tick, small, parasitic arachnid of the order Ixodida, closely related to the mites. Ticks, which are larger than the often microscopic mites, are all parasitic in at least one developmental stage; most parasitize mammals and birds although some have reptilian and amphibian hosts. The unsegmented body is typically oval and compact, and there are four pairs of legs. The movable head is attached to the body by a hinge. There are four stages in the tick life cycle: egg, larva, nymph, and adult, but soft ticks may go through several nymphal states. An anchoring structure in the tick's mouth enables it to embed its entire head under the skin of the host, where it sucks the host's blood. If a tick is pulled off the host, the head usually remains embedded in the skin. Members of the family (Argasidae) of soft ticks, with a membranous outer covering, hide in crevices and come out at night to suck blood; their bites are typically painful. Hard ticks (family Ixodidae), which have thickened outer plates made of chitin, remain attached to the host for long periods; their bites are typically painless.
Ticks transmit Rocky Mountain spotted fever, tularemia, Lyme disease, equine encephalitis, several forms of ehrlichiosis, and other diseases. Tick-borne diseases of livestock (e.g., babesiosis, anaplasmosis) are of great economic significance. Each species needs three different hosts to complete its life cycle. Typically the larval stage will feed on small reptiles, birds, or mammals; the nymph stage will parasitize larger vertebrates; and adults will parasitize large herbivores and livestock. The adult of the ixodid species Ixodes scapularis, the vector of Lyme disease and babesiosis in the E United States and Canada, usually chooses deer as its host (I. scapularis of all stages will feed on humans). The closely related I. pacificus, which transmits Lyme disease and Rocky Mountain spotted fever in the western states, prefers livestock in the adult stage. Ticks can sometimes harbor more than one disease organism at a time. Rapidly multiplying Asian long-horned ticks, Haemaphysalis longicornis, can also kill a young animal when the growing offspring consume a large quantity of its blood.
Ticks belong in the phylum Arthropoda, subphylum Chelicerata, class Arachnida, superorder Parasitiformes, order Ixodida (or Metastigmata).
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