All insects belong to the phylum Arthropoda. But unlike other arthropods—like lobsters, spiders, or millipedes—insects have three pairs of jointed legs, segmented bodies, an exoskeleton, one pair of antennae, and (usually) one or two pairs of wings.
Insects live in nearly every habitat, and it’s estimated that there are currently 10 quintillion insects on the globe. So far scientists who study bugs, called entomologists, have named one million insect species but studies estimate that four million are still uncategorized.
The oldest insect fossil—a mandible (or jaw) found in Scotland—is between 408 and 438 million years old. The oldest winged fossil dates back 330 million years ago, suggesting that insects were among the first animals to leave the oceans for land during the Devonian period some 400 million years ago.
Insects are vital to every ecosystem. They pollinate plants, decompose plant and animal matter, and are themselves a source of food. Birds alone are estimated to eat 400 to 500 million tons of insects per year.