Spider dragline silk is of practical interest because of its excellent mechanical properties. However, the structure of this material is still largely unknown. In this article, we report what we believe is a new model of the hierarchical structure of silk based on scanning electron microscope and atomic force microscope images. This hierarchical structure includes beta-sheet, polypeptide chain network, and silk fibril. It turns out that an exceptionally high strength of the spider dragline silk can be obtained by decreasing the size of the crystalline nodes in the polypeptide chain network while increasing the degree of orientation of the crystalline nodes. Based on this understanding, how the reeling speed affects mechanical properties of spider dragline silk can be understood properly. Hopefully, the understanding obtained in this study will shed light on the formation of spider silk, and consequently, on the principles for the design of ultrastrong silk.
No animals are known to possess both ultraviolet (UV) reflectance and fluorescence that are sex-specific. We provide evidence for this separation in the jumping spider Cosmophasis umbratica, which has UV reflectance and UV-induced green fluorescence restricted to males and females, respectively. During courtship, many of the studied pairs failed to show typical display posturing when UV light was blocked. Occluding the UV component of sunlight to only one of each pair also caused atypical behavior: Females showed no interest in non-UV-reflective courting males, and males either ignored or were lackluster in courting nonfluorescing females. These results demonstrate the importance of both sex-specific hues as sexual signals for effective intraspecific communication.
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