Ticks, like mites, belong to the subclass Acari that lack their primary somatic segmentation of the abdomen (or opisthosoma), rather these parasitic arachnids present a subsequent fusion of the abdomen with the cephalothorax (or prosoma). The tagmata typical of other Chelicerata have developed into the gnathosoma (head), which is retractable and contains the mouthparts, and idiosoma (body), which contains the legs, digestive tract, and reproductive organs. The gnathosoma is a feeding structure with mouthparts adapted for piercing skin and sucking blood; it is the front of the head and contains neither the brain nor the eyes. Features of the gnathosoma include two palps, two chelicerae, and hypostome. The hypostome acts as stabilizer and helps to anchor the tick's mouthparts to the host. The chelicerae are specialized appendages used for cutting and piercing into the host's skin while palps are leglike appendages that are sensory in function.
The ventral side of the idiosoma bears sclerites, and the gonopore is located between the fourth pair of legs. In the absence of segmentation, the positioning of the eyes, limbs, and gonopore on the idiosoma provide the only locational guidance.
Most ticks are inornate and appear to be brown or reddish brown in color. However, some species are ornate and have distinctive white patterns on the scutum.
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