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If these all survived to reproduce, the resulting progeny would number 5, flies! Carried out to the 25th generation, there would be 1. Fortunately this population explosion is held in check by many factors. Insects are at once entirely familiar, yet completely alien. Their jaws work from side to side, not up and down. Insect eyes, if present, are each unblinking and composed of dozens, hundreds, or even thousands of individual lenses. Although they lack nostrils or lungs, insects still breathe, thanks to small holes located on the sides of their bodies behind their heads, connected to an internal network of finely branched tubes.

This external skeleton is quite pliable and consists of a series of body divisions and plates joined with flexible hinges that allow for considerable movement. The orders Protura, Collembola, and Diplura, formerly considered insects, now make up the class Entognatha.

Grzimeks Animal Life Encyclopedia: Mammals V

Entognaths have mouthparts recessed into the head capsule, reduced Malpighian tubules excretory tubes , and reduced or absent compound eyes. The remaining orders treated in this volume are in the class Insecta. Insects are one of only four classes of animals with pterosaurs, birds, and bats to have achieved true flight, and were the first to take to the air. In-stead, insect wings evolved from structures present in addition to their legs, not unlike Pegasus, the winged horse of Greek mythology. Long extinct dragonflies winged their way through Carboniferous forests some million years ago and had wings measuring Insects are limited in size by their external skeletons and their mode of breathing.

While most species range in length from 0. The parasitic wasps that at-tack the eggs of other insects are less than 0. Some giant tropical insects, measuring 6. The small size of insects has allowed them to colonize and exploit innumerable habitats not available to larger animals.

Most species live among the canopies of lush tropical forests. Some species are permanent residents of towering peaks some 19, ft 6, m above sea level.

Fewer still have conquered the oceans, skating along the swelling surface. No insects have managed to penetrate and conquer the depths of freshwater lakes and oceans. The feeding ecologies of insects are extremely varied, and insects often dominate food webs in terms of both population size and species richness.

Plant-feeding insects attack all vegetative and reproductive structures, while scavengers plumb the soil and leaf litter for organic matter. Some species collect plant and animal materials—not to eat, but to feed to their young or use as mulch to grow fungus as food. Predatory species generally kill their prey outright; parasites and parasitoids feed internally or externally on their hosts over a period of time or make brief visits to acquire their blood meals.

Volume For example, from a chef's point of view, fishes come in two basic varieties—shellfish and finfish. Scientists eschew such groupings of distantly related creatures. However, lest they be hoisted with their own petards, ichthyologists might tread gently on the many concepts of fish, for they must ac-knowledge science's inability to form an absolute taxonomic definition of "fish" based on biological characteristics that are shared by all fishes and yet not shared with any "nonfish.

Widespread views of the particular characteristics that de-fine fishes, of course, are biased by general familiarity with extant i. Certainly, most fishes possess a well-developed vertebral column; however,hagfishes Myxinidae lack well-defined vertebrae, and there is disagreement among scientists regarding whether this characteristic exists because the ancestors of these fishes were similar or, antithetically, because vertebrae were "lost" from this lineage through evolutionary modification.

Unlike worms, fishes are chordates phylum Chordata , and they possess skeletal components that form a cranium i. This characteristic as well as many others distinguishes them from some fishlike chordates, such as the lancelets Amphioxiformes , but, of course, amphibians, reptiles, birds, and mammals also have a cranium.

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At first glance, fins seem to define fishes. Several unrelated groups of nonfishes e. Furthermore, althoughsome fishes, such as hagfishes and lampreys Petromyzontidae , lack paired fins, the paired appendages of amphibians, reptiles, birds, and mammals are considered homologous to the paired fins of fishes. These differences in the scales of fishes point to the fact that some other aquatic chordates, such as sea snakes, also have scales, even though the outer coverings of reptiles, birds, and mammals are heavily keratinized, whereas those of fishes are not.

Superclass Pisces as a polyphyletic group. Given that no one characteristic distinguishes all fishes from all other organisms, even the most committed ichthyologist must admit that the superclass Pisces an assemblage that ineludes all fishes represents an unnatural or polyphyletic group. Most biologists probably would agree that the consideration of all craniates as fishes would be of little scientific value and would betray the longstanding and widespread conception of a fish. Those not willing to endorse this definition might rest easy by considering "fish" as the raison d'etre for ichthyologists.

Volume 6. Amphibians edited by Neil Schlager Gale Group Excerpt: Almost everyone recognizes a fish, a bird, or a mammal, even a reptile.

