I’ve heard of the existence of these mites on odd occasions for decades, but have only just got some hard information on them. If you are of a nervous disposition, look away now!
Below is a classic model of our skin. You can see a hair follicle on the left & 2 sebaceous glands (vertical yellow tubes) which produce sebum (the oily stuff that keeps our skin moist). The surface of the skin is at top:-
What they never told me at school is that there are little--in fact, almost microscopic (0.3–0.4mm (0.012–0.016 in) long)--mites that live within those follicles & glands. Here is a photograph of the ones that live within the hair follicles:-
The above photo has been taken with a ‘scanning electron microscope’ and, because of that, the colours are false colours (chosen so as to make their subject very clear). Now, just in case you think that this photo is the product of some mad seamstress that likes to make strange-looking animals from lengths of left-over cloth, here is another micrograph of some human eyelashes; you can clearly see some D. folliculorum lurking inside each follicle:-
Only 14% of people have lots & lots of D. folliculorum like the photo above; most folks have just two D. folliculorum per eyelash (1 or 2 per square centimetre of skin). Men have more than women (men have more sebaceous glands than women) and older folks have more than younger folks. The infestation seems to become 100% at puberty (the sebaceous glands become especially active at that time).
So, D. folliculorum resides in pores and hair follicles, while D. brevis prefers to settle deeper, within the sebaceous glands. They are each arthropods (jointed-legged animals such as insects and crabs; as mites, their closest relatives are spiders and ticks) (they have 8 short legs, near their head).
derived from Greek ‘dēmos’ fat + ‘dēx’, a wood worm
It is reckoned that 90% of the human being is actually composed of non-human micro-organisms (the so-called microbiome). As just one example, we could not digest the food that we eat without a vast mass of bugs that infest the whole of our guts from mouth to anus. In similar fashion, the whole of our skin is covered in bacteria and, at a more macro level, the Demodex mites.
The first report of human Demodex is from 1842 in France.
The genus Demodex was named in 1840 after a parasitic species was found that was responsible for mange in dogs; there are currently about 65 species of Demodex known. Then, in 1842, a Frenchman named Berger discovered Demodex folliculorum in human earwax (the two human Demodex species are not parasitic and appear to be commensal - they neither harm nor assist).
Humans acquire Demodex either during or shortly after birth. Their life-cycle is 18-24 days. Demodex are sexual; the female is a little bigger than the male, and D. brevis is a little smaller than Demodex folliculorum. As adults they are 0.3–0.4mm (0.012–0.016 in) long.
They spend their day within pores & hair follicles (D. folliculorum) or sebum ducts (D. brevis). All 3 of these homes are both larger & more common on the face than elsewhere, so there is a greater density of Demodex upon the face, and also the genitals & breast, though Demodex occupy the whole of the human skin surface.
Demodex come out to play at night; they travel around the skin (at about 8-16mm per hour; Usain Bolt they are not). Procreation takes place at the follicle/pore openings whilst the eggs are laid inside the follicles/pores (a YouTube video is here). The eggs are huge - about 1/3 the size of the mite. A female adult lays 20–24 eggs in a single hair follicle. 7 days later the larva has become old enough to reproduce itself.
The final (revolting) surprise comes at Demodex death, and is due to the fact that Demodex do not possess an anus. Yes, you read that right, they cannot poo. Now, no-one is perfectly certain exactly what they eat, though the best guesses are bacteria and/or sebum and/or dead skin. However, one thing that everyone is certain about is that if they cannot get the end results of that out, then it will all stack up inside. Sure enough, at death they appear to be just one huge bag of compacted oh-dear!
And, with that final, wonderful image I shall bid you good night.
--------- Alex Kemp