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Ticks

Ticks are small arachnids in the order Parasitiformes. Along with mites, they constitute the subclass Acari. Ticks are ectoparasites (external parasites), living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians. Ticks are vectors of a number of diseases that affect both humans and other animals.

Despite their poor reputation among human communities, ticks may play an ecological role by culling infirm animals and preventing overgrazing of plant resources.

Taxonomy

Of the three families of ticks, one – Nuttalliellidae – comprises a single species, Nuttalliella namaqua. The remaining two families contain the hard ticks (Ixodidae) and the soft ticks (Argasidae). Ticks are closely related to the numerous families of mites, within the subclass Acarina (see article: Mites of livestock).

The Ixodidae include over 700 species. They are known as 'hard ticks' because they differ from the Argasidae in having a scutum or hard shield. This shield generally can resist the force of a human's soft-soled footwear, especially on soft ground; it requires a hard sole on a hard surface to crush the tick. However, stepping on an engorged tick, filled with blood, kills it easily, though messily. In nymphs and adults of the Ixodidae, a prominent capitulum (head) projects forwards from the body; in this they differ from the Argasidae. They differ too, in their life cycle; Ixodidae that attach to a host will bite painlessly and generally unnoticed, and they remain in place until they engorge and are ready to change their skin; this process may take days or weeks. Some species drop off the host to moult in a safe place, whereas others remain on the same host and only drop off once they are ready to lay their eggs.

The Argasidae are known regionally as 'soft ticks' or 'tampans'. The family includes about 200 species, but the proper composition of the genus is under review. The following genera were accepted in 2010:

Antricola, Argas, Nothaspis, Ornithodoros, Otobius

The most obvious distinctions between the Argasidae and the Ixodidae are that: they have no scutum and the capitulum is concealed beneath the body.

The Argasidae also differ from the Ixodidae in their habits and ecology. Many of them feed primarily on birds, though some Ornithodoros for example feed on mammals and are extremely harmful. Both groups feed rapidly, typically biting painfully and gorging within minutes and none of the species will stick to the host in the way that hard ticks do. Unlike the Ixodidae that have no fixed dwelling place except on the host, they live in sand or in crevices or similar shelters near animal dens or nests, or in human dwellings where they might come out nightly to attack roosting birds, or emerge only when they smell carbon dioxide in the breath of their hosts and emerge from the sand to attack them. Species common in North America primarily parasitise birds, and very rarely attack humans or other mammals.

The family Nuttalliellidae contains only a single species, Nuttalliella namaqua, a tick found in southern Africa from Tanzania to Namibia and South Africa,. It can be distinguished from Ixodidae ticks and Argasidae ticks by a combination of characteristics, including the position of the stigmata, lack of setae, strongly corrugated integument, and the form of the fenestrated plates.

Fossilized ticks are common. Recent hypotheses based on total-evidence approach analysis place the origin of ticks in the Cretaceous (65 to 146 million years ago), with most of the evolution and dispersal occurring during the Tertiary (5 to 65 million years ago).[12] The oldest example is an argasid (bird) tick from Cretaceous New Jersey amber. The younger Baltic and Dominican ambers have also yielded examples, all of which can be placed in living genera.

Range and habitat[edit]

Tick species are widely distributed around the world, but they tend to flourish more in countries with warm, humid climates, because they require a certain amount of moisture in the air to undergo metamorphosis, and because low temperatures inhibit their development from egg to larva. Ticks of domestic animals are especially common and varied in tropical countries, where they cause considerable harm to livestock by transmission of many species of pathogens and also causing direct parasitic damage.

For an ecosystem to support ticks, it must satisfy two requirements: the population density of host species in the area must be high enough, and humidity must be high enough for ticks to remain hydrated. Due to their role in transmitting Lyme disease, ixodid ticks, particularly I. scapularis, have been studied using geographic information systems (GIS), to develop predictive models for ideal tick habitats. According to these studies, certain features of a given microclimate – such as sandy soil, hardwood trees, rivers, and the presence of deer – were determined to be good predictors of dense tick populations.

 

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