The Metal-enriched Head Cuticle of Velvet Ants (Mutillidae): A New Special Feature of the “Indestructible Insects”

One of the most variable traits associated with insect cuticle structure concerns the presence of both transition metals and alkaline earth metals, which are believed, and in few cases were proven, to increase hardness and resistance to wear. Velvet ants (Hymenoptera: Mutillidae) are peculiar aculeate parasitoids of mainly bees and wasps, and are known to possess a very strong cuticle. Here, we analyzed through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) the occurrence and abundance of metals in six species of velvet ants, as well as in five species of other aculeates belonging to families exploited by mutillids as hosts. We found evidence for a different profile of metals in the cuticle of mutillids compared to other aculeates. Overall, velvet ants present Al, Zn, Ca, Mg, and Fe in their cuticle, while the other species lack Fe and, with the exception of ants, also lack Zn. Aluminum, Zn, and Ca generally occur in greater abundance in velvet ants compared to the other species. The halogen Cl also occurs and co-varies with Zn across species. The metal enrichment profile, interestingly, considerably varied with body part. Velvet ants have especially the cuticle of eyes and of the head portion between eyes—but not the cuticle of mesosoma and gaster—richer in Al and Ca compared with the other species, while Zn and Fe were exclusive of mandibles and essentially lacking in non-mutillid aculeates. On the other hand, there was a weak tendency for a greater % of Mg in non-mutillids compared with mutillids. We suggest that velvet ants might take advantage of an overall metal-reinforced cuticle in the anterior body part (head and head structures) during host nest invasion and consequent encounters with the hosts.