There are 22 families of scale insects representing more than 7,600 species worldwide. They belong to the group of insects known as the Sternorrhyncha which include related insects such as aphids , psyllids , and whiteflies. The mouthparts are found on the ventral side of the insect between the fore-coxae and project backwards.
Scale insects are among the most unusual insects known.
Adult female scales appear quite unlike a typical insect. They are sacklike with no well-defined head, thorax, or abdomen. They are neotenic, that is they resemble the immature stages as an adult. Adult females are wingless and may or may not have legs. Female scales usually have two or three immature instars.