Abstract
The cuticle of certain insect body parts can be hardened by the addition of metals, and because niche separation may require morphological adaptations, inclusion of such metals may be linked to life history traits. Here, we analysed the distribution and enrichment of metals in the mandibles and ovipositors of a large family of gall-inducing wasps (Cynipidae, or Gall-Wasps) (plus one gall-inducing Chalcidoidea), and their associated wasps (gall-parasitoids and gall-inquilines) (Cynipidae, Chalcidoidea and Ichneumonoidea). Both plant types/organs where galls are induced, as well as galls themselves, vary considerably in hardness, thus making this group of wasps an ideal model to test if substrate hardness can predict metal enrichment. Non-galler, parasitic Cynipoidea attacking unconcealed hosts were used as ecological “outgroup”. With varying occurrence and concentration, Zn, Mn and Cu were detected in mandibles and ovipositors of the studied species. Zn tends be exclusively concentrated at the distal parts of the organs, while Mn and Cu showed a linear increase from the proximal to the distal parts of the organs. In general, we found that most of species having metal-enriched ovipositors (independently of metal type and concentration) were gall-invaders. Among gall-inducers, metals in the ovipositors were more likely to be found in species inducing galls in woody plants. Overall, a clear positive effect of substrate hardness on metal concentration was detected for all the three metals. Phylogenetic relationships among species, as suggested by the most recent estimates, seemed to have a weak role in explaining metal variation. On the other hand, no relationships were found between substrate hardness or gall-association type and concentration of metals in mandibles. We suggest that ecological pressures related to oviposition were sufficiently strong to drive changes in ovipositor elemental structure in these gall-associated Hymenoptera.
Highlights
The stiffness, hardness and thickness of arthropod cuticle is extremely variable [1], [2], and in certain species and body parts can be reinforced by the addition of Zn, Mn or other elements [3],[4]
The abundant Zn is even visible from Scanning Electron Microscopy (SEM) images of mandibles, in which a clearly whiter area is recognizable at the outer margins of the teeth (Fig. 2)
This seems to be true for the ovipositor only, since no links between life-history traits and variability of metal inclusion were observed in the mandibles of our sample
Summary
The stiffness, hardness and thickness of arthropod cuticle is extremely variable [1], [2], and in certain species and body parts can be reinforced by the addition of Zn, Mn or other elements [3],[4]. The stiffness, hardness and thickness of arthropod cuticle is extremely variable [1], [2], and in certain species and body parts can be reinforced by the addition of Zn, Mn or other elements [3],. Metals and halogens have been found in the mandibles, chelicerae, stings, pedipalps, forcipules, leg claws and ovipositors, and typically at prone-to-wear cutting edges of these organs [3],. [5], [6], [7], [8], [9] The inclusion of such elements can greatly improve cuticle hardness. For other metals, such as Mn, quite common but found in minor concentrations, it is still not clear the effect on cuticle mechanical properties [11].
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