Scale insects (which include mealybugs) are distinguished from other groups of Sternorrhyncha by possession of a single claw at the apex of each tarsus; all other insects possess double claws in this position. Immatures and adults may possess specialized hinged plates and other structures associated with the anus, concerned with safe disposal of sugary honeydew, but never possess a vasiform orifice or lingula. Apart from a very few Margarodidae, the stages that possess legs all have single-segmented tarsi. Adults are strongly sexually dimorphic; the ephemeral males have one pair of membranous wings (the hind-wings are reduced to hamulohalteres) and lack mouthparts; they are short-lived and seldom seen. Adult females are larviform (neotenous) and sedentary or sessile; adult females and immature stages of both sexes secrete a waxy covering over themselves.
Reproduction is usually sexual but parthenogenesis occurs in some species; hermaphrodity has been recorded. Ovipary, ovovivipary and vivipary occur; the eggs lack a peduncle and are laid under the protective scale cover or in a waxy ovisac produced by the female. There are two to four immature stages in the female and four in the male. Immature and adult females normally feed at least intermittently, but only the first two male instars feed.