Appearance
''Demodex folliculorum'' is adapted to live inside hair follicles, and therefore is thin and worm-like, with short legs. As an adult, ''D. folliculorum'' measures 0.3 to 0.4 mm long. Adults have four pairs of legs, larvae and nymphs have only three pairs. ''D. folliculorum'' has a rudimentary gut, and no anus.Behavior
The entire life cycle of ''D. folliculorum'' takes 14–16 days. Adult mites copulate at the top of the hair follicle, near the skin surface. Eggs are deposited in the sebaceous gland inside the hair follicle. The heart-shaped egg is 0.1 mm long, and hatches into a six-legged larva. It takes seven days for the larva to develop into a mature adult, with two intervening nymph stages. The adult lives for 4–6 days.Habitat
''Demodex folliculorum'' prefers areas where sebum production is high, and is typically found in hair follicles on the human face, generally in greater numbers around the cheeks, nose, and forehead, but also elsewhere on the face, eyelids and ears. The mites may also be found on other parts of the body, such as the chest and buttocks.Within the hair follicle, ''D. folliculorum'' is found above the sebaceous gland, positioned head downward, with the end of abdomen often protruding from the hair follicle. Inhabited follicles usually contain 2–6 mites, but greater numbers can occur.
In one hour, ''D. folliculorum'' can travel 8 to 16 mm ; the mites usually travel at night.
The mites are obligate commensals of humans, and can only live on the skin; they soon dry out and die if they leave the host. Higher numbers of ''D. folliculorum'' are found in the spring and summer than at other times of the year.
Reproduction
The entire life cycle of ''D. folliculorum'' takes 14–16 days. Adult mites copulate at the top of the hair follicle, near the skin surface. Eggs are deposited in the sebaceous gland inside the hair follicle. The heart-shaped egg is 0.1 mm long, and hatches into a six-legged larva. It takes seven days for the larva to develop into a mature adult, with two intervening nymph stages. The adult lives for 4–6 days.Evolution
The first report of ''Demodex folliculorum'' was made by German scientist Jakob Henle in 1841, but his presentation to the Natural Sciences Society of Zurich, reported in a local newspaper, attracted little attention at the time. In 1842, German dermatologist Gustav Simon gave a full report of the appearance of ''Demodex folliculorum'', naming it ''Acarus folliculorum''. The following year, 1843, the genus was named ''Demodex'' by English scientist Richard Owen. From Simon's initial description of ''D. folliculorum'' onwards, two forms were recognized, a long form and a short form. In 1963, it was suggested that these long and short forms were two subspecies of ''D. folliculorum'', and that the smaller mite be named ''Demodex brevis'', with the larger mite retaining the name ''D. folliculorum''. It was not until 1972 that the existence of two separate species was confirmed.References:
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