Scale insects (Hemiptera: Sternorrhyncha: Coccoidea) are a speciose and morphologically specialized group of plant-feeding bugs for which evolutionary relationships, and thus higher classification, are controversial. Historically, scale insect systematics has been based on the sedentary and highly reduced adult females, which has led to there being few phylogenetically informative features that can be scored for taxa across all recognized families. Major areas of controversy concern the number of families that should be recognized and the relationships of lineages of "primitive" scale insects (belonging to the informal group called the "archaeococcoids") to each other and to the "advanced" scale insects ("neococcoids"). Phylogenetic relationships of scale insect taxa are beginning to be studied using molecular and alternative morphological data sets, as well as by estimating relationships of the primary endosymbionts of selected scale insect groups. New data fail to resolve relationships among archaeococcoids and suggest an ancient radiation into a number of distinct lineages that currently are lumped into the Margarodidae sensu lato. In addition, recent data support the view that the New Zealand taxon Phenacoleachiidae is a phylogenetic relict of an archaic group and that the Holarctic and Neotropical genus Puto Signoret should be placed in its own family (Putoidae) rather than treated as a mealybug (Pseudococcidae). Among the neococcoids, the Pseudococcidae appears to be sister to all other advanced scale insects and the Eriococcidae is not monophyletic.
Keywords: coccoids, mealybugs
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