Many unusual features of scale insects (Hemiptera: Coccoidea) can be explained as historical legacy. Developmental specializations in ancestral coccoids resulted in a neotenous adult female and a drastic metamorphosis of the male. Subsequent evolution led to numerous, often convergently derived, adaptations to parasitic life on higher plants. The sedentary lifestyle of female scale insects has favored the evolution of appendage reduction or loss, gross changes in body shape, and protective wax secretions, tests, and other scale covers. Morphological peculiarities of adult males relate to flight or to mating with concealed females. Scale insects have diverse egg-protecting methods, a range of chromosome behaviors (including several methods of sex determination), marked sexual dimorphism [even sometimes in first-instar nymphs (crawlers)], and more rarely sexual dichronism. Crawlers have evolved as the main agents of dispersal. The biotic interactions of scale insects include diverse endosymbioses with microorganisms, sometimes morphological and behavioral adaptations for obligate association with ants, and often highly specific host-plant associations that may lead to demic adaptation or the evolution of complex galls.

]. Four-kilobase fragments containing 16S-23S ribosomal DNA (rDNA) were obtained from the P-endosymbionts of 22 mealybug species and the S-endosymbionts of 12 representative species. Phylogenetic analyses of the P-endosymbionts indicated that they have a monophyletic origin and are members of the ␤-subdivision of the Proteobacteria; these organisms were subdivided into five different clusters. The Sendosymbionts were members of the ␥-subdivision of the Proteobacteria and were grouped into clusters similar to those observed with the P-endosymbionts. The S-endosymbiont clusters were distinct from each other and from other insect-associated bacteria. The similarity of the clusters formed by the P-and S-endosymbionts suggests that the P-endosymbionts of mealybugs were infected multiple times with different precursors of the S-endosymbionts and once the association was established, the P-and S-endosymbionts were transmitted together. The lineage consisting of the P-endosymbionts of mealybugs was given the designation "Candidatus Tremblaya" gen. nov., with a single species, "Candidatus Tremblaya princeps" sp. nov. The results of phylogenetic analyses of mitochondrial DNA fragments encoding cytochrome oxidase subunits I and II from four representative mealybug species were in agreement with the results of 16S-23S rDNA analyses, suggesting that relationships among strains of "Candidatus T. princeps" are useful in inferring the phylogeny of their mealybug hosts.

The mealybugs (Hemiptera: Coccoidea: Pseudococcidae) are a speciose and ubiquitous group of sap‐sucking plant parasites, many of which are very serious agricultural pests. There has been much work on the alpha‐level taxonomy, amounting to the description of more than 2000 species, but suprageneric relationships remain poorly known. Downie & Gullan reviewed proposed schemes for a mealybug subfamily‐level classification and used DNA sequence data from three nuclear genes to infer the mealybug phylogeny. They recognized three subfamilies: the Rhizoecinae, the Phenacoccinae, and the Pseudococcinae; and within the Pseudococcinae, recognized the tribes Pseudococcini, Trabutinini, and Planococcini. Excepting the Trabutinini, none of these groupings was well‐supported. We improve our estimation of the mealybug phylogeny by: (i) increasing the taxon sampling by 50%; (ii) adding a morphological character matrix; and (iii) performing mixed model Bayesian and maximum likelihood inference procedures. We recover two primary clades within the Pseudococcidae, to which we apply the subfamily names Phenacoccinae and Pseudococcinae. Within the Phenacoccinae, we recover support for the monophyly of the hypogaeic mealybugs (Rhizoecini), and within the Pseudococcinae, we find support for the tribes Pseudococcini, Trabutinini, and Planococcini. Our results suggest a clear sequence of (i) β‐Proteobacteria primary endosymbionts infecting the mycetome of the most recent common ancestor of the Pseudococcinae, followed by (ii) γ‐Proteobacteria secondary endosymbionts infecting the primary β‐Proteobacteria endosymbionts. For each subfamily, we provide a morphological diagnosis and a comprehensive list of included genera. We resurrect the genus Ceroputo Šulc, currently under synonymy with the genus Puto Signoret (Putoidae), and place it in the mealybug subfamily Phenacoccinae. Greenoripersia kaiseri Bodenheimer is transferred from the Pseudococcidae to the Eriococcidae.

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