent, and are beautiful when observed closely. The green frog has a dark bronze iris with a gleaming gold edge around the pupil, and around the outer margin. The eye of the leopard frog is darker; the iris seems to be black, with specks of ruddy gold scattered through it, and there is an outer band of red-gold around the margin. When the frog winks, the nictitating membrane rises from below and covers the whole eye; and when the frog makes a special effort of any sort, it has a comical way of drawing its eyes back into its head. When trying to hide at the bottom of the aquarium, the leopard species lets the eye-lids fall over the eyes, so that they do not shine up and attract pursuers.
The ear is in a similar position to that of the toad, and in the bullfrog, is larger than the eye. In the green frog, it is a dull grayish disc, almost as large as the eye. In the leopard frog, it is not so large as the eye, and has a giltish spot at the center.
The nostrils are small and are closed when below the water, as may be easily seen by a lens. The mouth opens widely, the corners extending back under the eye. The jaws are horny and are armed with teeth, which are for the purpose of biting off food rather than for chewing it. When above water, the throat keeps up a rythmic motion which is the process of breathing; but when below water this motion ceases. The food of frogs is largely composed of insects, that frequent damp places or that live in the water.
The sound-sacs of the frogs, instead of being beneath the throat, as is the case with toads and tree-frogs, are at the side of the throat; and when inflated, may extend from just back of the eyes, out above the front legs. The song is characteristic, and pleasant to listen to, if not too close by. Perhaps exception should be made to the lay of the bullfrog, which like the song of some noted opera singers, is more wonderful than musical; the boom of the bullfrog makes the earth fairly quake. If we seize the frog by the hind leg, it will usually croak and thus demonstrate for us, the position of its sound-sacs.
In addition to the snakes, the frogs have inveterate enemies in the herons which frequent shallow water, and eat them in great numbers. The frogs hibernate in mud and about ponds, burrowing deep enough to escape freezing. In the spring, they come up and sing their spring songs and the mother frogs lay their eggs in masses of jelly on the bottom of the pond, usually where the water is deeper than in the situations where the toads' eggs are laid. The eggs of the two can always be distinguished, since the toads' are laid in strings of jelly, while the frogs' are laid in masses.
It is amusing to watch with a lens, the frog tadpoles seeking for their microscopic food along the glass of the aquarium. There are horny upper and lower jaws, the latter being below and back of the former. The upper jaw moves back and forth slightly and rythmically, but the dropping of the lower jaw opens the mouth. There are three rows of tiny black teeth below the mouth and one row above; at the sides and below these teeth are little, finger-like fringes. Fringes, rows of teeth and jaws all work together, up and down, out and in, in the process of breathing. The nostrils, although minute, are present in the tadpole in its early stages. The pupil of the eye is almost circular and the iris is usually yellow or copper-bronze, with black mottling. The eyes do not wink nor withdraw. The breathing-pore on the left side, is a hole in a slight protuberance.
At first, the tadpoles of the frogs and toads are very much alike; but later, most of the frog tadpoles are lighter in color, usually being olive-green, mottled with specks of black and white. The frog tadpoles usually remain much longer than the toads in the tadpole stage, and when finally they change to adults, they are far larger in size than the toads are, when they attain their jumping legs.
Leading thought—The frog lives near or in ponds or streams. It is a powerful jumper and has a slippery body. Its eggs are laid in masses of jelly at the bottom of ponds.
Method—The frog may be studied in its native situation by the pupils or it may be brought to the school and placed in an aquarium; however, to make a frog aquarium there needs to be a stick or stone projecting above the water, for the frog likes to spend part of the time entirely out of water or only partially submerged.
1. Where is the frog found? Does it live all its life in the water? When found on land how and where does it seek to escape?
2. Compare the form of the frog with that of the toad. Describe the skin, its color and texture. Compare the skin of the two.
3. Describe the colors and markings of the frog on the upper and on the under side. How do these protect it from observation from above? From below? How do we usually discover that we are in the vicinity of a frog?
4. Describe the frog's ears, eyes, nostrils and mouth.
5. Compare its "hands and feet" with those of the toad. Why the difference in the hind legs and feet?
6. How does the frog feel to your hand? Is it easy to hold him? How does this slipperiness of the frog benefit it?
7. On what does the frog feed? What feeds on it? How does it escape its enemies?
8. What sounds does the frog make? Where are its sound sacs located? How do they look when they are inflated?
9. Is the frog a good swimmer? Is it a better jumper than the toad? Why?
10. Where are the frog's eggs laid? How do they look?
11. Can you tell the frog tadpoles from those of the toad? Which remains longer in the tadpole stage? Study the frog tadpoles, following the questions given in Lesson XLIV.
12. What happens to the frog in winter?