Stable isotope analysis reveals the importance of plant-based diets for tropical ant-mimicking spiders

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.