Two kinds of hungry pests gnaw away at the pilings that hold up structures like the FDR Drive, the U.N. school on East 25th Street, and the Con Ed plant at 14th. Teredos, which start life looking like tiny clams, grow up to be worms "as big around as your thumb, and nearly four feet long, with little triangular teeth," says commercial diver Lenny Speregen. Like underwater termites, they devour wood. And Limnoria tripunctata, a.k.a. "gribbles," are bugs about the size of a pencil dot that look like tiny armadillos, and eat not only wood but also concrete. Speregen says he's seen fifteen-inch-diameter columns that have been gnawed down, hourglass style, to three inches. The city has tried jacketing pilings in heavy plastic to keep the critters out, but it hasn't worked well: Floating ice tears up the jackets in winter. "I never said this wasn't a war," says Speregen.