It\'s been a lazy day. I reclined in front of the fire to stretch an aching back against the old birch and maple floor. I looked across what appeared to be a clean wooden surface, but my biologist\'s imagination soon went into high gear. What other living creatures might be there, unseen by human eyes?

After the usual candidates came to mind (minuscule spiders and perhaps the smallest ants hiding in cracks, and of course harmless free-living bacteria everywhere, no matter how scrupulous the house cleaning), I settled upon a group of tiny animals we never see, and in fact cannot be seen unless they are brightly illuminated against a dark background, and then only if we almost place our noses on top of them.

Among the world\'s smallest creatures with legs, collectively their several species are known as dust mites. Dust is where they live, and they aren\'t any bigger than the inanimate particles around them. A female dust mite is about a quarter of a millimeter long (less than 1/16 of an inch), and the males are smaller still. Because miniaturization has undisputed survival value, it is a path taken to the extreme by these tiny relatives of spiders.

Dust mites dwell actively on every kind of surface, understandably in greater abundance in protective carpets and mats, upholstery, and bedding, than on hard smooth surfaces. They are always present, even after diligent sweeping and vacuuming.

Sometimes a dust mite population can be temporarily reduced with a potent cleaning solution, but most mites survive the onslaught. No matter, they are prolific and quickly return to their usual abundance. They aren\'t bad little creatures. The only undesirable effect they have is upon those unfortunates among us with allergies and asthma who react violently to such minute household presences - and don\'t know the cause.

Dust mites are found everywhere in the world: in mansions and huts, polar regions and humid tropics. Of 13 known species, six commonly live in every home under conditions best suited to their needs. As a group they are known as pyroglyphids, which translates roughly to \"notched pear.\" In this case \"pyro\" refers not to fire, but to the grooved fruit-like shape of the animal.

Why do they prefer houses to outdoor life? Nutrition dictates their habitat. Up to 90 percent of all dust mites depend upon scales shed from human skin. The dry outer layers of our skin are constantly sloughing off in minute flakes (five grams a week per person), each particle far smaller than more visible dandruff from our scalps.

Appreciative of this constant rain of manna from the giants in whose homes they dwell, dust mites live abundantly and well-nourished beneath our feet. Scientists studying them claim one gram of dried human skin can feed thousands of dust mites for months. Even the Latin name given the dust mite genus, Dermatophagoides, translates as \"skin-eaters.\"

But these scavenging mites also seek variety in their diet and consume other floor-level organic delicacies such as pollen grains, bits of mold, plant fragments, pieces of dead insects or cast-off insect skins, and even clumps of bacteria. In almost every instance what they eat is completely dry, so dust mites must first liquefy their food (as their spider cousins do) by injecting enzymes to start digesting a substance before it is sucked in.

It\'s been said they are not much more than perambulating stomachs, sort of eight-legged mini-vacuum cleaners. Their internal organs (which are surprisingly complex for so small an animal) seem mostly to do with ingesting and digesting food, then excreting the residue. They have a nervous system adequate to their way of life, but no eyes or poor ones at best, and no respiratory organs. They breathe through their skin cuticle.

A dust mite\'s life cycle isn\'t complicated. They do best in moderate household temperatures with fairly high humidity. When conditions are favorable and after mating takes place, a female lays an egg a day for almost a month. A larva emerging from an egg remains in this state for about a week, then passes through two immature stages, each lasting less than a week. Twenty-four hours after adulthood has been reached, dust mites are able to reproduce and start the repetitive egg-laying process. Ripe old age is a little over four weeks for a geriatric dust mite.

What about their effect upon us? Allergies are on the increase worldwide and one of the most common of these is reacting to house dust. What this really means is reacting to dust mites, their molted skins, and their waste products. What to do about it?

Whatever can be dry-cleaned, steam cleaned, or washed in hot water will be rid of mites, but only temporarily. Even cold water washing kills many of the little animals, yet at least 10 percent always survive to repopulate their underfoot world.

There may be more dust mites in bedding than anywhere else, up to 2 million in the average double bed. After all, sheets and pillow cases are in contact with human skin for hours at a time, so the flake-off banquet is a feast for hungry dust mites. Even though the feeding frenzy goes on day after day, we\'re never aware of the tiny diners unless severely allergic to them, and then we usually ascribe the malady to something else - synthetic fabrics, detergents, cat, dog, or just plain dust, with no idea what its components may be. The Institute of Food and Agricultural Sciences of the University of Florida estimates 4 percent of humans suffer from dust mite allergies.

A person known to be specifically allergic to dust mites at least has a chance to minimize the misery. Washable rugs can be substituted for carpeting, vacuuming can be done by someone else when the sufferer isn\'t around, and humidity should be kept low. Household cleaning chemicals of various sorts can be used, and for the severely afflicted there are chemicals (acaricides) that are specially formulated to kill mites.

Even if a house should be relatively free of dust mites, schools and work places usually are not, especially if they are carpeted. Work-place allergies are often in the news, but complaints and claims seem to be against industrial pollutants and contaminants, or against the chemical backing of carpeting. I\'ve never heard of a school\'s carpeting being ripped up just to get rid of tiny creatures living deep in the pile, or someone suing a major company for failure to dispose of a microscopic zoo tenaciously hunkered down in its corporate headquarters.

This essay is sure to have an unhappy effect upon readers. Learning about dust mites for the first time, even the most non-allergic among us is likely to go on a rampage - vacuuming, washing, and scrubbing to rid our homes of these invisible, uninvited little co-inhabitants. By all means do so. But don\'t for a moment believe you will ever be completely rid of dust mite companions. Within hours of your best effort, pioneers will arrive on your or someone else\'s clothes, on pets, or will waft in on the breeze through an open door. Best not to worry, because these little eight-legged beasties have been with us as long as we have been human and are here to stay.

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