Most people use the word "bug" when talking about insects like beetles, bees, and butterflies, and other small, many-legged creatures that crawl, jump, or fly, such as spiders and centipedes. Although for scientists, the word "bug" does correspond with a very specific group of insects – and not all insects are bugs. Of the millions of insect species on earth, only about 50,000 are true bugs. In the scientific world, a "true bug" is classified as an insect species that belongs to the order Hemiptera, including bed bugs, fire bugs, and some water bugs. Insects in this order are different from insect in other orders, such as those for ants and bees (Hymenoptera), butterflies and moths (Lepidoptera), or flies and mosquitoes (Diptera).