Description
Scale insects show very great sexual dimorphism. Female scale insects, unusually for Hemiptera, retain the immature external morphology at sexual maturity (neoteny). Adult males can have wings but never feed and die within a day or two. Male scale insects may or may not have wings, depending on their species. Species in which males do have wings generally possess only one pair of fully functional wings. This is unusual among insects; it does resemble the situation in the true flies, the (Diptera). However, their tail filaments do not resemble anything in the morphology of flies, and the respective orders are not closely related. The hind (metathoracic) wings of scale insects are reduced, commonly to the point that they generally are overlooked. In some species the hind wings have hamuli, hooklets, that couple the hind wings to the main wings, which usually is a condition associated with the Hymenoptera. The vestigial wings often are reduced to the point where they are referred to as halteres or pseudohalteres. It is not at present clear to what extent the pseudohalteres have any substantial control function to match the true halteres of the flies.
The first instars of most species of scale insects emerge from the egg with functional legs and are informally called "crawlers". They immediately crawl around in search of a favourable spot to settle down and feed. In some species they delay settling down either until they are starving, or until they have been blown away by wind onto what presumably is another plant, where they may establish a colony separate from the parent. There are many variations on such themes, such as scale insects that are associated with species of ants that act as herders and carry the young ones to favourable protected sites to feed. In either case, many such species of crawlers, when they change their skins, lose the use of their legs if they are female, and stay put for life. Only the males retain their legs and use them in seeking females or mating.
The specifics of their reproductive systems vary considerably within the group, including hermaphroditism and at least seven forms of parthenogenesis.
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