The effect of host web complexity on prey-stealing success in a kleptoparasitic spider mediated by locomotor ability

We examined how the web structure of host spiders affects the locomotor ability of the kleptoparasitic spider . Argyrodes kumadai. We compared two populations of . A. kumadai differing with respect to whether the host used was . Agelena silvatica (Agelenidae) or . Cyrtophora ikomosanensis (Araneidae). The barrier web of . Agelena had a higher thread density than that of . Cyrtophora, which presumably constrains locomotion of . A. kumadai in . Agelena webs. A reciprocal transplant experiment into different webs revealed that, in complex webs, the . A. kumadai population using . Agelena as a host walked faster than the . A. kumadai population using . Cyrtophora; however, no such difference was found in simple webs. This suggested that . A. kumadai using . Agelena are better adapted to the webs of their native host with regard to their locomotor ability than are . A. kumadai using . Cyrtophora. The prey capture success of . A. kumadai, as estimated by the foraging response of the host to prey and the walking speed of . A. kumadai, revealed that the population of . A. kumadai using . Agelena captured larger prey much more frequently than the population using . Cyrtophora. Thus, the greater locomotor ability of . A. kumadai using . Agelena appears to be adaptive for enhancing prey acquisition in . Agelena webs where prey-stealing opportunities are severely limited. We conclude that both physical web architecture and the foraging behaviour of the host are important selective agents on locomotor ability in . Argyrodes.