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Keywords:

  • Acacia ant;
  • Acacia melanocerus;
  • anti-predator behaviour;
  • Eustala oblonga;
  • exploitation;
  • mutualism;
  • myrmecophily;
  • Pseudomyrmex satanicus

Abstract

  1. Myrmecophily is uncommon in spiders and adaptations that allow spider infiltration of ant colonies are poorly studied. Here, a novel interaction between the orb-weaver spider, Eustala oblonga Chickering, and the acacia ant, Pseudomyrmex satanicus Wheeler, in central Panama is documented.

  2. These spiders occupy webs at night, but spend most of the day crouched directly against the surface of their host acacias (Acacia melanocerus Fabaceae) amidst the plant-defending ants. Detailed behavioural observations indicated that the spiders generally occupied areas on the acacias patrolled more actively by ants, but were attacked only if the spiders moved, which happened very infrequently. We hypothesized, therefore, that the spiders avoid ant aggression behaviourally by being still and not reacting to encounters by patrolling ants.

  3. We tested this hypothesis experimentally by comparing ant responses to moving versus immobilised E. oblonga and moving versus immobilised individuals of another plant-inhabiting, orb-weaver spider (Argiope argentata Fabricius) not naturally found on ant-acacias. Consistent with the hypothesis, ants responded significantly more aggressively to moving spiders of both species than to immobilised spiders.

  4. Further, moving E. oblonga utilised a particular method of escape in which they suspended themselves on a dragline until ant activity waned before returning to the plant surface and crouching quietly without further agitating the ants. In contrast, moving A. argentata attempted to outrun the ants, thus, continuing to agitate them until the spiders were killed or dropped to the ground.

  5. Our results suggest that E. oblonga may be able to inhabit ant-defended acacias essentially by hiding in plain sight.