Ticks are small arachnids in the order Parasitiformes. Along with mites, they constitute the subclass Acarina. 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 ailing infirm animals and preventing overgrazing of plant resources.
A questing tick, fingers for scale.
Ticks satisfy all of their nutritional requirements as ectoparasites, feeding on a diet of blood in a practice known as hematophagy. They are obligate hematophages, needing blood to survive and move from one stage of life to another. Ticks unable to find a host to feed on will die.This behavior is estimated to have evolved approximately 120 million years ago through adapative pressures to a blood-feeding environment. Evidence suggests this behavior evolved independently in the separate tick families, with differing host-tick interactions driving the adapative change.
Ticks extract the blood by cutting a hole in the host’s epidermis, into which they insert their hypostome, and keep the blood from clotting by excreting an anticoagulant or platelet aggregation inhibitor.
Ticks find their hosts by detecting animals’ breath and body odors, or by sensing body heat, moisture and vibrations. They are incapable of flying or jumping, but many tick species wait in a position known as “questing”. While questing, ticks hold on to leaves and grass by their third and fourth pair of legs. They hold the first pair of legs outstretched, waiting to climb on to the host. When a host brushes the spot where a tick is waiting, it quickly climbs onto the host. Some ticks will attach quickly while others will wander looking for thinner skin like the ear. Depending on the species and the life stage, preparing to feed can take from ten minutes to two hours. On locating a suitable feeding spot, the tick grasps the skin and cuts into the surface.