Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that survive as external parasites of mammals and birds. Fleas live by consuming blood, or hematophagy, from their hosts. Adult fleas grow to about 3 mm (0.12 in) long, are usually brown, and have bodies that are “flattened” sideways or narrow, enabling them to move through their host’s fur or feathers.