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But what about an amphibian? There are three living groups of amphibians. Most salamanders are terrestrial, but some are aquatic, a few are burrowers, and some others are arboreal. Most frogs are terrestrial or arboreal, but many are aquatic, and a few are burrowers. A few caecilians are aquatic, but most burrow in soil in tropical regions of the world. In some ways amphibians are intermediate between the fully aquatic fishes and the terrestrial amniotes reptiles, birds, and mammals , but they are not simply transitional in their morphology, life history, ecology, and behavior.

The attachment of the pelvic girdle to the vertebral column incorporates only one sacral vertebra. Caecilians and some salamanders lack limbs and girdles, whereas in anurans the hind limbs are elongated and modified for jumping. The skin is glandular and contains both mucous and poison glands but lacks external structures such as scales, feathers, or hair, characteristic of other groups of tetrapods.

The heart has three chambers, two atria and one ventricle,which may be partially divided.

Grzimek's Animal Life Encyclopedia - Wikipedia

Typically, amphibians have two lungs, but the lungs may be reduced or absent in some salamanders, and the left lung is proportionately small in most caecilians as it is in snakes. Amphibians are ectotherms cold-blooded. The life histories of amphibians are highly diverse. The eggs of amphibians lack a shell and the embryonic membranes e. Phylogenetic relationships and classification. The living groups of amphibians are most closely allied with diverse fossils, the basal tetrapod vertebrates commonly placed in the class Amphibia. The phylogenetic relationships among these groups of fossils is equivocal.

The relationships of some groups of living amphibians have not been resolved with a high level of confidence. For example, a group of frogs endemic to Madagascar has been recognized as a family, Mantellidae, a subfamily of Ranidae, and a subfamily of Rhacophoridae adopted herein. The distributions of the families of amphibians reflect the history of Earth, especially from the time of the breakup of the supercontinent Pangaea, beginning about million years ago.

The early fragmentation resulted in two major land masses: Laurasia, consisting of what is now North America , Europe , and most of Asia ; and Gondwana, which included what are now South America , Africa , Madagascar , the Indian subcontinent, Australia , New Zealand , and Antarctica.

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Although a fossil caecilian is known from the Jurassic of North America, these amphibians now all live in regions that were part of Gondwana. Two families are restricted to the Indian subcontinent one in adjacent southeastern Asia , one family is endemic to Africa , and another to South America. Salamanders evolved in Laurasia. One early lineage containing the living Ascaphidae in North America and Leiopelmatidae in New Zealand has been allied with fossils from the Jurassic of South America, therebyindicating that this lineage had diversified prior to the breakup of Pangaea.

Many lineages are restricted to one continent: six families in South America , three in Africa , two in Australia , and one each in Madagascar and the Seychelles.

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  • Microhylidae is present on all Gondwanan land masses except the Seychelles , and it has dispersed into southeastern Asia and southern North America. True frogs Ranidae occur throughout the world, though only in northern Australia and northern South America on these continents, and toads Bufonidae occur on all continents, except Australia one species introduced. As a group, amphibians are distributed throughout the world, except for polar regions, most oceanic islands, and some desert regions.

    However, the patterns of distribution differ among the three living groups of amphibians. Globally, except for the Arctic and Antarctic regions which are not inhabited by amphibians , six biogeographic regions are recognized. The largest of these, the Palearctic Europe and temperate Asia has the fewest species of amphibians , followed by the Nearctic temperate North America with species. In contrast, the amphibian faunas of the southern continents consist mainly of anurans.

    The Australo-Papuan region Australia , New Zealand , New Guinea , and associated islands has species of anurans, but no salamanders or caecilians. The Ethiopian or Afrotropical region sub-Saharan Africa and Madagascar has species, of which 29 are caecilians. The Oriental region tropical and subtropical southeastern Asia, India, and associated islands harbors species, of which 29 are salamanders and 44 are caecilians.

    Ichthyophiidae and Uraeotyphlidae are endemic to the Oriental region, Scolecomorphidae to the Ethiopian region, and Rhinatrematidae to the Neotropical region. Most salamanders live in the Northern Hemisphere; they are absent in the Australo-Papuan and Ethiopian regions. Cryptobranchidae and Proteidae are represented by one genus each in the Nearctic and Palearctic regions. Salamandridae is the most widespread family of salamanders with nine genera in the Palearctic region, and two genera in the Nearctic region.