The arachnids (spiders, scorpions, ticks and acari) (Class: Arachnida), are distinguished from other arthropods by having eight legs, they do not have antenna. Their heads are not differentiated from their bodies which are typically divided in to two principal regions: the cephalothorax (prosoma) and the abdomen (opisthosoma). On the cephalothorax there are four pairs of legs and other appendages called chelicerae and pedipalps. The abdomen may have no appendages, or may have modified appendages such as in the spiders and scorpions (Brusca and Brusca, 2002). In the spiders, the chelicerae function as fangs and inject venom, while in the scorpions the pedipalps are modified as large pincers to catch prey.
The great majority of arachnids are predators of insects and other arachnids. The exceptions are the harvestmen and many of the acari which feed on plant material and which reintegrate this material to the soil. Many acari and the ticks are parasites of larger animals.
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