Experimental Evidence of a Mutualistic Association between Two Web-Building Spiders

1. The large sheet- and tangle-webs of the tropical spider Psechrus argentatus provide structural support to two small spiders, the orb-weaver 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 result of lower food intake rates, when individuals of P. undulata were experimentally excluded from the host web. The control spiders may have had higher food intake rates because the webs of P. undulata attract more prey items and/or increase the probability that prey fall into the sheet-web of P. argentatus. These field experiments, together with field observations, indicate that the relationship between P. argentatus and its web guest P. undulata is mutualistic. Mutualistic relationships between two species of spiders have not been previously recorded. 3. The higher growth rate of P. argentatus with P. undulata guests may substantially enhance the reproductive success of the host spider, either through reducing the time taken to form a clutch of eggs, or increasing the clutch size. 4. When P. argentatus was experimentally removed from its web, the numbers of P. undulata and A. fissifrons dropped, and eventually no guests were found in the web complex. Experimental manipulation of the number of each guest species per web and observations of natural populations, revealed no correlation between the numbers of each guest species per host web.