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Ezekiel Turner
Ezekiel Turner


Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. The timing of the origin of ticks is uncertain, though the oldest known tick fossils are from the Cretaceous period, around 100 million years old. Ticks are widely distributed around the world, especially in warm, humid climates.



Ticks belong to two major families, the Ixodidae or hard ticks, and the Argasidae, or soft ticks. Nuttalliella, a genus of tick from southern Africa, is the only member of the family Nuttalliellidae, and represents the most primitive living lineage of ticks. Adults have ovoid/pear-shaped bodies (idiosomas) which become engorged with blood when they feed, and eight legs. Their cephalothorax and abdomen are completely fused. In addition to having a hard shield on their dorsal surfaces, known as the scutum, 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. Ticks locate potential hosts by sensing odor, body heat, moisture, and/or vibrations in the environment.[1]

Ticks have four stages to their lifecycle, namely egg, larva, nymph, and adult. Ticks belonging to the Ixodidae family undergo either a one-host, two-host, or three-host lifestyle.[2] Argasid ticks have up to seven nymphal stages (instars), each one requiring blood ingestion, Argasid ticks undergo a multihost lifestyle. Because of their hematophagous (blood-ingesting) diets, ticks act as vectors of many serious diseases that affect humans and other animals.

Ticks belong to the Parasitiformes, a distinctive group of mites that are separate from the main group of mites, the Acariformes. Whether the two groups are more closely related to each other than to other arachnids is uncertain, and studies often recover them as not closely related.[3] Within the Parasitiformes, ticks are most closely related to the Holothyrida, a small group of free living scavengers with 32 described species confined to the landmasses that formed the supercontinent Gondwana.[4]

Fossilized ticks have been discovered from the end of the Early Cretaceous onwards, most commonly in amber. The oldest discovered tick fossils are an argasid bird tick from Late Cretaceous (Turonian 94-90 million years ago) aged New Jersey amber,[5] and various ticks found in Burmese amber, including Khimaira and Deinocroton, which do not belong to any living family of tick, and members of the living ixodid genera Amblyomma and Ixodes dating the earliest Cenomanian stage of the Late Cretaceous, around 99 million years ago.[6][4][7] An undescribed juvenile tick is known from late Albian Spanish amber, dating to 105 million years ago.[6] The younger Baltic and Dominican ambers have also yielded examples that can be placed in living genera.[8] A phylogenetic analysis suggests that the last common ancestor of all living ticks likely lived around 195 million years ago in the Southern Hemisphere, in what was then Gondwana.[4]

Ticks belong to three different families. The majority of tick species belong to the two families: Ixodidae (hard ticks) and Argasidae (soft ticks). The third living family is Nuttalliellidae, named for the bacteriologist George Nuttall. It comprises a single species, Nuttalliella namaqua,[9][10] and as such is a monotypic taxon. Nuttalliella namaqua is found in southern Africa ranging from Tanzania to Namibia and South Africa.[9][11]

The Ixodidae contain over 700 species of hard ticks with a scutum or hard shield, which the Argasidae lack. The Argasidae contain about 200 species; the genera accepted as of 2010[update] are Antricola, Argas, Nothoaspis, Ornithodoros, and Otobius.[9] They have no scutum, and the capitulum (mouth and feeding parts) is concealed beneath the body.[12] The phylogeny of the Ixodida within the Acari is shown in the cladogram, based on a 2014 maximum parsimony study of amino acid sequences of 12 mitochondrial proteins. The Argasidae appear monophyletic in this study.[13]

Larval ticks hatch with six legs, acquiring the other two after a blood meal and molting into the nymph stage.[18] In the nymphal and adult stages, ticks have eight legs, each of which has seven segments and is tipped with a pair of claws. The legs are sometimes ornamented and usually bear sensory or tactile hairs.[19] In addition to being used for locomotion, the tarsus of leg I contains a unique sensory structure, Haller's organ, which can detect odors and chemicals emanating from the host, as well as sensing changes in temperature and air currents.[20][21][22] Ticks can also use Haller's organs to perceive infrared light emanating from a host.[23] When stationary, their legs remain tightly folded against the body.[20][21]

