Of dramatically variable appearance and extreme sexual dimorphism, they comprise the infraorder Coccomorpha which is considered a more convenient grouping than the superfamily Coccoidea due to taxonomic uncertainties.

Adult females typically have soft bodies and no limbs, and are concealed underneath domed scales, extruding quantities of wax for protection.

Scale insects are herbivores, piercing plant tissues with their mouthparts and remaining in one place, feeding on sap.

The majority of female scale insects remain in one place as adults, with newly hatched nymphs, known as "crawlers", being the only mobile life stage, apart from the short-lived males.

They secrete a waxy coating for defence, making them resemble reptilian or fish scales, and giving them their common name.

[2] The key character that sets apart the Coccomorpha from all other Hemiptera is the single segmented tarsus on the legs with only one claw at the tip.

Legs are absent in the females of some families, and when present vary from single segment stubs to five-segmented limbs.

[2] Adult males in contrast have the typical head, thorax and abdomen of other insect groups, and are so different from females that pairing them as a species is challenging.

The adult possesses an ovotestis, consisting of both female and male reproductive tissue, and sperm is transmitted to the young for their future use.

The fact that a new population can be founded by a single individual may have contributed to the success of the cottony cushion scale which has spread around the world.

[2] The first instars of most species of scale insects emerge from the egg with functional legs, and are informally called "crawlers".

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 protected sites to feed.

[2] Scale insects show a very wide range of variations in the genetics of sex determination and the modes of reproduction.

The commonest (known as the lecanoid system) involved deactivation of the paternal genome and elimination at the time of sperm production in males, this is seen in Pseudococcidae, Kerriidae and some Eriococcidae.

A third variant found in Diaspididae involves the paternal genome being completely removed at an early stage making males haploid both in somatic and germ cells even though they are formed from diploids, i.e., from fertilized eggs.

In Parthenolecanium, males are born from unfertilized eggs but diploidy is briefly restored by fusion of haploid cleave nuclei and then one sex chromosome is lost through heterochromatinization.

Some species have certain habitat requirements; some Ortheziidae occur in damp meadows, among mosses and in woodland soil, and the boreal ensign scale (Newsteadia floccosa) inhabits plant litter.

[2] A Hawaiian mealybug Clavicoccus erinaceus that fed solely on the now critically endangered Abutilon sandwicense has gone extinct as has another species Phyllococcus oahuensis.

[9] Several other monophagous scale insects, especially those on islands, are threatened by coextinction due to threats faced by their host plants.

This includes sugars, amino acids and minerals, and is attractive to ants as well as acting as a substrate on which sooty mould can grow.

[13] Many tropical plants need ants to survive which in turn cultivate scale insects thus forming a tripartite symbiosis.

[15] Scale insects have various natural enemies, and research in this field is largely directed at the species that are crop pests.

[2] Ladybirds feed on aphids and scale insects, laying their eggs near their prey to ensure their larvae have immediate access to food.

[22] One species, the cottony cushion scale, is a serious commercial pest on 65 families of woody plants, including Citrus fruits.

[31] The colour name "scarlet" is similarly derived from Arabic siklāt, denoting extremely expensive luxury silks dyed red using kermes.

The phylogeny of the extant groups, inferred from analysis of small subunit (18S) ribosomal RNA, is shown in the first cladogram.

[citation needed] Psylloidea (jumping plant lice, etc) Aleyrodoidea (whiteflies) Coccomorpha (scale insects) Phylloxeroidea (phylloxera bugs) Aphididae (aphids) The timing of phylogenetic diversification within the Coccomorpha was estimated in a 2016 study based on molecular clock divergence time estimates, along with fossils being used for calibration.

[37] Scale insects are very well represented in the fossil record, with the oldest known member of the group reported from the Late Jurassic amber from Lebanon.