Host specificity and dispersal behavior in acacia orb-web spiders

In the ant-acacia mutualism (Pseudomyrmex – Vachellia), the plant offers food and shelter to the ants in exchange for protection. In Panama, the acacia V. collinsii found in the Pacific side hosts the ant P. spinicola. Interestingly, this ant-plant relationship can be exploited by the spider Eustala illicita. Normally, the ants are aggressive against any organism (plant or animal) that stays on the plant. However, Eustala spiders seem to use some type of mimicry to avoid detection by ants, but it is unknown how they do it, and how spiders identify and locate host plants. Regarding selection of the host plant, laboratory experiments with Y-maze show that spiders prefer the side with the plant over the empty side or another type of plant or an object. Mark and release of Eustala illicita at different distances from the host plant show a higher proportion of spiders returning to the same host acacia or another acacia, as compared to other orb-web spiders that mostly did not return to the acacias. This would indicate that E. illicita spiders actively search for the acacia plants and other orb-web spiders arrive by chance to the plant. Field and laboratory experiments in which we placed transplanted plants in pots with and without ants strongly suggest that spiders prefer plants with ants present. Altogether, our works provides evidence of an orb-web spider that have a strong relationship with a plant, which is a unique system in the world.