Colour-assisted variation in elytral ICP-OES-based ionomics in an aposematic beetle

Very little is known about how the elemental composition (ionome) of an insect cuticle varies as a result of different colouration. Using inductively-coupled plasma optical emission spectrometry (ICP-OES), we established ionomic profiles in microsamples of two adjacent regions of an insect cuticle with a contrasting colour pattern, namely, the black and orange regions of the elytra of the aposematic burying beetle Nicrophorus vespillo. The analysis revealed 53 elements (ranging in atomic weight from Na to Bi) occurring above the detection limit. The frequency of detectability of individual elements varied strongly, and only ten elements (Ba, Cu, Fe, K, Mg, Mn, P, Rb, Sb and Zn) were present in concentrations exceeding the detection limit in all the samples. The sum of concentrations of all elements in the orange regions of the elytra was 9% lower than in the black ones. The opposite distribution was displayed by the rare earth elements (REEs), the sum of which was 17% lower in the black elytral regions than in the orange ones. The concentrations of six elements were significantly higher in the black than in the orange regions: Al (by 97%), Cu (41%), Mn (14%), Na (46%), Se (97%) and W (47%). The concentrations of essential elements measured in both the black and orange regions exhibited very considerable variance: Ca (σ(2) = 1834; 1882, respectively), K (145; 82) P (97; 76), Na (84; 53), Mg (24; 26) and Ba (9; 13). This, in part, could be attributed to individual differences, e.g. those resulting from the consumption of animal carcasses of different quality/chemical composition, but interference between elements and the consequent lowering of measurement quality are also possible. We highlight the fact that deeper insight into the basic relationship between insect colouration and variation in elemental composition requires micro-sampling of the homogeneous layers of an exoskeleton.