Class Amphibia include the toads and frogs (Superorder Salienta), salamanders, and worm-like tropical caecilians, and various fossil forms from the Devonian Period (408-360 mya) forward. The class nameAmphibia appropriately indicates that most of the species live partly in fresh water and partly on land. In both structure and function, the Amphibians stand between fishes and reptiles, being the first group among the chordates to live out of water. There are several new features that adapted them for terrestrial life, such as legs, lungs, nostrils connecting the mouth cavity, and sense organs that can function in both water and air.

Class Amphibians are the lowest and earliest Tetrapoda, or land vertebrates. They undoubtedly derived from some fish-like ancestors, possibly in Devonian (408-360 mya) times. The transition from water to land involved:

- modification of the body for travel on land while retaining the ability to swim

- Development of limbs in place of paired fins

- change of the skin to permit exposure to the air

- Replacement of the gills by lungs

- changes in the circulatory system to provide for respiration by the lungs and skin

- and acquisition of sense organs that function in both air and water

The earliest recognized amphibians (Ichtyostega) are in the Devonian (408-360 mya) rocks of Greenland and were of some size, as the skulls are 6 inches long. They possibly derived form crossopterygian fishes, had both aquatic and aerial respiration, and possessed fins with skeletal supports from which the tetrapod limb could be derived. The Devonian (408-360 mya) was evidently a time of seasonal droughts, when the chances of survival were greater for an animal that could leave a drying pool to travel over land and seek water elsewhere. By Carboniferous (360-286 mya) time, with its coal swamps, there were three distinct orders of ancient amphibians. The skull became flattened and roofed with bone, and many had external armor of bony plates, possibly as protection against large eurypterids (arthropods) then in fresh waters. Those amphibians were from a few inches to 15 feet in length, some aquatic and some terrestrial. Some of the large types (Triassic) were permanently aquatic, and several had degenerated to become limbless, like eels or snakes. Salamanders appeared in the Cretaceous (144-65 mya) and were distinct from earliest frogs, which appeared in the Triassic. Both show degenerative trends in the skull by loss of bones.

The typical toad (Bufo) has a warty skin and short legs for hopping, and the typical frog (Rana) has a relatively smooth skin and long legs for leaping, but there are numerous variations among the other genera. Common names for members of this group have been acquired through usage. There are no hard and fast rules for distinguishing a "toad" from a "frog."

They must avoid temperature extremes and drought because they have no regulation of body temperature and can lose water easily from the soft skin. In winter toads hibernate deep in the lakes or streams that do not freeze; then they burrow or go below the frost line. During hibernation all bodily processes are lessened, the heartbeat is slow, and the animal subsists on material stored within its body, including glycogen in the liver. In some southern states most of the toads and frogs (amphibians) are active all seasons, but in the hot dry lowlands of California some aestivate during summer.

The Tropical Tree Toad are staple foods of turtles, snakes, herons, certain hawks, raccoons, and large fishes. The acrid secretions of the warts on toads protect these animals from many predators, but not all. Raccoons and skunks reportedly roll toads underfoot to get rid of the secretions before killing and eating them. The larvae are preyed on by large water bugs, beetle, dragon-fly nymphs, and the enemies of adult amphibians.

The earliest known fossil insects are Collembola from the middle Devonian (408-360 mya) Of Scotland, but that group is off the main line of insect evolution, having only six abdominal segments and lacking Malpighian tubules. The sudden appearance of winged insects in Carboniferous (360-286 mya) rocks is a spectacular feature of fossil record. Several theories have been proposed to account for the origin of insect wings. The paranotal theory is based on the presence in Carboniferous insects (Palaeodictyoptera) of lateral expansions of the prothoracic terga, suggesting that the functional wings of meso- and metathorax may have arisen from similar broad flaps. Presumably these could have been used for guiding or planing, even before they became movable as true wings. The paranotal theory would derive winged insects from forms like the present-day Diplura and these, in turn, are derivable from myriapod-like ancestors that resembled present-day Symphyla. The tracheal gill hypothesis is based on the resemblance between wings and the movable gills found on present-day May-fly nymphs. Primitive May-flies were present in Palezoic (544-245 mya) times, but this theory would require an unknown aquatic ancestor for winged insects on a line separate from terrestrial Aperygota.

