FREE ESSAY ON HISTORY OF FROGS

HISTORY OF FROGS

A Frog is a small, tail less animal that has bulging eyes. Almost all frogs have long back
legs. The strong hind legs make the frog able to leap farther than the length of its
body. Frogs live on every continent except Antarctica, but tropical regions have the
greatest number of species. Frogs are classified as amphibians. Most amphibians,
including most frogs, spend part of their life as a water animal and part as a land
animal. Frogs are related to toads, but are different from them in a few ways. The giant
frog of west-central Africa ranks as the largest frog. It measures nearly a foot (30
centimeters) long. The smallest species grow only 1/2 inch (1.3 centimeters) long. Frogs
also differ in color. Most kinds are green or brown, but some have colorful markings.
Although different species may vary in size or color, almost all frogs have the same
basic body structure. They have large hind legs, short front legs, and a flat head and
body with no neck. Adult frogs have no tail, though one North American species has a
short, tail like structure. Most frogs have a sticky tongue attached to the front part of
the mouth. They can rapidly flip out the tongue to capture prey. Frogs have such internal
organs as a heart, liver, lungs, and kidneys. Some of the internal organs differ from
those of higher animals. A frog's heart has three chambers instead of four. And although
adult frogs breathe by means of lungs, they also breathe through their skin. The eggs of
different species vary in size, color, and shape. A jelly like substance covers frog
eggs, providing a protective coating. This jelly also differs from species to species.
Some species of frogs lay several thousand eggs at a time. But only a few of these eggs
develop into adult frogs. Ducks, fish, insects, and other water creatures eat many of the
eggs. Even if the eggs hatch, the tadpoles also face the danger of being eaten by larger
water animals. The pond or stream in which the eggs were laid sometimes dries up. As a
result, the tadpoles die. Certain tropical frogs lay their eggs in rain water that
collects among the leaves of plants or in holes in trees. Other tropical species attach
their eggs to the underside of leaves that grow over water. When the eggs hatch, the
tadpoles fall into the water. Among some species, one of the parents carries the eggs
until they hatch. For example, the female of certain South American tree frogs carries
the eggs on her back. Among another species of frog, the midwife toad, the male carries
the eggs wound around his hind legs. Males of another species, Darwin's frog, carry the
eggs in their vocal pouch. Some tropical frogs lay their eggs on land. They lay them
under logs or dead leaves. These frogs have no tadpole stage. A young frog hatches from
the egg and begins life as a land animal. Tadpoles are not completely developed when they
hatch. At first, the tadpole clings to some support in the water, using its mouth or a
tiny sucker. A tadpole has no neck, and so its head and body look like one round form.
The animal has a long tail and resembles a little fish. It breathes by means of gills,
which are hidden by a covering of skin. A tadpole's form changes as the animal grows. The
tail becomes larger and makes it possible for the animal to swim about to obtain food.
Tadpoles eat plants and decaying animal matter. Some tadpoles eat frog eggs and other
tadpoles. In time, the tadpole begins to grow legs. The hind legs appear first. Then the
lungs begin to develop and the front legs appear. The digestive system changes, enabling
the frog that develops to eat live animals. Just before its change into a frog, the
tadpole loses its gills. Finally, a tiny frog, still bearing a stump of a tail, comes up
from the water. Eventually, the animal absorbs its tail and assumes its adult form. After
a frog becomes an adult, it may take a few months to a few years before the animal is
mature enough to breed. The green frog and the pickerel frog mature in about three years.
In captivity, a bullfrog may live more than 15 years. But few species of frogs live
longer than 6 to 8 years in the wild. Many are eaten by such enemies as bats, herons,
raccoons, snakes, turtles, and fish. Adult frogs eat mainly insects and other small
animals, including earthworms, minnows, and spiders. Most frogs use their sticky tongue
to capture prey. The tongue is flipped out of the mouth in response to movement by the
prey. Most frogs have teeth only on their upper jaw. Toads lack teeth altogether. As a
result, frogs and toads swallow their prey in one piece. To aid in the swallowing
process, the frog's eyes sink through openings in the skull and force the food down the
throat. More than 20 kinds of true frogs live in the United States. Many of these frogs
also live in Canada. A group of related species known as leopard frogs are the most
widespread. Leopard frogs range from the Atlantic coast to eastern California and from
northern Canada to the Mexican border. The bullfrog, which may grow up to 8 inches (20
centimeters) long, ranks as the largest American and Canadian frog. Other common true
frogs of the United States and Canada include the green frog, the pickerel frog, and the
wood frog. Unlike most other true frogs, the wood frog spends much of its time away from
water. It lives in damp wooded areas of Alaska, Canada, and the Midwestern and Eastern
United States. Tree frogs, like true frogs, live on all continents except Antarctica.
Most tree frogs measure less than 2 inches (5 centimeters) long and dwell in trees. About
25 species of tree frogs live in the United States. Some of these species are also found
in Canada. Common species in the Eastern United States include the green tree frog, the
gray tree frog, and the spring peeper. Western tree frogs include the California tree
frog, the canyon tree frog, and the Pacific tree frog. Some North American tree frogs,
called chorus frogs and cricket frogs, live mainly on the ground. Other frogs of the
United States include leptodactylid frogs, narrow-mouthed toads, spadefoot toads, and
tailed frogs. Leptodactylid frogs make up a large family of frogs that live mainly in
Australia and South America. Those found in the United States include the barking frog,
the cliff frog, and the white-lipped frog. The barking frog and the cliff frog live on
rocky cliffs in Texas. These frogs lay their eggs under rocks. Tiny frogs hatch from the
eggs, without going through the tadpole stage. The white-lipped frog lives in the
southern Rio Grande Valley area of Texas. The female white-lipped frog lays her eggs in a
hole near water. She then beats the egg jelly into a foam. The tadpoles live in the foam
nest until rain washes them into the nearby water. Narrow-mouthed toads live throughout
most tropical and subtropical regions. As their name suggests, these frogs have an
extremely narrow mouth. The eastern narrow-mouthed toad, the Great Plains narrow-mouthed
toad, and the sheep frog are the only members of this family that live in the United
States. All three species live in burrows and eat ants and termites. Spadefoot toads live
in Asia, Europe, North America, and northwestern Africa. These frogs are called
spadefoots because most of them have a sharp-edged spadelike growth on each hind foot.
They use this growth as a digging tool. Spadefoot toads live throughout much of the
United States. They dwell underground and are usually seen only after a rain. Several
species live in dry regions of the Great Plains and the Southwest. These spadefoots may
remain in their burrows for weeks at a time to stay moist. They breed following heavy
rains, often laying their eggs in temporary ponds. The tadpoles develop rapidly. If
enough food is available, tiny adults may emerge in only 12 days. Tailed frogs live in
swift mountain streams of the northwestern United States and southwestern Canada. The
moving water makes external fertilization of the eggs difficult. Instead, the male uses a
tail like structure to fertilize the eggs while they are inside the female. Tadpoles of
tailed frogs have a large sucker that enables them to hold on to rocks even in the
strongest current.