Physico-chemical Studies of Spider's Silks Resistant to Ultra-violet Rays

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It is interesting to study the effects of ultraviolet rays upon the physical properties of spider silks because the ultraviolet (UV) rays are usually irradiated to the orb-web outdoors. Especially, it is important to measure the mechanical properties of dragline before and after the UV-irradiation to study the effects of the irradiation.The samples used here were the draglines from Nephila clavata at different stages of growth. The breaking strength and the elastic limit strength were determined from the stress-strain curve of dragline. The fine structures were also observed by electron microscope to determine the diameter of cross section of dragline.The author measured the mechanical properties of dragline at different stages of spider's growth before the UV were not incident. The safety coefficient, which is defined as the ratio of the elastic limit strength to the spider's weight, was determined to be about 2 for mature spiders and to be about 3 for immature spiders.UV-irradiation decreased the breaking strength while it did not change the elastic limit strength even after the UV-irradiation of a long time. It was made clear that the UV-irradiation decomposed the protein molecules constituting dragline and might decrease the molecular weight.The present study shows that the spider silks are stronger to the UV-rays than the silks from silkworm and that they are stronger for juvenile spiders than form mature ones.