A picture of a Sheep Tick
Photo by:
London Scientific Films/
Oxford Scientific Films
Tick, common name for members of a group of large mitelike arachnids parasitic on mammals, birds, and reptiles. All ticks are bloodsucking parasites. Ticks are found in most parts of the world but are generally limited to those habitats frequented by their hosts-namely, woods, tall grass, and shrubby vegetation-where they climb onto plants and wait to jump on a passing host.

Body Structure
Ticks are actually a specialized group of mites and share many features with other mites. In general, they are larger than most mites, ranging from about 0.2 to 0.6 cm (about 0.08 to 0.24 in) in length, although females may be 1 cm (0.4 in) or more in diameter when fully engorged with blood. The adult tick has a mitelike body with a tough skin and four pairs of clawed legs; tick larvae have only three pairs of legs. The mouthparts consist of a paired anchoring organ, or rostrum, covered with backward-curving hooks and equivalent to the pedipalps of other arachnids; and a pair of sharp mandibles that move back and forth in two longitudinal channels on the rostrum.

Ticks spend a great deal of time waiting for their hosts. They are particularly sensitive to carbon dioxide and movement-signals that a host is nearby. Their grasping forelegs allow them to climb on a host. They quickly find a protected spot on the host's body, sink their mouthparts into the flesh, and begin to feed. When full, they drop off the host. In some species, adult males and some nymph stages do not feed.

Ticks are divided into two families: hard ticks and soft ticks. In hard ticks, the mouthparts are visible from above. Hard ticks are parasites primarily of mammals but are also found on birds and reptiles. The nymphs may feed on a different host species in each developmental stage; in each stage, the nymph feeds only once. The adult female lays a single large batch of eggs after her final meal. The American dog tick is perhaps the most familiar North American hard tick. Another important species is the deer tick, which is known to transmit Lyme disease to humans.

In soft ticks, the mouthparts are hidden underneath the body. In general, soft ticks are parasites of birds, but some feed on other hosts. Usually all the developmental stages feed on a single host species. Each stage may feed many times over a period of at least several days, taking refuge in nearby crevices or under rocks when not feeding. The adult female soft tick lays relatively few eggs over an extended period. The relapsing-fever tick is a soft tick that occasionally bites humans.

Several diseases are transmitted to humans and domestic animals through tick bites or tick excrement. The most important of these are spotted fever, relapsing fever, Lyme disease, tularemia, some forms of encephalitis, and Texas cattle fever. Usually, a particular tick-and-host combination harbors a specific disease organism, and often an infestation is limited to a relatively small geographical area.

Scientific classification: Ticks belong to the suborder Metastigmata in the order Acari, class Arachnida, phylum Arthropoda. The American dog tick is classified as Dermacentor variabilis, and the deer tick as Ixodes dammini. Both are in the hard-tick family, Ixodidae. The relapsing-fever tick belongs to the soft-tick family, Argasidae, and is classified as Ornithodoros turicata.

Contributed by: Evan A. Sugden

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