SMELLS LIKE TROUBLE: HARVESTMAN CHEMORECEPTION AND THE PHEROMONE AVOIDANCE HYPOTHESIS
The pheromone avoidance hypothesis holds that prey animals can detect chemical signals (or pheromones) that are produced by their predators, and that prey should avoid areas where they detect pheromones because they recognize them as a threat of a nearby predator. While a variety of prey animals have been tested under this hypothesis, few behavioral studies have examined how arachnids may exhibit this avoidance behavior, as most studies of arthropods have focused on insects and crustaceans. Harvestmen (Prionostemma spp.), which rely primarily on chemical detection (chemoreception) for navigating their world, and use a variety of defensive behaviors to evade predators, are amenable test subjects for manipulative studies of pheromone avoidance. For this experiment, I sampled harvestmen at the La Selva Biological Station in Costa Rica, where these arachnids are abundant and coexist among a diversity of ant species, which lay down pheromone trails for other ants to follow, and include known harvestmen predators. Through three-way choice trials, in which I allowed a harvestman to move around an enclosure laced with two types of ant pheromone-treated filter paper and an odorless control, I discovered that Costa Rican harvestmen did not significantly avoid paper laced with trail pheromone of the predatory army ant (Eciton burchellii) over the course of several days after the ant pheromones were collected. However, on the first day of the choice trials, harvestmen did avoid army ant trail pheromone more than the trail pheromone of the harmless leafcutter ant (Atta cephalotes), and avoided both pheromones more than the odorless control paper. As harvestman avoidance of Eciton burchellii trail pheromone fell with each subsequent day of choice trials, it appears that these arachnids can identify the chemical cues of predatory ants as dangerous and will take evasive action, but harvestman avoidance responses should be strongest when this ant pheromone is freshest.
Goldberg, Daniel, "SMELLS LIKE TROUBLE: HARVESTMAN CHEMORECEPTION AND THE PHEROMONE AVOIDANCE HYPOTHESIS" (2019). University Research Symposium. 226.