Most people are familiar with those arachnids called spiders and scorpions, but the ticks and mites are important parasitic members. Ticks are exclusively parasitic whereas mites have parasitic and free-living representatives.
All species are dioecious. Parasitic forms infect various vertebrates and invertebrates. The female produces and lays eggs. A six-legged larva hatches from the egg. The larva molts into an eight-legged nymph. After several molts the nymph transforms into an adult.
The legs are segmented but body segmentation is lacking. The adults have eight legs. Mouthparts are variable, but usually comprise at least a pair of lateral palps and chelicerae, used to cut or pierce the host's skin. The body is usually covered by a dorsal carapace. The genital aperture is usually on the mid-ventral surface of the body.