The lateral eyes of the horseshoe crab, Limulus polyphemus, are the largest compound eyes within recent Arthropoda. The cornea of these eyes contains hundreds of inward projecting elongated cuticular cones and concentrate light onto proximal photoreceptor cells. Although this visual system has been extensively studied before, the precise mechanism allowing vision has remained controversial. Correlating high-resolution quantitative refractive index (RI) mapping and structural analysis, it is demonstrated how gradients of RI in the cornea stem from structural and compositional gradients in the cornea. In particular, these RI variations result from the chitin-protein fibers architecture, heterogeneity in protein composition, and bromine doping, as well as spatial variation in water content resulting from matrix cross-linking on the one hand and cuticle porosity on the other hand. Combining the realistic cornea structure and measured RI gradients with full-wave optical modeling and ray tracing, it is revealed that the light collection mechanism switches from refraction-based graded index (GRIN) optics at normal light incidence to combined GRIN and total internal reflection mechanism at high incident angles. The optical properties of the cornea are governed by different mechanisms at different hierarchical levels, demonstrating the remarkable versatility of arthropod cuticle.

Adolphe Merkle Institute University of Fribourg Chemin des Verdiers 4 Fribourg 1700 Switzerland

B CUBE Center for Molecular Bioengineering Technische Universität Dresden 01307 Dresden Germany

Chemistry and Physics of Materials University of Salzburg Jakob Haringer Str 2a Salzburg 5020 Austria

Department of Biomaterials Max Planck Institute of Colloids and Interfaces 14476 Potsdam Germany

European Synchrotron Radiation Facility 71 avenue des Martyrs CS 40220 Grenoble Cedex 9 38043 France

Humboldt University Berlin Institute of Biology Philippstraße 13 10115 Berlin Germany

Institute for Globally Distributed Open Research and Education Ribeirão Preto 14091 310 Brazil

TELIGHT Libušina třída 21 Brno 623 00 Czech Republic

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