Tick

Ticks are parasitic arachnids belonging to the order Ixodida. They are part of a larger group of mites called the Parasitiformes. Most ticks are external parasites, meaning they live on the outside of a host. They survive by feeding on the blood of mammals, birds, and occasionally reptiles or amphibians. This blood-feeding diet makes them hematophagous. Because they consume blood, ticks act as vectors, which are organisms that carry and transmit diseases to humans and animals.

Anatomy is specialized for a life of parasitism. A tick's body is divided into two main parts: the gnathosoma and the idiosoma. The gnathosoma is the feeding structure at the front of the head. It contains the mouthparts but does not contain the brain or eyes. Within the gnathosoma, two chelicerae are used to cut and pierce the host's skin. A structure called the hypostome acts as an anchor to stabilize the mouthparts. The idiosoma is the main body, which holds the legs, digestive tract, and reproductive organs. Unlike many other arachnids, the tick's cephalothorax and abdomen are completely fused.

Ticks follow a specific life cycle consisting of four stages: egg, larva, nymph, and adult. When they first hatch as larvae, they have only six legs. After a blood meal, they molt, or shed their skin, to become nymphs. Once they reach the nymph stage, they possess eight legs. Adult ticks also have eight legs, and each leg has seven segments ending in claws. Ticks locate hosts using a sensory structure on their first pair of legs called Haller's organ. This organ allows them to detect odors, body heat, moisture, vibrations, and even infrared light.

There are three distinct living families of ticks. The largest is Ixodidae, known as hard ticks. These possess a hard dorsal shield called a scutum. In males, the scutum covers nearly the entire back, while in females, it is a smaller structure.

The second family is Argasidae, or soft ticks. These lack a scutum and have a leathery skin. Their mouthparts, or capitulum, are located on the underside of the body rather than at the front.

Finally, there is the Nuttalliellidae family. This is a monotypic taxon, meaning it contains only one living species, *Nuttalliella namaqua*. This species is found in southern Africa and represents the most primitive living lineage of ticks.

Finding a host often involves a behavior called questing. Ticks do not jump; instead, they cling to vegetation using their third and fourth pairs of legs. They hold their first pair of legs outward to grasp passing animals. Once they contact a host, they pierce the epidermis. To ensure a steady meal, they excrete an anticoagulant, which is a substance that prevents blood from clotting. This allows the tick to engorge, or swell with blood, over several days or weeks.

Ticks have a very long evolutionary history. Fossil evidence shows ticks existed during the Cretaceous period, with some fossils found in amber.

Some studies suggest the last common ancestor of all living ticks lived around 195 million years ago in the supercontinent Gondwana. Other research places their origin even earlier, around 270 million years ago during the Permian period. This long history has allowed them to adapt to many different environments.

These animals are incredibly resilient and can survive in extreme conditions. They can endure a near vacuum for thirty minutes and survive in Antarctica by feeding on penguins. Ticks have a slow metabolism, which helps them survive long periods without food. They can survive 18 weeks of starvation and endure bouts of dehydration. To prevent drying out, they can secrete a hygroscopic fluid from their salivary glands to absorb moisture from the air. This ability to wait for a host makes them highly successful survivors in the natural world.