Most scale insects are parasites of plants, feeding on sap drawn directly from the plant's vascular system. A few species feed on fungal mats and fungi, e.g., some species in the genus Newsteadia in the family Ortheziidae. Scale insects vary dramatically in their appearance from very small organisms (1-2 mm) that occur under wax covers (some look like oyster shells), to shiny pearl-like objects (about 5 mm), to creatures covered with mealy wax. Adult female scales are almost always immobile (aside from mealybugs) and permanently attached to the plant they have parasitized. They secrete a waxy coating for defense; this coating causes them to resemble reptilian scales or fish scales, hence the name.
Scale insects feed on a wide variety of plants, and many scale species are considered pests. Some types are economically valuable, such as the cochineal, Polish cochineal and lac scales. Scale insects' waxy covering makes them quite resistant to pesticides, which are only effective against the first-instar nymph crawler stage. However, scales are often controlled with horticultural oils, which suffocates them, or through biological control. Soapy water is also reported to be effective against infestations on houseplants.
Female scale insects, unusually for Hemiptera, retain the immature external morphology at sexual maturity (neoteny). Adult males have wings but never feed and die within a day or two. Male scale insects are unusual in possessing only one pair of wings, thus making them resemble true flies (Diptera), though they lack the halteres (rudimentary hind wings) seen in flies, and have tail filaments, which do not occur in flies. The specifics of their reproductive systems vary considerably within the group, including hermaphroditism and at least seven forms of parthenogenesis.