Ant‐mimicking spiders are a well‐known example of Batesian mimicry. Earlier studies have proposed that plant‐based diets (e.g. extrafloral nectar) are an important food source for ant‐mimicking spiders. However, it is still unclear whether the plant‐based diets are a main food source or supplementary energy source to the spiders. To examine the feeding habits, we measured stable nitrogen and carbon isotope ratios (δ15N and δ13C) of ant‐mimicking spiders (six species of Myrmarachne; one species of Agorius, Salticidae; and two species of Corinnidae) collected from a tropical rain forest in Malaysia and a dry evergreen forest in Thailand. We also investigated the isotopic signatures of several ant species sampled from the two forests for comparison. In general, the ant‐mimicking spiders had relatively low δ15N values, which were comparable to those of the nectar‐feeding and omnivorous ants. The δ15N values of the ant‐mimicking spiders differed significantly among the species: some species (e.g. Myrmarachne sp. H and Myrmarachne sp. 2) showed δ15N values similar to those of the nectar‐feeding ants, whereas the δ15N values of others (e.g. Myrmarachne maxillosa and Myrmarachne malayana) were close to those of the omnivorous ants. The δ13C values of the ant‐mimicking spiders did not differ significantly among the species, but tended to be similar to those of the nectar‐ and honeydew‐feeding ants rather than the predatory ants. These results indicate that plant‐based diets are important for the nutrition of ant‐mimicking spiders and suggest that the importance would differ among the ant‐mimicking spider species.

Complicated three-dimensional webs of silk threads produced by Tetranychus spider mites provide protection from predation by many generalist phytoseiid mite species. Accessibility to prey may be the most significant criterion for successful predation in complicated webs. To assess the protective effects of combination between web size and density, we compared predation on eggs of the two-spotted spider mite, Tetranychus urticae, by a generalist (Euseius sojaensis) and a specialist (Neoseiulus womersleyi) phytoseiid mite in the laboratory under manipulated web size and density. Web sizes negatively affected to the predation. Significant interactions were found between phytoseiid mite species and prey distribution; egg consumption by E. sojaensis was more in uniform distribution, but that by N. womersleyi, in contrast, was larger in contagious distribution. However, the egg consumption by E. sojaensis and N. womersleyi were both mitigated in larger webs. This area size depending mitigation was more effective to the predation by E. sojaensis. Although the mechanism of web size depending mitigation is unknown, web sizes might affect prey searching efficiency of this phytoseiid mite. Consequently, combination between web density and size are likely to affect not only a generalist E. sojaensis but also a specialist N. wormersleyi.