Differences facilitating the coexistence of two sympatric, orb-web spiders, Argiope aurantia Lucas and Argiope trifasciata (Forskal) (Araneidae, Araneae)

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Two closely-related, sympatric, orb-web spiders, Argiope aurantia and Argiope trifasciata, take prey which differ in size. In accordance with the often-assumed direct relationship between the size of predators and their prey, A. aurantia is larger than A. trifasciata at any single point in time, largely a result of their asynchronous reproductive cycles. The ratio of their sizes exceeds the 1.28 proposed by Hutchinson (1959) for coexistence. Vertical and horizontal differences in their use of the microhabitat also occur and may further reduce the overlap in their use of food or reduce the frequency of interspecific interactions. In this study, vertical stratification of webs occurred only late in the season, with A. aurantia higher than A. trifasciata. These results contrast with those of Enders (1974), probably due to different densities of the two species in our study areas. Experiments show that the differences in the size of prey taken by these Argiope spiders were due, in part, to dissimilarities in the filtering properties of their webs and to differences in the ability of the two species to capture prey of the same relative size. However, the differences were mainly due to the spiders' rejection of a large and different portion of the available prey.