Tick
Ticks (Ixodida) are arachnids, typically 3 to 5 mm long, part of the superorder Parasitiformes. Along with mites, they constitute the subclass Acari. Ticks are external parasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. Ticks evolved by the Cretaceous period, the most common form of fossilisation being amber immersion. Ticks are widely distributed around the world, especially in warm, humid climates.
Almost all ticks belong to one of two major families, the Ixodidae or hard ticks, and the Argasidae or soft ticks. Adults have ovoid or pear-shaped bodies, which become engorged with blood when they feed, and eight legs. In addition to having a hard shield on their dorsal surfaces, hard ticks have a beak-like structure at the front containing the mouthparts, whereas soft ticks have their mouthparts on the underside of their bodies. Both families locate a potential host by odour or from changes in the environment.
Ticks have four stages to their lifecycle, namely egg, larva, nymph, and adult. Ixodid ticks have three hosts, taking at least a year to complete their lifecycle. Argasid ticks have up to seven nymphal stages (instars), each one requiring a blood meal. Because of their habit of ingesting blood, ticks are vectors of many diseases that affect humans and other animals.
Mites and nematodes feed on ticks, which are also a minor nutritional resource for birds. More importantly, they carry diseases as the primary hosts of pathogens such as spirochaetes, and without their agency, the organisms could not reach their secondary hosts. The diseases caused may debilitate their victims, and ticks may thus be assisting in controlling animal populations and preventing overgrazing.
Certain infectious diseases of humans and other animals can be transmitted by ticks, with the species of tick involved tending to be those with a wide host range. Spread of disease in this way is enhanced by the extended time during which a tick remains attached, during which time the mobile host can be carried long distances, or in the case of bird hosts, across the sea. The infective agents can be present not only in the adult tick, but also in the eggs produced plentifully by the females. Many tick species have extended their ranges as a result of the movements of people, their pets, and livestock. With increasing participation in outdoor activities such as wilderness hikes, more people and their dogs may find themselves exposed to attack.
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