The grasshoppers, flies, lice, butterflies, beetles, bees, and a host of similar small creatures that comprise the CLASS INSECTA number full 800,000 species. They are the most abundant and widespread of all land animals, being the principal invertebrates that can live in dry environments and the only ones able to fly. These habits are made possible by the chitinious body covering that protects the internal organs against injury and loss of moisture, by the extensions of this covering that form the wings, and the system of tracheal tubes that enable insects to breathe air. The ability to fly helps them to find food and mates and to escape enemies. Because their life cycles usually are short they can multiply rapidly under favorable conditions. Insects abound in all habitats except the sea; various kinds live in fresh and brackish waters, in soil, on and about plants of all kinds, and on or in other animals. Different species eat all sorts and parts of plants-roots, stems or leaves, sap or blossoms, seeds or fruits; many flowers-visiting insect aid in pollination. Others utilize the tissues, fluids, and excretions of animals, and the scavenger insects consume dead animals and plants. Parasitic insects live in the eggs, larvae, or adults of other insects and on many other animals and plants. Some insects transmit disease-virus, bacterial, protozoan, or others-to plants, animals, and man. Insects in turn are eaten by other insects, spiders, scorpions, and many vertebrates from fishes to mammals. The predaceous and parasitic species serve importantly to regulate the numbers of other insects.

Insects along with birds and bats are the only animals capable of true flight. The wings of insects are unique structures in being derived as extensions of the body integument quite unlike the limb-wings of the vertebrates. The ability to fly enables insects to extend their feeding ranges and to disperse and occupy new territory.

Insects occur from sea level to well above 20,000 feet on the highest mountains. Surveys by airplanes show that many inhabit the air, especially by day in summer; they are mostly below 1,000 feet, but some have been taken up to 14,000 feet. By such means, species are constantly being carried to new localities.

Life on land exposes insects to greater extremes of environmental conditions than are experienced by most inhabitants of the sea and fresh waters, They have to withstand changes in temperature and food supply, as between the summer and winter and in the abundance or scarcity of plants. Different species are adapted to these problems in various ways. Many are abundant in warm seasons and are far reduced at other times. Some, during the winter as adults hide away to hibernate in shelters where their bodily metabolism is far reduced. Others survive as pupae or larvae. In many species all individuals die at close of the warm season and then are represented only by eggs that will develop and hatch the following spring.

Insects responds to many stimuli that provoke sensations in man, including light, chemical stimuli (taste, smell), touch, and sound, but their perceptions differ in both kind and magnitude. They detect chemical stimuli far too delicate for the human nose or tongue, and some react to ultraviolet rays but not to red or infrared.

When the ancestors of insects left the water and moved to the land and air, they experienced changes analogous to those of amphibians and reptiles as compared with fishes. The sense organs became adapted to function in air, the cuticle served to resist the loss of body fluids by evaporation, and the trachea provided a means for breathing air. All insects, save those in humid environments, have a problem in the conservation of body water that does not pose as a problem for aquatic animals, since moisture may be lost in respiration and in evacuating food residues. The valves in the spicules may limit the amount of air (and moisture) exhaled under some conditions, but the more vigorously respiratory movements during flight serve incidentally to deplete the supply of fluids. Insects in dry situations extract water from food residues in the rectum, and they gain some metabolic water as a by-product of the oxidation of food materials within the body.

Because of their abundance, the insects are preyed upon by a great variety of other insects and invertebrates of the land and fresh waters. Insects are subject to many diseases that act to reduce their numbers. Grasshopper eggs are eaten by some beetles, bee flies, moles, skunks, and mice, the nymphs by robber flies and digger wasps, and both nymphs and adults by large predatory insects and by frogs, reptiles, birds, and mammals. One-tenth of all insects found in bird stomachs examined the U.S Biological Survey were grasshoppers and their close allies. Eggs of grasshoppers are parasitized by certain insects. Flesh flies (Sarcophaga) lay living maggots on adults, and tachinid flies deposit their eggs on grasshoppers in flight; the larvae of both burrow into their hosts and consume the fat tissues. Parasitized grasshoppers become logy and fail to reproduce, or die. The parasitic insects thus constitute a factor in grasshopper control. Both fungus and bacterial diseases also destroy numbers of grasshoppers at times. Man practices control by using chemical sprays and poisoned bait on fields where nymphs and adults feed and also by plowing weed or stubble fields to expose the egg masses.

The grasshopper is generalized as to anatomy, has chewing mouth parts, undergoes a gradual or incomplete metamorphosis from the young to nymph stages to the adult, and lives independently for a single season. They occur over the world, mainly in open grasslands, where they eat leafy vegetation (a great variety of plants).

BIOLOGY (Fourth Edition), Neil A. Campbell University Of California, Riverside

General Zoology (Fourth Edition), Storer and Usinger McGraw-Hill Book Company

http://biodiversity.uno.edu (Biodiversity Collection)

http://www.oit.itd. umich.edu/Bio108 (Animal Diversity Web)