Arachnid, term for animals in the class including the scorpions, spiders, daddy longlegs, mites, and ticks, and certain other eight-legged land invertebrates. Fossils suggest that arachnids were among the first animals to live on land, perhaps in the early Devonian Period, nearly 400 million years ago. About 60,000 species are known, although many, especially mites, remain undiscovered or undescribed. Arachnids are found throughout the world in nearly every habitat, but they reach their greatest size and diversity in warm arid and tropical regions.
The arachnid body is divided into two parts: anterior and posterior. The anterior part, called the cephalothorax, contains sense organs, mouthparts, and limbs in pairs. The first pair of limbs-the chelicerae-may form pincers or poison fangs, and the second pair-the pedipalps-may serve as pincers, feelers, or legs. The other limb pairs, generally four, are used for walking. The posterior part of the body, the abdomen, bears the genital opening and other structures. It is usually equipped with modified gills called book lungs. Most arachnids are solitary except at the time of mating, when a variety of complex behavior patterns may be observed. Females may guard eggs or young, which are often born live.
Arachnids are usually predaceous. Often they hunt or lie in wait for small animals such as insects. Food may be partly or wholly broken down by secreted fluids and then sucked in. Arachnids have simple eyes and various prey-subduing structures, such as the segmented, stinging tail of scorpions and the abdominal spinnerets with which spiders construct elaborate insect traps (orbs or webs).
Mites constitute the largest and most diverse order of arachnids, followed by the spiders. Some mites feed on plants and a few species are serious agricultural pests. Some are predaceous, often feeding on other species of mites. Parasitic lifestyles are common among mites and many are of veterinary and medical importance. Ticks are a distinct bloodsucking subgroup of mites specialized for parasitizing reptiles, birds, and mammals. Ticks carry organisms that cause serious human diseases, such as Lyme Disease.
The bites of some spiders and the stings of a few species of scorpions are dangerously poisonous to humans. However, most arachnids are harmless and contribute to the balance of nature by controlling the populations of the insects they prey on or the plants, reptiles, birds, or mammals that serve as their hosts.