The candiru, a tiny catfish found in South America, seems to be more feared than the piranha. Why?
-- Charles N. Cooper, Baltimore, Maryland

The candiru is one creature you definitely don't want to get too close to. Like many of the more than 2,000 species of catfish in the world (there are about 1,200 in South America alone), Vandellia cirrhosa possesses a set of barbels that resemble feline whiskers. But this diminutive specimen -- it is only about half an inch to three inches long -- has a rather dicey trait not shared with others in its family: It is the world's only vertebrate that parasitizes people. It can anchor itself in a most intimate part of the human anatomy, leading one jokester to dub it Urinophilus diabolicus. In English it is candidly known as the urethra fish, because it can actually swim up the urethra of an unsuspecting bather and lodge itself firmly in place by flaring sharp spines along its gills. Not only does this make for a most excruciating ordeal, but once embedded, the cold-blooded little terror can't be removed by anything short of surgery.

Candirus primarily set up house inside larger fish, where they feast on the host's blood. They are attracted to nitrogen, which usually leads them to a gill chamber, but apparently they can't distinguish between one nitrogen-emitting orifice and another: They have been known to follow a stream of urine right to its source. Candirus are hard to spot because they are nearly transparent. So if you're swimming in an Amazon jungle, here's a tip: Don't micturate in the water, and be sure to wear tight swimwear.

What are the seven natural wonders of the world?
-- Mary Murphy, Maynard, Massachusetts

That would make a terrific category for Jeopardy!, but unfortunately, it's not as cut and dried as, say, the seven seas, the seven deadly sins, or the Seven Sisters. No single list has won unanimous agreement, and to further complicate the matter, tourist boards around the world are fond of citing local phenomena as part of this visitor-drawing septet. As a result, there are many variations on the theme. One roster comprises the Grand Canyon, Mount Everest, the northern lights (aurora borealis), Australia's Great Barrier Reef, the harbor of Rio de Janeiro, Victoria Falls in Zimbabwe, and the Paricutin volcano in Mexico. Other lists substitute one or more of the following: the Petrified Forest in Arizona, Niagara Falls, Rainbow Natural Bridge in Utah, Carlsbad Caverns in New Mexico, Mount Fuji in Japan, Mount Kilimanjaro in Tanzania, Indonesia's Krakatoa Island, the Bay of Fundy in Canada, and Iguacu Falls, between Brazil and Argentina.The practice of enumerating seven wonders started with the ancient Greeks, who were giving their citizens tips on travel destinations. The first compendium included the most soaring monuments known at the time, such as the Pyramids of Giza in Egypt (the only survivor), the Hanging Gardens of Babylon, and the Colossus of Rhodes.

One inventory of natural wonders is kept by the United Nations Educational, Scientific, and Cultural Organization (UNESCO). To date, 114 places have been designated as World Heritage Sites, worthy of being preserved for their "outstanding universal value." For more information, visit the World Heritage web site (http://www.unesco.org/whc/).

How does nature protect woodpeckers from scrambling their brains as they hammer at trees?
-- Theodore L. Gaillard Jr., Philadelphia, Pennsylvania

Given that a woodpecker can hammer 25 strokes in a second and that its head travels at a speed of more than 20 feet per second, it's a wonder the bird doesn't beat out its brains. Luckily, it happens to be superbly suited to its ecological niche.

Woodpeckers have developed a number of dandy features to withstand the impact of rapid-fire pecking, tapping, and drumming. The bird's skull is thick and heavily ossified to keep it from shattering. Shock-absorbing tissue between its eyes and around its skull cushions its brain, much as a crash helmet would. The skull and bill are separated by spongy material, which also lessens the impact of blows. Unlike humans, whose gray matter is enclosed in a sac of fluid, woodpeckers have a brain surrounded by a tough outer membrane, so it doesn't bounce around. The bird's sharp, chisellike bill is extremely sturdy and is built to transmit the tremendous force of repeated beating to strengthened areas of bone at the front of the skull. Even its nostrils are well shielded: In some species they are covered with fine bristly feathers; in others, reduced to narrow slits to keep them from becoming inundated with the sawdust and wood chips generated by the bird's woodworking.