This article is about parasitic arachnids. For the symbol, see Check mark. How to insert tick mark in pdf to be confused with Tic. Ticks are small arachnids, part of the order Parasitiformes.
Along with mites, they constitute the subclass Acari. Ticks had evolved by the Cretaceous period, the most common form of fossilisation being immersed in amber.
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, which are difficult to crush, and the Argasidae or soft ticks. Adults have ovoid or pear-shaped bodies which become engorged with blood when they feed, and eight legs. As well as 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 the body.
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.
Because of their habit of ingesting blood, ticks are vectors of at least twelve diseases that affect humans and other animals. Fossilized ticks are known from the Cretaceous onwards, most commonly in amber. The oldest example is an argasid bird tick from Cretaceous New Jersey amber. The younger Baltic and Dominican ambers have also yielded examples which can be placed in living genera.
There are three families of ticks. The third is Nuttalliellidae, named for the bacteriologist George Nuttall. It comprises a single species, Nuttalliella namaqua, and is the most basal lineage.
Ticks are closely related to the mites, within the subclass Acarina. DNA analysis suggests that the Ixodidae are a clade, but that the Argasidae may be paraphyletic. The Ixodidae contains over 700 species of hard ticks with a scutum or hard shield, which the Argasidae lack. 2010 are Antricola, Argas, Nothoaspis, Ornithodoros and Otobius.
The family Nuttalliellidae contains only a single species, Nuttalliella namaqua, a tick found in southern Africa from Tanzania to Namibia and South Africa. 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 twelve mitochondrial proteins.
The Argasidae appear monophyletic in this study. 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 are also widely distributed among host taxa, which include marsupial and placental mammals, birds, reptiles such as snakes, iguanas and lizards, and amphibians. Ticks of domestic animals cause considerable harm to livestock by transmission of many species of pathogen, as well as causing anaemia and damaging wool and hides.