Testing the Limits of Morphology: A Comprehensive Morphometric Study of the Sister Lineages <i>Lasiocyano</i> Galleti‐Lima, Hamilton, Borges and Guadanucci, 2023 and <i>Lasiodora</i> C. L. Koch, 1850 (Theraphosidae, Mygalomorphae)

Morphological conservatism and homoplasy pose significant challenges for the systematics of mygalomorph spiders, limiting the number of reliable morphological characters available for species identification, particularly in Theraphosidae. Closely related taxa frequently display high phenotypic similarity, which limits the resolution of morphology-based approaches. In this study, we conducted the most extensive morphometric analysis to date within Theraphosidae, with the objective of explicitly testing how much morphological information is retained within the Lasiocyano sazimai and Lasiodora lineage. We applied a morphometric framework combining linear morphometry and geometric morphometry, including multivariate statistics, discriminant analyses, and cross-validation, to evaluate interspecific differentiation in this group. Although some analyses recovered statistically significant differences among taxa, extensive morphological overlap, high intraspecific variation, and pronounced overfitting in cross-validated classifications consistently reduced the discriminatory power of the methods. As a result, none of the approaches provided reliable diagnostic separation between Lasiocyano sazimai and Lasiodora species. Our results indicate that morphology alone reaches clear limits of resolution within this lineage. The only characters that remained consistently informative for distinguishing the genera were the presence of stridulatory setae in Lasiodora and the distinctive blue-purplish setae of Lasiocyano. By explicitly testing the limits of morphometric and morphological inference in a morphologically conservative group, this study helps clarify the actual scope and limitations of morphology-based systematics in Theraphosidae.