Taxonomy and phylogeny }} 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 resolve them as not closely related. 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. and as such is a
monotypic taxon.
Nuttalliella namaqua is found in southern Africa ranging from
Tanzania to
Namibia and
South Africa. The fourth is an extinct family,
Khimairidae. Its only known species was
Khimaira fossus.
as well as identifying ancient species of the living ixodid genera Amblyomma, Ixodes, Haemaphysalis,
Bothriocroton and Archaeocroton''. Tick paleobiota is also known from late
Albian amber, (~105
Ma) as well as
Baltic (~56–34
Ma) and
Dominican amber (~40-20
Ma). A 2019 analysis suggested that the
last common ancestor of all living ticks likely lived around 195 million in the Southern Hemisphere, in what was then
Gondwana, although another 2018 study put the origin of ticks at closer to ~270 million years ago during the
Permian period. Almost all contemporary taxa fall into one of the two major tick families. The
Ixodidae contain 750 species over 18 genera, characterised by a
scutum or hard shield. The
Argasidae contain about 220 species over 15 genera. Argasid species have no scutum, and the
capitulum (mouth and feeding parts) is concealed beneath the body.
Anatomy and physiology Ticks, like
mites, belong to the subclass Acari that lack their primary somatic segmentation of the
abdomen (or
opisthosoma), rather these parasitic
arachnids present a subsequent fusion of the abdomen with the
cephalothorax (or
prosoma). Features of the gnathosoma include two
palps, two
chelicerae, and
hypostome. The hypostome acts as stabilizer and helps to anchor the tick's mouthparts to the host. The chelicerae are specialized appendages used for cutting and piercing into the host's skin while palps are leglike appendages that are sensory in function. The ventral side of the idiosoma bears
sclerites, and the gonopore is located between the fourth pair of legs. In the absence of segmentation, the positioning of the eyes, limbs, and
gonopore on the idiosoma provide the only locational guidance. 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. 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. Ticks can also use Haller's organs to perceive
infrared light emanating from a host. When stationary, their legs remain tightly folded against the body. Ticks are extremely resilient animals. They can survive in a near vacuum for as long as half an hour. Their slow metabolism during their
dormant periods enables them to go prolonged durations between meals. Even after 18 weeks of starvation, they can endure repeated two-day bouts of dehydration followed by rehydration, but their survivability against dehydration drops rapidly after 36 weeks of starvation. To keep from dehydrating, ticks hide in humid spots on the forest floor 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. Ticks can withstand temperatures just above for more than two hours and can survive temperatures between for at least two weeks. Ticks have even been found in Antarctica, where they feed on penguins. Most ticks are plain brown or reddish brown. However, the scuta of some species are decorated with white patterns.
Ixodidae In
nymphs and adults, the is prominent and projects forwards from the body. The eyes are close to the sides of the scutum and the large
spiracles are located just behind the
coxae of the fourth pair of legs. The hard protective
scutellum, a characteristic of this family, covers nearly the whole dorsal surface in males, but is restricted to a small, shield-like structure behind the capitulum in females and nymphs. When an ixodid attaches to a host the bite is typically painless and generally goes unnoticed. They remain in place until they engorge and are ready to
molt; this process may take days or weeks. Some species drop off the host to molt in a safe place, whereas others remain on the same host and only drop off once they are ready to lay their eggs.
Argasidae 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 is 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.
