Prey attraction as a possible function of the silk decoration of the uloborid spider Octonoba sybotides

Both laboratory experiments and field observations were usedto examine theprey-attraction hypothesis for the function ofthe silk decoration on the orbweb of Octonoba sybotides. Thereflectance spectrum of the decorativesilk showed that thedecorations reflect relatively more ultraviolet (UV)light.Choice experiments were conducted using Drosophilamelanogaster,a common prey species of the spider, to determine whetherwebswith silk decoration attract more flies than undecorated webs.The choiceexperiment showed that webs with silk decorationattract more flies in lightthat includes UV rays. However,flies choose their flight direction randomlyin light withoutUV rays. This suggests that the silk decoration might attractpreyinsects that tend to fly toward UV-reflecting objects. Fieldobservationscomparing the prey capture rate between webs withand without a silkdecoration showed that more prey are caughtin decorated webs. In this study,no difference between thetwo forms of silk decoration, linear and spiral, wasdetectedeither in prey attraction in the choice experiment or in thepreycapture rate in the field observations.