Our planet is home to about 50,000 species of vertebrates -- a group that includes mammals, amphibians and reptiles. We have about 400,000 species of plants. And while there is wiggle room in those numbers (mostly as genetic techniques redefine what it means to be a species), the biggest source of unknowns lies within the arthropods, which includes insects, spiders and mites.
Arthropods make up 80 percent of all known species (not including bacteria, which are not usually included in studies like these), and most arthropods live in the tropics. Scientists have named about 855,000 arthropod species, but that is just a small fraction of what's out there.
The first stab at estimating actual arthropod numbers came in 1982, when scientists counted the number of creatures living on a single species of tropical tree and multiplied that by the total number of tree species. They came up with an estimate of 30 million species on Earth.
Using the same kind of method, though, other researchers produced estimates ranging from 10 million to more than 100 million species. As researchers realized the extent of what they didn't know, the discussion gradually petered out.