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|Reproduction||Sexual; lays eggs|
|Average Length||0.012–0.016 in (0.3–0.4 mm)|
|Locomotion||Crawling about on eight legs|
|Prey||Skin cells, oils|
|Related Species||Demodex brevis|
Demodex folliculorum is a species of face mite commonly referred to as an eyelash mite or follicle mite, alongside its cousin species, D. brevis and, alongside this cousin, is one of the parasitic/commensal face mites that occur on people. It is the most common ectoparasite on the human skin.
They were first described by Dr. Simon in 1842.
D. folliculorum have a semitransparent, elongated body consisting of the two fused segments naturally seen in other arachnids, with their eight legs attached to the first segment. These segments are covered in scales to allow them to anchor onto the hair follicles themselves, which they then feed upon the skin cells and sebum (oils) that accumulate thereon using a set of pin-like mouthparts.
The species exhibits minor sexual dimorphism, with females appearing larger and rounder than their male counterparts.
The species as a whole have genital openings for breeding purposes, while fertilization is internal. Mating will occur within the follicle opening, and eggs will then be laid inside this follicle or along a sebaceous gland. Strangely, demodex species lack an anus and because of this, are completely incapable of getting rid of their feces. Instead, the abdomen continues to get larger and larger; when the mite then finally dies, it releases all of its feces all at once into the pore. Some scientists believe that this may be sufficient enough to stimulate an immune reaction, thereby leading to the cause of acne-like skin conditions such as rosacea. As for why they lack an anus when their ancestors would have one is not openly understood.
Young larvae are born between three and four days after being laid, with only six of their eight legs. It will take them around a full week to then mature into adulthood. A Demodex will only live for several weeks.
They particularly favor hair follicles, specifically of the eyebrows and eyelashes, and also being found around the nose. They may appear in other areas of the human body, as well. Measuring a mere fraction of a millimeter long, they live inside of the follicle until night, where they will leave their homes and then crawl around a human face during the night, where they mate and then proceed to lay their eggs in their host's pores before dying. A healthy human adult will have around one to two mites per centimeter of facial skin, but those with the condition known as rosacea can have as many as ten times more.
Folliculorum dislikes light and will try to avoid it if possible. Because of this they will rarely leave their follicle homes until nightfall, to crawl along the face or other skin sections they may inhabit. In terms of human observation of distance, they cannot travel far, only moving at approximately 8-16 cm per hour.
It is suggested that the older a human becomes, the more likely they are to carry the mites, with the lowest numbers being found in children due to their much lower production of sebum. While it is believed that only a third of children and young adults have them with half of the adults and two-thirds of the elderly being carriers, more recent studies have suggested that this may be a vast understatement.
As they transfer between hosts via direct contact of hair, eyebrows, and sebaceous glands of the face, this may also indicate how the longer a person is alive the more opportunities will arise for them to contract the mites.
As a pest
Generally speaking, they are harmless but in some instances have been found to cause skin disease. This is usually caused by their host may have a suppressed immune system due to stress or unrelated illness, leading to drastic increases in overall Demodex population. This may lead to several problems, such as a skin condition known as demodicosis (also known as Demodex mite bite); this manifests itself in itching and inflammation. Another potential problem a human might experience due to these organisms is blepharitis, an inflammation of the eyelids. Anecdotal evidence suggests that they may also provide a role in the development of acne and even rosacea.