Cuticle function and homology in polyneopteran head capsules

The insect head capsule is a geometrically complex structure that is responsible for providing mechanical support during the generation of bite forces. The shape of the head is crucial for distributing stresses incurred during bites to prevent undue structural deformation and minimize loss of force while biting. Although the contribution of head capsule shape to biting mechanics has been considered in a handful of taxa, the role of cuticle composite structure in this context remains largely uncharacterized. Here we present evidence of composite-structure differentiation between regions of the head in the cuticle of four Polyneopteran species, as determined using a suite of electron and light microscopy techniques. Additionally, we present a preliminary analysis of nanoindentation data obtained from (1) isolated cuticle samples from individual regions of the head, and (2) entire head-capsules embedded in epoxy. Based on differences in composite-structure and stiffness of the cuticle across the head capsule, we advance the hypothesis that profound functional gradients in the cuticle are essential for the transmittance of bite forces to tough substrates. Finally, we consider the implications of our results for future work involving finite element modeling of cuticular structures in biomechanical studies of insects.