Trombiculidae

The Trombiculidae (/trɒmbɪˈkjuːlɪdiː/; also called berry bugs, harvest mites, red bugs, scrub-itch mites, and aoutas) are a family of mites. The best known of the Trombiculidae are chiggers. The two widely recognized definitions of 'chigger' are the scientific (or taxonomic) and the common, the latter of which can be found in English and medical dictionaries. According to most dictionaries, the several species of Trombiculidae that bite their host in their larval stage and cause 'intense irritation' or 'a wheal, usually with severe itching and dermatitis', are called chiggers. The scientific definition seemingly includes many more, but not all species of Trombiculidae. Trombiculidae live in forests and grasslands and are also found in the vegetation of low, damp areas such as woodlands, berry bushes, orchards, along lakes and streams, and even in drier places where vegetation is low, such as lawns, golf courses, and parks. They are most numerous in early summer when grass, weeds, and other vegetation are heaviest. In their larval stage, they attach to various animals, including humans, and feed on skin, often causing itching. These relatives of ticks are nearly microscopic, measuring 0.4 mm (1/60 of an inch) and have a chrome-orange hue. There is a marked constriction in the front part of the body in the nymph and adult stages. The best known species of chigger in North America is the hard-biting Trombicula alfreddugesi of the Southeastern United States, humid Midwest and Mexico; in the UK, the most prevalent chigger, called the 'harvest mite', is Trombicula autumnalis, with distribution through Western Europe to Eastern Asia. Trombiculid mites go through a lifecycle of egg, larva, nymph, and adult. The larval mites feed on the skin cells of animals. The six-legged parasitic larvae feed on a large variety of creatures, including humans, rabbits, toads, box turtles, quail, and even some insects. After crawling onto their hosts, they inject digestive enzymes into the skin that break down skin cells. They do not actually 'bite', but instead form a hole in the skin called a stylostome and chew up tiny parts of the inner skin, thus causing severe irritation and swelling. The severe itching is accompanied by red, pimple-like bumps (papules) or hives and skin rash or lesions on a sun-exposed area. For humans, itching usually occurs after the larvae detach from the skin. After feeding on their hosts, the larvae drop to the ground and become nymphs, then mature into adults, which have eight legs and are harmless to humans. In the postlarval stage, they are not parasitic and feed on plant material. The females lay three to eight eggs in a clutch, usually on a leaf or under the roots of a plant, and die by autumn. Trombiculidae, from Greek τρομειν ('to tremble') and Latin culex, genitive culicis ('gnat' or 'midge'), was first described as an independent family by Henry Ellsworth Ewing in 1944. Then, when the family was first described, it included two subfamilies, Hemitrombiculinae and Trombiculinae. Womersley added another, Leeuwenhoekiinae, which at the time contained only Leeuwenhoekia. Later, he erected the family Leeuwenhoekiidae for the genus and subfamily, having six genera; they have a pair of submedian setae present on the dorsal plate. References to chiggers, however, go as far back as sixth-century China, and by 1733, the first recognition of trombiculid mites in North America was made. In 1758, Carl Linnaeus described a single species, Acarus batatas (now Trombicula batatas). However, most information about chiggers came from problems that arose during and after World War II. Trombiculid mites are found throughout the world. In Europe and North America, they tend to be more prevalent in the hot and humid regions. In the more temperate regions, they are found only during the summer (in French, harvest mites are called aoûtat because they are common in August). In the United States, they are found mostly in the Southeast, the South, and the Midwest. They are not present, or barely found, in far northern areas, high mountains, and deserts. In the British Isles, the species Trombicula autumnalis is called harvest mites, in North America the species Trombicula alfreddugesi, and the species Trombicula (eutrombicula) hirsti, which are found in Australia and are commonly called the scrub-itch mite. The length of the mite's cycle depends on species and environment, but normally lasts two to 12 months. The number of cycles in a year depends on the region. For example, in a temperate region, only three per year may occur, but in tropical regions, the cycle might be continuous all year long. Adult harvest mites winter in protected places such as slightly below the soil surface. Females become active in the spring, and once the ground temperature is regularly above 60 °F (16 °C), they lay eggs in vegetation, up to 15 eggs per day. The eggs are round and are dormant for about six days, after which the nonfeeding prelarvae emerge, with only three pairs of legs. After about six days, the prelarvae grow into their larval stage.