Insects are small creatures with three pairs of legs, a body with three main parts (a head, thorax, and abdomen), and a tough shell-like outer covering, called an exoskeleton. Insects are arthropods, which means they do not have a backbone. Most have one or two pairs of wings and a pair of antennae. There are 900,000 known species of insects in the world, and entomologists (scientists who study bugs) estimate that there are millions (perhaps up to 10 million!) more yet to be discovered. Insects are everywhere—there are more bugs in 1 square mile (2.59 square kilometers) of rural land than there are human beings on the entire globe. Insects are divided into 32 orders, or groups. The largest insect order is the beetles (Coleoptera) with 125 different families and approximately 500,000 different species. In fact, one out of every four animals on Earth is some type of beetle. In the United States, there are some 73,000 species of insects: approximately 24,000 beetles, 19,500 flies, 17,500 ants, bees, and wasps, and 11,500 moths and butterflies.