An ecomorphological approach to understand parasitic adaptation in triungulin larvae of blister beetles (Coleoptera: Meloidae)

Blister beetles (Coleoptera: Meloidae; about 130 genera and 3000 species) are remarkable not only for cantharidin production, but also for the evolution of a highly specialized first instar larva, the triungulin, present in all subfamilies except Eleticinae. Depending on the taxon, these larvae feed mainly either on provisions and immature stages of bees (Hymenoptera: Apoidea) or on grasshopper egg pods (Orthoptera: Acrididae). The food source may be reached by actively crawling over the substrate or through phoresy, primarily on bees and, only in the genus Cyaneolytta, on Coleoptera Carabidae. These peculiar ecological adaptations appear to have evolved multiple times within the family, and triungulin morphology is thought to vary accordingly, although this correlation relationship has never been quantitatively tested. Here, we aim to test the association between ecological strategies and larval morphology across the Meloidae family within a robust phylogenetic framework. We analyzed variation in key morphological traits (including head capsule, thoracic segments, and leg appendages) using a 2D/3D geometric morphometric approach applied to a broad taxonomic dataset composed of 130 species belonging to 45 different genera. Phylogenetic comparative methods were employed to examine shape variation in relation to ecological categories (e.g., phoretic vs. non-phoretic behaviour, host type), and to assess patterns of morphological convergence and divergence. This study sheds light on the evolution of larval morphology as an adaptation to parasitism and contributes to unveiling the mechanisms adopted by insects for the exploitation of host resources.