EXPERIMENTAL-EVIDENCE OF A MUTUALISTIC ASSOCIATION BETWEEN 2 WEB-BUILDING SPIDERS

1. The large sheet- and tangle-webs of the tropical spider Psechrus ar gentatus provide structural support to two small spiders, the orb-weav er Philoponella undulata and the kleptoparasite Argyrodes fissifrons. In general, larger host webs have more guests than smaller host webs. 2. The growth rate of P. argentatus was reduced, apparently as a resul t of lower food intake rates, when individuals of P. undulata were exp erimentally excluded from the host web. The control spiders may have h ad higher food intake rates because the webs of P. undulata attract mo re prey items and/or increase the probability that prey fall into the sheet-web of P. argentatus. These field experiments, together with fie ld observations, indicate that the relationship between P. argentatus and its web guest P. undulata is mutualistic. Mutualistic relationship s between two species of spiders have not been previously recorded. 3. The higher growth rate of P. argentatus with P. undulata guests may s ubstantially enhance the reproductive success of the host spider, eith er through reducing the time taken to form a clutch of eggs, or increa sing the clutch size.4. When P. argentatus was experimentally removed from its web, the numbers of P. undulata and A. fissifrons dropped, an d eventually no guests were found in the web complex. Experimental man ipulation of the number of each guest species per web and observations of natural populations, revealed no correlation between the numbers o f each guest species per host web.