Demodex

Demodex (pronounced dem-ah-deks or dem-oh-deks)

A genus of parasitic, usually nonpathogenic, mites that invade the skin and are usually found in humans in the sebaceous glands of the face and hair follicles.

1839: First noted in the scientific literature in the publications of Richard Owen (1804–1892), a comparative anatomist and palaeontologist.  Name is said to be a construct of scientific Latin, from the ancient Greek δημός (dēmos) (fat) of unknown origin + the Byzantine Greek δήξ (dëx) (woodworm) of unknown origin.

Of the six dozen-odd known species, two live on human faces: Demodex folliculorum and Demodex brevis, both often referred to as eyelash mites.  A majority of humans are infested with Demodex, so it’s endemic but usually harmless except for a few where a range of skin diseases (called demodicosis) can eventuate.  Because most people asymptomatically co-exist with the creatures, some biologists once classified them an example of commensalism rather than parasitism but the modern scientific consensus is they must be considered parasitic.  

Demodex folliculorum was first described in 1842, Demodex brevis identified as distinct only in 1963, the former found in hair follicles, the latter in the sebaceous glands connected to these follicles, both inhabiting areas mostly near the nose, eyelashes, and eyebrows.  Adult mites are a tiny 0.3–0.4 mm (0.012–0.016 inch) long with Dempdex brevis slightly shorter than Demodex folliculorum.  Each has a semi-transparent, elongated body consisting of two fused segments with eight short, segmented legs attached to the upper part.  The body is covered with scales for anchoring in the hair follicle, and has pin-like mouthparts for eating skin cells and oils which accumulate in the follicles.  Preferring to avoid light, at night, they do move from the follicles, walking slowly across the skin, sometimes transferring between people through contact with hair, eyebrows, and the sebaceous glands of the face.

Females are larger and rounder than males, both sexes having a genital opening and fertilization is internal.  Mating is done in the follicle opening, eggs being laid inside the hair follicles or sebaceous glands.  The six-legged larvae hatch after three to four days and become adults in about seven days.  A typical lifespan is five-seven weeks.

In your face.  You’re never alone with a Demodex.