Recently, I was reading and interviewing and looking through microscopes to find out about acarology, the scientific study of ticks and mites. As part of my study, I was peering down at a tiny, elongated creature at the nether end of a binocular microscope. It was a human follicle mite, Demodex folliculorum hominis. Its shape suggested a tadpole with a very long tail and all its other important body parts crammed toward the head end. Eight stubby, clawed legs feebly groped in the immersion oil. It was out of place and far from home. I’d mined it from my forehead by dragging the curved end of a paper clip across the skin, unloaded the tiny wad of skin grease and evicted mites onto a microscope slide, added a drop of immersion oil, then placed the slide with its oily load under the microscope.
There were many more of my personal mites in the oil drop. I scanned and found a larva. It resembled a medieval mace with the spikes filed down to knobs. The human follicle mite, nearly microscopic, is specialized to live in the pits of human hair follicles. They prefer the face and ears, but may be found anywhere on a human body. A related species, Demodex brevis, lives in the sebaceous glands that link to the hair follicules. Both species are
shared by all of mankind, without exceptions, from Inuit to New Guinea highlanders, from African pygmies to Andamanese Islanders.
Mites are relatives of spiders. A few mite species even weave webs. Except for ticks, which are a type of mite, most mites are microscopic or nearly so. They swarm all over the planet, in land, in fresh water, and in the seas, as universal as bacteria, but like bacteria, are scarcely noticed unless they cause trouble, such as triggering asthma attacks (dust mites) or decimating honeybee colonies. There are over 200 families and 140,000 recognized species of mite, and these are probably no more than 5 percent of the total number of species.
Mites may be parasites, active hunters, or scavengers on junk organics like decaying vegetable matter and dead, sloughed human skin cells (e.g., dust mites). Parasitic mites, including ticks, feast on the blood and body fluids of vertebrates, insects and their relatives, and plants.
Some ticks and mites carry disease microorganisms to which humans are susceptible. These include the infamous Lyme disease, typhus, tularemia, and relapsing fever.
Demodex mites have tiny, simple eyes, no lungs, no anuses. Most mites have no eyes and don’t need them, and most are so small that they don’t need or have any elaborate breathing organs, but directly exchange gases through their body walls. In a Demodex mite, wastes are accumulated in a growing, crystallized mass within its body, never expelled lest they foul the cramped quarters where the mites live out their days. A mite dies still carrying the waste mass, and the tiny corpse, crystals and all, is expelled from the follicle by the outward flow of sebaceous oil. Males and females are born, live, feed, mate, and die within a hair follicle. A fertile female produces up to 25 eggs, but only one at a time, a third as long as herself. Since space and food are limited in this minute environment, no mass egg production is called for.
Ticks are a more familiar kind of mite because many species are large enough to be easily visible, especially after they’ve ballooned to many times their normal size when they load up with their standard food, blood. I saw a preserved, bloated specimen of the world’s largest tick species, the size of a large grape, that was plucked from a South American tree sloth. The creature’s eight black, wiry, many-jointed legs, like snippets of miniature barbed wire, added to the horror. My guide told me that the grape-sized mite was swollen to only half of its maximally extended self.
Ticks prey on reptiles, mammals, and birds. With their specialized piercing mouth parts, they can penetrate skin and, vampire-like, tap into capillaries and swill on the host’s blood. When they’ve fueled up completely, they may drop off and digest at leisure, or remain attached to the host, depending on the tick species’s life cycle.
Mite and tick body shapes run from fat and oval and stubby-legged, like dust mites, to vaguely crablike, like black-legged ticks (carriers of Lyme disease), to long, thin, hotdoggish forms, like the human skin mites. Eight legs are the norm, although some larval stages have only six legs, and some species have wound down to only four legs in the adult form. Sensory strands may cover parts or most of the body. Beyond those basics, the array of forms among mites is fantastically varied and bizarre, including creatures that only experts can deduce are mites.
Some mite species live among New World army ants. A few species disguise themselves as ant larvae, the shapes so close to those of true larvae that only the eight tiny legs of the mites give away the disguise. The nurse ants tote the mites about with the true larvae, feeding both and never realizing they’ve been had. Other army ant mites are engineered by nature to fit one small body part of an ant, including a species that gloms onto the foot pad of an ant and gets stepped on every time the ant moves, which is most of the time.
In the New World tropics, certain mites that live in flowers hitch rides on hummingbirds to travel among flowers.
Just as strange are species of ear mites that live on noctuid moths (also called owlet and miller moths) hunted by bats. Bats’ sonar and reflections can find the moths in flight and pinpoint their locations. Noctuid moths are sensitive to bat sonar, and take evasive action when they detect supersonic bat squeakings. The mites inhabit only one ear of an individual moth, leaving the other free and unimpeded to hear bat sonar. The survival stakes are obvious: if the mites clogged both ears, then the moth’s hearing would be too impaired to accurately hear bat sonar, and would be vulnerable to foraging bats. The mites would be equally vulnerable. Leaving one moth ear free to hear bat sonar increases the odds of survival for moth and mites. Most unbelievable about this arrangement is that scientists have experimentally extracted some of the mites from the infected ear of a captured moth and moved them into the opposite, uninfected ear. The misplaced mites immediately crawl out of that ear, over the exterior of the moth, and back into the infected ear, ensuring that the other ear remains uninfected.