Ant-acacia-inhabiting Eustala spiders (Aranaeidae) potentially employ chemical camouflage to avoid ant aggression

Background/Question/Methods

Neotropical ant-acacia mutualisms involve several Acacia species that provide food and shelter for Pseudomyrmex ant species in return for ant protection from plant-feeding animals. We have documented a novel interaction between two closely related orb-weaving spiders (Eustala oblonga and E. illicita) and two ant-acacia systems (A. melanocerus and A. collinsii, respectively) in central Panama in which the spiders exploit the ant-acacia mutualisms for enemy-free space. In these particular systems, the spiders spin their webs among the branches at night, but inhabit the plant surface by day without being attacked by patrolling ants. We hypothesized that the spiders mimic hydrocarbons produced by the ants or the acacias, thus rendering the spiders chemically camouflaged to the ants. In a preliminary test of this hypothesis, individual E. oblonga and E. illicita were swapped between host and not-host acacias in an attempt to present ‘chemically familiar’ and ‘chemically unfamiliar’ spiders to patrolling acacia ants. We recorded the length of time until the translocated spiders were detected and attacked by ants on host and non-host acacias. We also categorized the behavioral response of ants (ignore, antennate, or lunge) that encountered the spiders and recorded the frequency of these behaviors over the course of the trials.

Results/Conclusions

E. oblonga spiders were detected and attacked significantly more rapidly on non-host acacias (A. collinsii) than they were on their host acacias (A. melanocerus). Further, E. oblonga spiders were lunged at significantly more often by acacia ants on non-host acacias (A. collinsii) than they were on their host acacias (A. melanocerus). In contrast, there was no difference in acacia ant response to E. illicita spiders on host acacias (A. collinsii) versus non-host acacias (A. melanocerus). The results of these preliminary behavioral assays partially support the chemical camouflage hypothesis and are being followed up in 2011 with additional behavioral assays as well as chemical assays comparing acacia plant volatiles, and ant and spider cuticular hydrocarbons.