Metal mouths: Analytical microscopy methods reveal new morphological characters and physiological properties in beetle mandibles

It is well-documented that arthropods incorporate various elements into their cuticle for reinforcement. High concentrations of zinc, manganese, and iron are often found in the cuticle of various arthropod structures. Numerous studies have focused on the types and locations of cuticular metals in particular groups of arthropods. None of these studies, however, investigated the types of metals across a large group of arthropods or examined their correlation with adult habits, largely because these metals cannot be detected with conventional light microscopy techniques. Similarly, the coating of specimens for scanning electron microscopy (SEM) conceals cuticular metals. This study, the first large-scale survey of incorporated metals, was conducted using advanced microscopy techniques. Regions of metal incorporation on the adult mandible were revealed and analyzed for several coleopteran groups. Images were generated utilizing a variable pressure SEM with backscatter detectors on uncoated specimens. Metals were identified using energy dispersive X-Ray spectroscopy (EDS) detectors. Several of the metals detected were not previously known to be incorporated into insect cuticles. The types of cuticular metals incorporated suggest potential correlations with adult food preference. The location and type of cuticular metals are potentially valuable characters for taxonomic diagnoses and phylogenetic studies.