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.