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We provide objective criteria for assessing the taxonomic status of genera, especially those that are monotypic, using the coccid genus Taiwansaissetia Tao, Wong & Chang. This genus contains only the type species, Lecanium formicarii Green [currently Taiwansaissetia formicarii (Green)], known from the Afrotropical and Oriental regions. Here, Taiwansaissetia is synonymised (syn.nov.) with Coccus Linnaeus (Hemiptera: Coccidae) on the basis of phylogenetic analyses of DNA sequence data and morphological examination. All analyses (maximum parsimony, neighbour-joining and Bayesian inference) of four gene regions (18S, 28S, COI and EF-1α), and a concatenation of these regions, placed the clade including T. formicarii and three unidentified Coccus specimens as sister to the type species of Coccus, C. hesperidum Linnaeus, with high support. Taiwansaissetia formicarii is more closely related to C. hesperidum than C. hesperidum is to C. viridis (Green), which is considered to be a ‘typical’ species of Coccus, and several other current members of Coccus [C. longulus (Douglas), C. penangensis Morrison and C. pseudomagnoliarum (Kuwana)]. Explicit criteria, including monophyly, diagnosability, sister taxa being of equal rank, and the level of genetic divergence between T. formicarii and C. hesperidum relative to within-genus divergence of other scale insects, were used to assess the taxonomic status of Taiwansaissetia. The autapomorphic features of Taiwansaissetia that differentiate it from typical species of Coccus might be due to its myrmecophilous habit and adaptation to living inside ant nests – most other species of Coccus live externally on their host plants. Since its description in 1896, T. formicarii has been placed in four different genera: here we transfer it back to Coccus as C. formicarii (Green) stat.rev. We rediagnose the genus Coccus to accommodate the features of C. formicarii. This study also found that Coccus might not be monophyletic, warranting further study of the group.