Across the Spiderverse: comparative analysis of metals in spiders' mouthparts (Arachnida, Araneae)

Spiders are a group of widespread generalist predators, principally insectivorous. This study aims to compare for the first time the presence of metals on the feeding structures (fangs, cheliceral teeth and serrula) of Araneomorph spiders and exploring how this feature varies in relation with morphological and ecological traits of the species (body length and functional guild). A total of 127 spiders (39 species distributed among 20 families) were analysed. The presence of zinc, manganese, calcium and magnesium was searched through Energy Dispersive X-ray Spectroscopy. Calcium was recorded in all the mouthparts, manganese only in teeth, whereas magnesium was generally absent only in fangs. Zinc was found in all the mouthparts, including the serrula, where its presence is documented here for the first time. The highest zinc and manganese levels were found in fangs and teeth respectively, in accordance with previously published works. However, through Kruskal-Wallis test followed by Bonferroni-adjusted Dunn’s paired comparisons, it was found that only zinc and manganese levels were significantly higher in feeding structures than in not-feeding control body parts. Furthermore, the greatest zinc levels in fangs occur in the family Salticidae. Through General Linear Models, it was found that teeth’s manganese % was positively associated with body length, whereas calcium % was higher in in the serrula and the fangs of free-hunters than in web-makers, because probably the former require a stronger prey holding. In addition, zinc levels in both fangs and serrula did not significantly vary with the ecological traits, likely due to phylogenetic inertia.