Nuttalliellidae 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. Nuttalliellidae genera are grouped together with reference to the character of the pseudoscutum and hypostome, but especially the 'ball-and-socket-like' leg joints. 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. This behavior evolved independently within the separate tick families as well, with differing host-tick interactions driving the evolutionary change. 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. 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. While all species are
haematophagous at some point in their lifecycle, a few argasid taxa, such as
Antricola delacruzi, only take blood-meals as larvae, subsisting in the nymphal and adult stages by consuming guano. Ixodidae remain in place until they are completely engorged. Their weight may increase by 200 to 600 times compared to their prefeeding weight. To accommodate this expansion, cell division takes place to facilitate enlargement of the cuticle. In the Argasidae, the tick's cuticle stretches to accommodate the fluid ingested, but does not grow new cells, with the weight of the tick increasing five- to tenfold over the unfed state. The tick then drops off the host and typically remains in the nest or burrow until its host returns to provide its next meal. The saliva of ticks also contains
anticoagulant and
antiplatelet proteins (integrin inhibitors), to stop the blood from coagulating while they suck. Most ticks do not use any other food source than vertebrate blood and therefore ingest high levels of protein, iron and salt, but few carbohydrates, lipids or vitamins. Ticks' genomes have evolved large repertoires of genes related to this nutritional challenge, but they themselves cannot synthesize the essential vitamins that are lacking in blood meal. To overcome these nutritional deficiencies, ticks have evolved obligate interactions with nutritional
endosymbionts. These intracellular symbiotic microorganisms are specifically associated with ticks and use
transovarial transmission to ensure their persistence. Although
Coxiella and
Francisella endosymbionts are distantly related bacteria, they have converged towards an analogous B vitamin-based nutritional mutualism with ticks. The genome sequencing of
Coxiella and
Francisella endosymbionts confirmed that they consistently produce three B vitamin types, biotin (vitamin B7), riboflavin (B2) and folate (B9). As they are required for tick life cycle, these obligate endosymbionts are present in all individuals of the tick species they infect, at least at early stages of development since they may be secondarily lost in males during nymphal development.
Range and habitat Tick species are widely distributed around the world. 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. The occurrence of ticks and tick-borne illnesses in humans is increasing. Tick populations are spreading into new areas, due in part to the warming temperatures of
climate change. Tick parasitism is widely distributed among host taxa, including marsupial and placental mammals, birds, reptiles (snakes, iguanas, and lizards), and amphibians.
Ticks of domestic animals cause considerable harm to livestock through pathogenic transmission, causing anemia through blood loss, and damaging wool and hides. The
Tropical Bont tick wreaks havoc on livestock and wildlife in Africa, the Caribbean, and several other countries through the spread of disease, specifically
heartwater disease. The
spinose ear tick has a worldwide distribution, the young feed inside the ears of cattle and various wildlife. A habitat preferred by ticks is the interface where a lawn meets the forest, or more generally, the
ecotone, which is unmaintained transitional edge habitat between woodlands and open areas. Therefore, one tick management strategy is to remove leaf litter, brush, and weeds at the edge of the woods. Ticks like shady, moist leaf litter with an overstory of trees or shrubs and, in the spring, they deposit their eggs into such places allowing larvae to emerge in the fall and crawl into low-lying vegetation. The 3 meter boundary closest to the lawn's edge are a tick migration zone, where 82% of tick nymphs in lawns are found. For an ecosystem to support ticks, it must satisfy two requirements; the population density of host species in the area must be great enough and it must be humid enough for ticks to remain hydrated. Due to their role in transmitting
Lyme disease, Ixodid ticks, particularly the North American
I. scapularis, have been studied using
geographic information systems 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. Ticks can transmit an array of infectious diseases that affect humans and other animals. Ticks that carry
zoonotic pathogens often tend to have a wide host range. 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, domesticated pets, and
livestock. With increasing participation in outdoor activities such as
wilderness hikes, more people and their dogs may find themselves exposed to ticks.
Life cycle , all to same scale); E=eggs, L=larvae, N=nymphs, F=adult female, M=adult male; upper row are unfed ticks, lower row are fully engorged larvae, nymphs, and a female. All three tick families ticks have four life cycle stages: egg,
larva,
nymph, and adult.
Ixodidae Ixodidae ticks have three different life cycles. Depending on the species, Ixodids can either possess a one-host life cycle, two-host life cycle, or three-host life cycle.
One-host ticks In one-host ticks the tick remains on the host through the larval, nymphal, and adult stages, only to leave the host to lay eggs. Eggs laid in the environment hatch into larvae, which immediately seek out a host in which to attach and feed. Fed larvae molt into unfed nymphs that remain on the host. After engorging on the host's blood, the nymphs molt into sexually mature adults that remain on the host in order to feed and mate. Once a female is both fed and ready to lay eggs, only then does she leave the host in search of a suitable area to deposit her eggs. Ticks that follow this life cycle are called one-host ticks. The winter tick
Dermacentor albipictus and the cattle tick
Rhipicephalus microplus are examples of one-host ticks.
Two-host ticks The life cycle of a two-host tick often spans two years. Often, egg laying and mating occurs detached from the host in a safe environment. ==Relationship with humans==