Enlarging knowledge on cantharidin in blister beetles (Coleoptera: Meloidae)

Cantharidin (CA) is a toxic terpenoid produced by blister beetles, transferred from males to females during copulation. In addition, as a characteristic defensive behaviour, blister beetles can exudate CA-containing hemolymph by reflex bleeding from leg joints. Despite attracting the interest of many researchers for its properties (CA is a promising compound for many medical applications), some basic knowledge enlightening the role of CA in blister beetles’ ecology are still lacking. For instance, data on CA content in many meloid species are still unavailable and the quantification of CA in the exuded hemolymph, despite its renowned defensive role, has been completely ignored. The objective of this study was to quantify the CA content in the exuded hemolymph (to be compared with CA quantity in body tissues) of eight species never investigated so far, belonging to Mylabris, Lydus, Meloe and Cerocoma genera. Specimens were collected in Italy, identified and reared in fauna boxes. The exuded hemolymph from each specimen was collected from leg joints every two days using capillary tubes and stored at -70 °C. Insects body tissues were also stored at -70 °C for subsequent analyses. Available protocols were optimized for CA extraction from both body tissues and exuded hemolymph and CA content quantified in GC-MS using a calibration curve with an internal standard. CA content in the exuded hemolymph resulted highly variable among species (from ~ 100 ng/mg in Cerocoma schreberi to ~2800 ng/mg in Mylabris fabricii) and typically less concentrated than in insect bodies. Interestingly, CA content in insect tissues greatly varied among individuals collected from a single locality (e.g. Lydus trimaculatus and Mylabris variabilis), with, apparently, no correlation with sex. The implications of our findings are discussed and future perspective to outline the chemical diversity and ecological role of CA and related-compounds are presented.