Biologists classify animals and plants according to characteristics that they have in common.
Organisms that move from place to place of their own accord and that do not manufacture their food from raw materials in their environment are classified as animals.
Organisms that move from place to place of their own accord and that do not manufacture their food from raw materials in their environment are classified as animals.
Organisms that cannot move from place to place of their own accord (but can be moved by forces of nature or by animals) and that can manufacture their own food from raw materials in their environment, are called plants. Within the huge group called animals are many that have no backbones. These are called invertebrates. The invertebrates include mostly small animals. Some live in water and some on land. Some move around by wriggling, some walk, and some fly. One major group or phylum of invertebrates is called the Arthropoda. Biologists include insects and spiders in this group.
Spiders are not insects because they have some characteristics that are different from insects.
Amongst the vast numbers of invertebrate animals in the Phylum Arthopoda, more than a million different kinds have bodies with three main parts--head, thorax, and abdomen. The head has eyes, antennae and mouthparts. The thorax has three pairs of legs. The entire body is protected by a tough outer covering called an exoskeleton. Animals that share these characteristics are called insects. The group to which they belong is called the Insecta.
Another, smaller, group of invertebrate animals has only two main body parts. The body consists of a combined head and thorax called the cephalothorax, and the abdomen. The cephalothorax has the eyes, mouthparts (no antennae) and four pairs of legs. Animals that share these characteristics include ticks, mites, scorpions and spiders. The group is called the Arachnida.
THE SPIDER
Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other orders of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exceptions of air and sea colonization.
THE SCORPION
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are easily recognised by the pair of grasping pedipalps and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger. Scorpions range in size from 9 mm / 0.3 in. (Typhlochactas mitchelli) to 23 cm / 9 in. (Heterometrus swammerdami).
The evolutionary history of scorpions goes back to the Silurian era 430 million years ago.
They have adapted to a wide range of environmental conditions and can now be found on all continents except Antarctica. Scorpions number about 1750 described species, with 13 extant families recognised to date. Only about 25 of these species are known to have venom capable of killing a human being.The taxonomy has undergone changes and is likely to change further, as genetic studies are bringing forth new information.
