Gradients of orientation, composition and hydration of proteins for efficient light collection by the cornea of the horseshoe crab

The lateral eyes of the horseshoe crab, Limulus polyphemus , are the largest compound eyes within recent Arthropoda. While this visual system has been extensively described before, the precise mechanism allowing vision has remained controversial. Correlating quantitative refractive index (RI) mapping and detailed structural analysis, we demonstrate how gradients of RI in the cornea result from the hierarchical organization of chitin-protein fibers, heterogeneity in protein composition and bromine doping, as well as spatial variation in water content. Combining the realistic cornea structure and measured RI gradients with full-wave optical modelling and ray-tracing approaches, we show that the light collection mechanism depends on both refraction-based graded index (GRIN) optics and total internal reflection. The optical properties of the cornea are governed by different mechanisms at different hierarchical levels, demonstrating the remarkable versatility of arthropod cuticle. One-sentence summary Structural hierarchy and protein hydration determine the optical performance of the cornea of L. polyphemus . ### Competing Interest Statement TS and MAKAR are employed by TELIGHT. All other authors declare that they have no competing interests.