Wildlife [antikvár]

There are over one million different types of animal on Earth today. Each kind of animal has unique features. The types look different from each other, find their food in different ways and arc quite distinct from other species. There arc however, certain features which group animals together into genus, family or class. Scientists make use of such features to categorise animals into 'cats', 'dogs' or 'lizards'. Each animal in the group shares characteristics with the other which are not found in animals of other groups.The most basic grouping is between animals with a backbone and those without. The invertebrates, which lack a backbone, are by far the most numerous both in absolute numbers and in the variety of species. Some species lack a skeleton at all, such as the jellyfish and amoebas, but most have a skeletal system which encloses the body. These are the crabs, crustaceans and insects.Of these the most numerous are the insects, and in fact the insects can be said to be the most successful group of creatures the world has ever known. They first evolved many hundreds of millions of years ago, before the first dinosaurs stalked the Earth, and have flourished ever since. There are thought to be about one million species of insects, though nobody is entirely certain how many diere really are. It has been said that any naturalist wanting to discover a new species need only walk through the Congo rain forest armed with a butterfly net for an hour or two to be certain of catching at least one previously undescribed type of insect. At the same time the destruction of large areas of wilderness means that several species are exterminated each year, often without anyone being aware that they existed.The incredible variety of insect life is, in part, due to the extraordinary degree of specialisation which the different species can show. The yucca moth (Tegeticula alba), for example is adapted to co-exist with the yucca plant. When ready to lay its eggs the female moth visits a yucca flower and collects a quantity of pollen. It then flies to a different yucca and deposits the pollen on the stigma, thus fertilising the seeds. The moth then lays its eggs in the flower and departs. The eggs are timed to hatch at the moment when the seeds have developed enough to provide food for the caterpillars. The moth larvae therefore have a ready-made source of nourishment on which they thrive before pupating into adults. The yucca also benefits for some seeds are usually left to develop into seedlings.Such diversity and adaptability is made possible by the tough exoskeleton which the insects have developed. Completely encasing the insect's body is a layer of strong chiton. This acts not only as armour, but also as an anchorage for the muscles, giving shape to the whole animal. As such the exoskeleton is remarkably successful and insects can withstand pressures, relative to their body size, far greater than any other animal. But it has its drawbacks, one of which is that the animal cannot grow and expand because it is encased in a solid shell of armour. Insects get around this problem by periodically moulting their skin. The old, rigid skin is split open and the insect, surrounded by a soft new skin emerges. Puffing its body up, the insect pushes its new skin into a larger shape before it dries and hardens into a rigid shell. At such a time the insect is almost shapeless and powerless to move. This makes it vulnerable to attack, and many fall victim to predators.The higher animals lack the exoskeleton and instead have an internal skeleton. These are the vertebrates, animals with backbones. The earliest vertebrates were fish, which first appeared over 500 million years ago and are still the most numerous. Vertebrates have the enormous advantage that their skeleton can grow with them, so they never need undergo the vulnerable stage of skin-shedding. When vertebrates moved on to the land in the form of amphibians some 390 million years ago they were faced with the problem of supporting their bodies. Denied the support of water, the land-based amphibians had to keep themselves upright and be able to move around. They solved both problems by converting their fins into legs.Amphibians were the most advanced animals on earth for millions of years, but have now been overtaken by several other groups. They do however remain an important part of the fauna in many areas. Amphibians are, by definition, tied to the water both during their everyday lives and for reproduction. Indeed some species are never found more than a few feet from ponds or streams.