Really now, what would you expect from a guy who once wrote an article called "Plants in Heat" and is best known for his first book, "Flattened Fauna: A Field Guide to Common Animals of Roads, Streets and Highways"? Roger Knutson has again stretched biological boundaries with his latest book, "Furtive Fauna" (Penguin Books, $8). This one's a guide to fleas, ticks, face mites, body lice, tooth amoebas and the whole collection of critters that live on or around people. Polite society may prefer not to know that squiggly mites spend most of their lives face down in our follicles or that fungus grows on nearly everyone's hair nearly all the time. But like it or not, people have been playing host to parasites throughout history. One of the first words in the human language, in fact, may have been a term for flea or tick, linguists reckon. Mr. Knutson even speculates that early humans' need to groom one another may have set them on the evolutionary course that led to modern society. "Someone who can talk another person into looking them over for tiny ectoparasites would have no trouble selling Veg-o-matics to the unwary," he writes. From years of teaching biology, botany and ecology classes at Luther College in Decorah, Iowa, Mr. Knutson has learned to use humor and examples that are "just a little bit gross" to convey real scientific facts. In "Furtive Fauna," he presents a mite's-eye view of the human body; then offers tidbits: * Ticks anesthetize a person's skin before biting. That's why you usually don't know you're a victim until you find the blood-engorged tick. * None of the millions of bacteria that houseflies spread is likely to cause disease. * Fleas are into sleaze. The flat-bodied insects possess the most elaborate sexual apparatus known among animals and can copulate for as long as four or five hours. * One small thing to be grateful for: Follicle mites, which live on people's faces, have no anus. They apparently die of constipation before reaching old age. Not to change the subject, but what about that article on plants in heat? For the past 20 years, Mr. Knutson has been studying flowers of eastern skunk cabbage plants growing on Lake Michigan's Beaver Island. The flower spike that pokes up in early spring generates enough heat to melt snow for six inches around the plant. What's more, it maintains a constant temperature of 75 degrees Fahrenheit for weeks -- an extraordinary feat for a plant. Enough about fervid flora -- back to furtive fauna. Odd facts aside, the book has a message. "The important ideas aren't about wiggly creatures and itchy things," Mr. Knutson says. "The important ideas are really ecological things -- habitat and the relationship of creatures to habitat." The success of "Flattened Fauna" convinced him that "people want to know about the natural world, but relate better to highways or to themselves than they do to tropical rain forests or the deserts of the world," he explains. "If people can see themselves as an interesting environment, they stand a much better chance of seeing other places as environments with value."