It has been reported that the mechanical properties of spider threads are almost comparable to the best man-made fibers such as aramid fibers. In this paper, the effects of ultraviolet ray (UV) and quasi-acid rain on the mechanical properties of spider silk produced by Nephila clavata, so called Jorougumo, were investigated to elucidate the environmental effect on the fibers. It was shown that (1) acid rain lower than pH 4, which is often the case at present, did not reduce the tensile strength of the drag line much and (2) UV irradiation induced the degradation of the spider thread. These results may be of value in the assessment of the environment of spider inhabitant.

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