Species composition of arboreal spiders well responds to the environment, but it is unclear how composition of the spider community determines their prey. This study focused on the compositional changes of prey for spiders through succession and intended to examine dietary shift of the spiders along the environmental gradients.

We took two variables to relate spider’s characters to dietary composition. Feeding guilds (FGs), which categorize way of foraging, orient the spiders to catch particular behavior of the prey, whereas body length affects optimum size of the prey. For four FGs with various body lengths in early to late successional forests, we measured C, N stable isotope ratios to estimate relative contributions of the prey.

Prey samples showed 1.Diptera from detrital food web is subsidized to the spiders, 2.Diptera was dominant among aerial insects, which was highly available in old growth forest, 3.Diptera was not the smallest but relatively small prey. Therefore, we hypothesized that 1.Contribution of Diptera is higher in the spider’s diets in old growth forests, 2.Variations of the contributions within a forest among individual spiders depend on the FG and the body length of spiders, 3.The contribution in every FG is unimodal to spider body length. We also discuss whether spider species that are dominant in old growth forest are well adapted to Diptera.