Body of fully mature adult female swells to form a rotund structure and looks like a gall. Newly formed adult females often are ornately colored with stripes, spots, or stippling, but as the body enlarges these patterns disappear. Some species produce wax over the body but the secretion disappears with age. No filamentous ovisac is produced, instead a brood chamber is formed under the body of the adult female.
There are some exceptions to the characters listed above. Nidularia species are reasonably typical of other kermesids but lack legs entirely, have setae and pores in the anal ring, and produce an ovisac. Physericoccus also seems to fit fairly comfortably into the Kermesidae but lacks a well defined band of invaginated tubular ducts. Eriokermes species are by far the most different morphologically from other gall-like scales having virtually none of the characteristics given in the diagnosis. However, molecular data and first-instar and adult male morphology support the association. The genera Fulbrightia and Reynvaania which have been treated as kermesids, seem to have closer affinities with the eriococcids. Kermesidae Signoret was first used as a family by Lobdell (1929).
Gall-like scales occur in the Afrotropical region, Nearctic region, Oriental region, and Palaearctic region. They are most speciose in the Palaearctic region, have only a single odd species in the Afrotropical region and are absent from the Australasian and Neotropical areas.
Kermesids are primarily found on Quercus but occasionally are recorded on other fagaceous hosts such as Chrysolepis and Lithocarpus. The odd genus Eriokermes is restricted to the conifer genus Juniperus.
Gall-like scales have 4 instars in the female and 5 in the male. Life history data have been gathered for only a few taxa. There is a single annual generation and overwintering stages occur on the large stems or trunk of the host in cracks and crevices. In early spring, those species that overwinter as first instars molt and second-instars females migrate to the new growth or leaves and feed; second-instar males either remain in the overwintering area or migrate to the duff beneath the tree and develop into adults. Those species that overwinter as second instars remain and feed at the overwintering site as females or migrate to large branches on the bole of the tree as males. In late spring or early summer adults occur and mating takes place. Females enlarge rapidly, assuming the appearance of a gall or leaf bud. Egg laying occurs in summer inside of a special brood chamber formed by lobe-like extensions that develop from the body wall. As many as 3,000 eggs can be laid by a single female. The eggs hatch, and the first instars leave the brood chamber through a small opening in the posterior end of the female body. The life history of Eriokermes gillettei is quite different. It forms a felted ovisac in the adult female; remains pear shaped and looks like a typical eriococcid, and has only 3 female instars. Most species of kermesids occur on the trunks, branches, twigs, or leaves of the host.