Ticks are extremely tough, hardy, and resilient animals. They can survive in a near vacuum for as long as half an hour.[24] Their slow metabolism during their dormant periods enables them to go prolonged durations between meals.[25] During droughts, they can endure dehydration without feeding for as long as eighteen weeks, however, ticks with limited energy reserves may succumb to desiccation after thirty-six weeks.[26] To keep from dehydrating, ticks hide in humid spots on the forest floor[27] or absorb water from subsaturated air by secreting hygroscopic fluid produced by the salivary glands onto the external mouthparts and then reingesting the water-enriched fluid.[28]

The body of a soft tick is pear-shaped or oval with a rounded anterior portion. The mouthparts cannot be seen from above, as they are on the ventral surface. A centrally positioned dorsal plate with ridges projecting slightly above the surrounding surface, but with no decoration are often present. Soft ticks possess a leathery cuticle as well. A pattern of small, circular depressions expose where muscles are attached to the interior of the integument. The eyes are on the sides of the body, the spiracles open between legs 3 and 4, and males and females only differ in the structure of the genital pore.[32]

Nuttalliellidae can be distinguished from both ixodid and argasid ticks by a combination of a projecting gnathosoma and a soft leathery skin. Other distinguishing characteristics include the position of the stigmata, the lack of setae, the strongly corrugated integument, and the form of the fenestrated plates.[33][34]

Ticks are ectoparasites and consume blood to satisfy all of their nutritional requirements. They are obligate hematophages, and require blood to survive and move from one stage of life to another. Ticks can fast for long periods of time, but eventually die if unable to find a host.[35] Hematophagy evolved independently at least six times in arthropods living during the late Cretaceous; in ticks it is thought to have evolved 120 million years ago through adaptation to blood-feeding.[5][36] This behavior evolved independently within the separate tick families as well, with differing host-tick interactions driving the evolutionary change.[5]

Some ticks attach to their host rapidly, while others wander around searching for thinner skin, such as that in the ears of mammals. Depending on the species and life stage, preparing to feed can take from ten minutes to two hours. On locating a suitable feeding spot, the tick grasps the host's skin and cuts into the surface.[35] It extracts blood by cutting a hole in the host's epidermis, into which it inserts its hypostome and prevents the blood from clotting by excreting an anticoagulant or platelet aggregation inhibitor.[37][36]

Ticks find their hosts by detecting an animals' breath and body odors, sensing body heat, moisture, or vibrations.[38] A common misconception about ticks is they jump onto their host or they fall from trees, however, they are incapable of flying or jumping. Many tick species, particularly Ixodidae, lie in wait in a position known as "questing". While questing, ticks cling to leaves and grasses by their third and fourth pairs of legs. They hold the first pair of legs outstretched, waiting to grasp and climb on to any passing host. Tick questing heights tend to be correlated with the size of the desired host; nymphs and small species tend to quest close to the ground, where they may encounter small mammalian or bird hosts; adults climb higher into the vegetation, where larger hosts may be encountered. Some species are hunters and lurk near places where hosts may rest. Upon receiving an olfactory stimulus or other environmental indication, they crawl or run across the intervening surface.[38]

Other ticks, mainly the Argasidae, are nidicolous, finding hosts in their nests, burrows, or caves. They use the same stimuli as non-nidicolous species to identify hosts, with body heat and odors often being the main factors.[38] Many of them feed primarily on birds, though some Ornithodoros species, for example, feed on small mammals. Both groups of soft tick feed rapidly, typically biting painfully and drinking their fill within minutes. Unlike the Ixodidae that have no fixed dwelling place except on the host, they live in sand, in crevices near animal dens or nests, or in human dwellings, where they come out nightly to attack roosting birds or emerge when they detect carbon dioxide in the breath of their hosts.[39]

Tick saliva contains about 1,500 to 3,000 proteins, depending on the tick species. The proteins with anti-inflammatory properties, called evasins, allow ticks to feed for eight to ten days without being perceived by the host animal. Researchers are studying these evasins with the goal of developing drugs to neutralise the chemokines that cause myocarditis, heart attack, and stroke.[41]

Tick species are widely distributed around the world.[53] They tend to flourish more in warm, humid climates, because they require a certain amount of moisture in the air to undergo metamorphosis, and low temperatures inhibit their development of eggs to larvae.[54] The occurrence of ticks and tick-borne illnesses in humans is increasing.[55] Tick populations are spreading into new areas, due in part to the warming temperatures of climate change.[56][57] 041b061a72


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