1 of 10

Darwin's Bark SpiderCreated by the Darwin's bark spider—called one of the top ten new species of 2010—a river-spanning web dwarfs a park ranger in Madagascar in 2008. Each May the International Institute for Species Exploration at Arizona State University (ASU), along with an international committee of taxonomists, announces their choices for the top ten species that were formally recognized during roughly the previous year. Participants draw up their own criteria, and selections can be made based on anything from unique attributes to odd names.The announcement is timed to celebrate the May 23 birthday of Carolus Linnaeus, who developed the scientific system of plant and animal names more than 250 years ago.Darwin's bark makes the world's largest webs of any single spider—as wide as 82 feet (25 meters), or about as long as two city buses.(See related pictures: "World's Biggest, Strongest Spider Webs Found.")The annual list draws attention to how little we know about Earth's species, said Quentin Wheeler, director of the ASU institute. (See pictures of the top ten new species of 2009.)So far, scientists have documented about two million species, but another ten million may still be unknown."For us to sit back and think we understand evolutionary history—how life arose and why it's as diverse as it is—is a joke when we're missing 80 percent of the evidence," Wheeler said."In reality we've just scratched the surface of that fascinating story."(Related: "Ten Weirdest New Animals of 2010: Editors' Picks.")—Christine Dell'Amore

Photograph courtesy Matjaz Kuntner