Power and Syred/Science Photo Library
By Daniel Cossins
Last night, while you were sleeping, legions of eight-legged creatures had an orgy between your eyebrows. No, you haven’t suddenly been invaded by sex tourists. Demodex mites, close relatives of ticks and spiders, are permanent and mostly harmless residents of the human face.
“Every person we’ve looked at, we’ve found evidence of face mites,” says Megan Thoemmes at North Carolina State University in Raleigh. “You can have thousands living on you and never even know they’re there.”
Growing up to 0.4 millimetres long, these beasts spend their days buried head-down in hair follicles gorging on who-knows-what and crawling out under cover of darkness to copulate. They have no anus, so on death disgorge a lifetime of faeces into your pores.
Before you lunge for the exfoliating brush: Demodex mites are far from your only microscopic residents. You host astonishing biodiversity, from anus-less arthropods to pubic lice to all manner of bacteria and fungi, and without it you wouldn’t be who you are. “Each of us is really a complex consortium of different organisms, one of which is human,” says Justin Sonnenburg at Stanford University in California.
Our resident aliens aren’t all benign. There are big beasts like parasitic worms: roundworm, hookworm and whipworm are prevalent in the developing world, and pinworm still infects kids in the West. Then there are hidden viruses such as Herpes simplex, which lies dormant inside the nerve cells of two-thirds of people until it mistakes your sniffles for a deadly fever and attempts to save itself by rushing outwards, causing cold sores.
“Last night, legions of eight-legged creatures ...
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