Structural flexibility in the optical design of the arthopod cornea

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The arthropod cuticle is a versatile biocomposite made of chitin fibers embedded in a protein matrix. The cuticle also forms the cornea of the arthropod compound eye, a characteristic feature of the phylum that has largely contributed to its incredible evolutionary success. In the compound eye structure each photoreceptor unit (ommatidium) carries its own optics. Arthropod compound eyes are homologous and show a similar general structure. However, in particular the basic dioptric unit underwent considerable evolutionary modifications. Here we propose to study the different structural solutions observed in the cornea cuticle, all of which were successful for the animals in their respective lifestyle. The goal is to improve our understanding of how the corneas optical performance emerges from their shape and the underlying chitin architecture. A large body of previous work has focused on understanding the compound eye optics in relation to the animals habitat, diurnal adaptations, ontogeny and evolutionary adaptations. Much less effort has been taken to understand the fundamental relation between the cuticle structure and chitin fiber organization and the emerging optical properties. In our approach, we have selected three species: a horseshoe crab, a crayfish and an amphipod, and we characterize their structure and composition and monitor the optical and mechanical properties of the cornea exploiting diverse advanced techniques developed within the collaborating groups. In the ommatidia found in the horseshoe crab Limulus polyphemus (Chelicerata) each cornea possesses a cone-like process, made entirely of chitin-based cuticle. In the Mandibulata (myriapods, crustaceans, hexapods), in contrast, this process is replaced by an intracellular crystalline cone and the ommatidial cornea is either lentiform or smooth, as exemplified by the chosen crayfish and amphipod. The cornea has to serve both an optical and a mechanical function leading to adjusted structures between these constraints. We wish to draw correlations between the underlying chitin-structure, the cuticle composition of the cornea and the emerging materials properties in order to draw conclusions relevant for polymer materials design. We shall exploit the diverse expertise of the partners in biological characterization and histology, as well as analytical techniques such as diffuse X ray scattering and microdiffraction, Raman spectroscopy, cryo electron microscopy and more. The significance of this proposal is in the attempt to explore the boundaries of structural flexibility as observed in biology as a source of design strategies for bioinspired material synthesis.