Size comparison of some commonly found ticks.
A tick is a type of parasitic arachnid belonging to the same order or subclass (Acarina) as mites. Ticks are larger than mites and all of them are external parasites, or ectoparasites, of various animals, including humans. They are also vectors for a number of dangerous diseases.
Although small, ticks are bigger than mites and large enough to be seen with the unaided eye; the largest reach up to about 1 cm in length. Generally they are black, brown, or red in color, and some species have markings. When engorged they may be purple/red, blue/gray, or creamy/pink in color, and may swell to several times their original size.
Ticks may seek hosts at any time but tend to be more active during periods of warm weather, and a change in temperature and length of day act as signals for ticks to find a host. They are able to sense the nearby presence of a potential host by the heat or carbon dioxide that the animal gives off. Not being able to fly or jump, ticks must wait for a host to brush past and then grasp onto them using their clawed legs.
Unlike other arachnids, ticks and mites appear to have undivided bodies. In fact, the cephalothorax and the abdomen are fused and a region of flexible cuticle, known as the circumcapitular furrow, separates the chelicerae and pedipalps from the rest of the body. The anterior body section is known as the capitulum or gnathosoma, and the remainder of the body is called the idiosoma, a section that is unique to ticks and mites.
Ticks have a small, relatively flat body and four pairs of legs that are covered in short hairs and end in a claw. The claws and hairs enable ticks to grasp onto vegetation and their hosts.
The mouthparts of a tick include a pair of pedipalps, two chelicerae that are used to cut through the skin of the host, and a barbed, needle-like hypostome that is used for feeding. The barbs on the hypostome are like a fish hook in that they point backwards, making it very difficult to remove a tick without damaging the skin.
As ticks feed their idiosoma expands, although the amount of expansion depends on the tick. The hard shield, or scutum, of a male hard tick covers most of the back so that the body isn't able to expand much. Female hard ticks must store blood in order to lay eggs, so their bodies swell immensely. Soft ticks lack scutums, but as they don't need to store blood in order to breed, they don't expand in the same way as hard ticks do.
Distribution and habitat
Ticks are found throughout the world, inhabiting especially areas with tall grasses, dense vegetation, shrubs, forests, and woods. They are particularly common in areas with deer trails or tracks, and are abundant near water where mammals come to drink.
Reproduction and life cycle
Ticks begin life as an egg and emerge as a six-legged larva that closesly resembles the adult form. The larva must find its first host in order to be able to feed and grow. The first host is usually a small mammal, and after feeding on it the tick larva falls to the ground in order to digest its food and grow.
After 1–3 weeks the larva molts and becomes a nymph. At this stage is has developed another pair of legs and looks like a small version of the adult form. In order for the nymph to develop further it must feed again. After feeding the nymph drops to the ground once more to digest its food and grow. After this final molt the nymph becomes an adult. Some species of soft tick molt several times, feeding before each molt, and becoming an adult after the final molt.
As an adult, a hard tick attaches itself to a host and feeds before mating. Female hard ticks need to store blood in order to produce eggs and will often feed for more than 24 hr prior to mating. After mating, male ticks usually die immediately, whereas females die after they have laid 2,000–18,000 eggs.
Soft ticks, by contrast, may feed and lay eggs several times before they die. This is the main reason they don't expand or consume as much in one feed as hard ticks do.
Ticks generally have a life expectancy of 1–2 years depending on the species. Some species survive the winter months by going into a sleep known as diapause.
Diet and methods of finding a host
Ticks are external parasites, feeding on the blood of mammals, birds, reptiles, and amphibians.
Soft ticks usually inhabit the nests or burrows of their hosts. They usually feed at night and only for short periods of time. Hard ticks find their hosts in a different way, using a behavior known as questing. They position themselves on some vegetation and stretch their limbs outward waiting for a host to pass by. When a host brushes past the tick it simply hansg on. As larvae, hard ticks usually quest at ground level; as nymphs they climb a little higher to find larger hosts, but adults climb the highest in the hope that they will find a large animal to use as a host.
Some hard ticks are known as one-host ticks, which means they find a host when they are in their larval stage and remain attached to it for their whole lives. A few species, called two-host ticks, remain on one host during their larval and nymph stages then find a second host as adults. Most species are three-host ticks and feed and drop to the ground at each developmental stage of their lifecycle.
All ticks and mites belong to the order or subclass Acarina. Withing this, ticks make up the superfamily Ixodoidea which comprises three families: Ixodidae which contains the hard ticks, Argasidae which contains the soft ticks, and Nuttalliellidea which contains just one species of tick. The three families are further subdivided into 18 genera and around 900 described species.
American dog tick (Dermacentor variabilis)
This species is probably the most well known of the North American hard ticks. It does not carry lyme disease but can be a vector for Rocky Mountain spotted fever.
Deer tick (Ixodes scapularis)
This tick, also known as the black-legged tick or bear tick, is hard-bodied and widely distributed throughout the eastern and north midwestern United States. It is the main vector for lyme disease in the US.
Western black-legged tick (Ixodes pacificus)
This species can be found in the western parts of North America and is a vector for lyme disease and Rocky Mountain spotted fever. It prefers livestock, such as cattle, as its host.
Paralysis tick of Australia
(Ixodes holocyclus) This species is found in a 20 km band that follows the eastern coastline of Australia. Encounters with it are relatively common and in most cases uneventful, although some bites have resulted in life-threatening conditions including paralysis, tick typhus, and severe allergic reactions.
Southern cattle tick (Boophilus microplus)
This species ranks globally as the most economically significant tick. It causes annual economic losses amounting to hundreds of millions of dollars to cattle producers throughout the world, and also affects horses, sheep, goats, and other related species.
Lone star tick (Amblyomma americanum)
Adult females can be distinguished by the white dot or "lone star" on their backs. They are associated with the transmission of southern tick-associated rash illness (STARI).