Integrative Taxonomy Clarifies the Taxonomic Status of the Morphologically Intermediate Form Between Tropidothorax cruciger and T. sinensis (Hemiptera: Lygaeidae)

(1) Background: The identification of <i>Tropidothorax cruciger</i> and <i>T. sinensis</i> is often complicated by the presence of the "intermediate form". Due to the lack of molecular data, the taxonomic status of the "intermediate form" and the species boundaries between <i>T. cruciger</i> and <i>T. sinensis</i> remain uncertain; (2) Methods: In this study, we integrated morphological, molecular, and ecological data to delimit species boundaries of these two species using multiple species delimitation approaches; (3) Results: Most species delimitation analyses based on the cytochrome c oxidase subunit I (COI) fragment suggested that <i>T. cruciger</i> and the "intermediate form" comprised a single species, with <i>T. sinensis</i> representing a separate species. This delimitation result was also supported by the analyses of BFD* and genetic clustering based on genome-wide SNPs. Under this species delimitation scenario, a clear-cut barcode gap was discovered between the interspecific and intraspecific genetic distances. In addition, environmental-related analyses showed highly similar ecological requirements of <i>T. cruciger</i> and the "intermediate form", supporting their recognition as a single species; (4) Conclusions: This study clarifies the taxonomic status of the "intermediate form" and the species boundaries between <i>T. cruciger</i> and <i>T. sinensis</i>, which is essential for further studies of ecology and evolution of these species.