Zoologists estimate that the arthropod population of the world, including crustaceans, spiders, and insects, numbers about a billion million (10¹⁸) individuals. More than one million arthropod species have been described, with insects making up the vast majority of them. In fact, two out of every three organisms known on Earth are arthropods, and the phylum is represented in nearly all habitats of the biosphere. About 90 percent of all arthropods are insects, and about half of the named species of insects are beetles.