Evolution of spiders from nocturnal to diurnal gave spider silks mechanical resistance against UV irradiation

During their evolutionary history, some species of spiders have changed from a nocturnal to a diurnal lifestyle, and the important change in their environment was irradiation by sunlight. Orb webs of diurnal spiders may be markedly affected by exposure to ultraviolet (UV) irradiation, whereas those of nocturnal spiders may be unaffected. It is of great interest to consider the effects of UV rays on the mechanical properties of spider silks from the viewpoint of evolution. The webs of orb-weaving spiders must trap insect prey, which closely relates to the breaking energy of the spider silk constituting the orb webs. The effects of UV rays on the mechanical properties, particularly the mechanical breaking energy, of the silks of diurnal and nocturnal spiders were investigated. UV rays mechanically strengthened the draglines of diurnal Nephila clavata and two other kinds of diurnal spiders ( Leucauge blanda and Argiope bruennichii ), suggesting that the ability of the orb web to capture insects requires less maintenance. However, UV rays mechanically weakened the draglines of nocturnal Yaginumia sia and one other kind of nocturnal spider ( Neosona nautica ), suggesting a decrease in the ability of the orb web to capture insects. The results provide strong support that diurnal spiders are in a more evolved stage than nocturnal spiders, so they secrete silks with an evolved mechanical resistance against UV irradiation. This study suggests a means to search for biological materials with resistance to UV rays. Time dependencies of the mechanical breaking stress (BS) of draglines secreted by a diurnal Nephila clavata spider and a nocturnal Yaginumia sia spider. The draglines were irradiated with UV-A* rays, which were prepared by cutting wavelengths shorter than 320 nm from a xenon-arc lamp with special filters. ○: N. clavata , •: Y. sia . In this study, the normalized BS is used by dividing the observed values of BS by the initial value before UV-A* irradiation.