Mites and Ticks

(Acarina), small arthropod animals of the class Arachnida of the subphylum Chelicerata. Some zoologists classify Acarina as a single order, comprising three suborders—Opilioacarina, Acariformes, and Parasitiformes. Other zoologists consider these suborders to be unrelated orders.

Mites and ticks measure 0.1–30 mm long. Acarina have six-legged larvae; the nymphs and adults have eight legs. There is a special anterior section, the capitulum (gnathosoma), which is movable and often hinged to the body. The capitulum bears the two anterior pairs of appendages, the chelicerae and pedipalpi. Segmentation is preserved only in a few primitive forms of Acarina, including the order Opilioacarina. The bodies of mites and ticks have pronounced divisions. The Acariformes have a head section with four pairs of appendages and a rear section with two posterior pairs of legs. Parasitiformes have a legless abdomen (opisthosoma) and a cephalothorax (prosoma) that bears all six pairs of appendages. In the higher representatives of these two suborders the body divisions are usually coalesced. The integument is thin and leathery; sometimes it thickens into plates. The coloring, which varies, is either monochromatic or mottled.

The structure of the appendages varies according to living habits and means of feeding. The chelicerae have claws on the end and are either curved or needle-shaped. They are used to grasp and grind food and to pierce and fasten onto the skin of the host animal. In some mites and ticks the chelicerae are adapted for mating. The pedipalpi are simple and resemble legs. Their basal segments form the preoral cavity. The legs have a tarsal claw and suckers (some Acarina lack these). The dermal sense organs are bristles and lyrate organs, which are distributed over the entire body and appendages. There are one or two pairs of eyes; sometimes there is a single eye. Many mites and ticks do not have eyes. Respiration is cutaneous or tracheal. The tracheae open through one to four pairs of spiracles, or stigmata, which are located in the anterior part of the body or on the sides.

Mites and ticks are dioecious. Sexual dimorphism is expressed in many forms of Acarina. The position of the genital aperture varies greatly. Females sometimes have paired copulatory orifices and an oviduct aperture. Fertilization is usually spermatophoral. The male inserts the spermatophore into the genital aperture of the female; often he leaves it on the substrate, where it can be picked up by the female. Fertilization may also be internal, by copulation. Parthenogenesis is also possible. Most mites and ticks are oviparous, although some are viviparous. The stages of development include the egg, prolarva, larva, proto-nymph, deutonymph, tritonymph, and adult. In many Acarina certain phases of development are absent; sometimes the nymphs molt several times, and the number of nymphal stages varies (for example, species of the genus Argas, or tick).

There are more than 10,000 known species of mites and ticks. The order Acariformes contains the suborder Sarcoptiformes, which includes oribatid mites (Oribatei), grain-mites (Tyroglyphoidea or Acaroidea), hair ticks (Listrophoridae), feather mites (Analgesoidea), and itch mites (Sarcoptidae). The suborder Trombidiformes is also of the order Acariformes; it includes red spider mites (Tetranychidae), water mites (Hydracarina), trombiculid mites (Trombiculidae), follicle mites (Demodex folliculorum), and gall mites (Tetrapodili). The order Parasitiformes comprises the gamasid mites (Gamasoidea), argasid ticks (Argasidae), and ixodid ticks (Ixodidae).

Mites and ticks are found throughout the world. Most species are terrestrial, but some live in fresh water or in the sea. They are especially numerous in temperate and tropical countries. Predacious and herbivorous mites and ticks live freely in soil and on the forest floor; they are also found on plants, on decaying organic remains, and in the burrows and nests of animals. Many Acarina parasitize plants, animals, and humans; some parasitize the body cavity.

Several species of mites and ticks are beneficial, assisting in soil formation, processing plant remains, and exterminating some plant pests. Many Acarina are extremely harmful. Ixodid ticks, argasid ticks, and some gamasid mites are particularly harmful to humans and animals; they are ectoparasites and carry the causative agents of such contagious diseases as encephali-tides, hemorrhagic fevers, and other febrile diseases. Mites and ticks carry viruses, bacteria, spirochetes, babesias, anaplasmata, theileriae, and microfilariae. Some oribatid mites are intermediate hosts of tapeworms, which are parasites of domestic animals. Grain mites damage stores of grain and other products. Gall mites, red spider mites, and some gamasid mites harm plants. Itch mites burrow in the skin of humans and animals, causing scabies. The species Acarapis Woodi Rennie causes Isle of Wight disease, or acariasis, which is a disease of bees. The bites of trombiculid larvae cause dermatitis in man and also transmit tsutsugamushi disease.

Mites and ticks are controlled by biological methods and the use of acaricides. The study of these animals is called acarology.