Abstract
We report field observations of the cosmopolitan terrestrial isopod species Porcellionides pruinosus both inside multiple nests and following foraging trails of the seed harvester ant Messor ebeninus in the spring of 2022 and 2023. To our knowledge, this is the first observation of P. pruinosus in association with any ant species. Isopods inside the nest either travelled to deeper tunnels or joined foraging trails. The density of isopods along foraging trails was as high as sixty individuals along one 22meter foraging trail. Results from twelve preliminary homing assays revealed that the distance isopods were displaced did not affect whether they returned to the trail, but the farther away they were displaced the longer it took them to return to the trail. Two of the four isopods introduced into a foraging trail of a new nest continued walking with the foreign trail. Isopods were met with minimal aggression from the ants along the trail, and there were no instances of nest guarding observed. The presence of P. pruinosus inside M. ebeninus. nests could be driven by a need for thermal refuge and access to subterranean granaries. The known ability of isopods to cue into ant pheromones is a plausible mechanism for allowing isopods to follow foraging trails. The benefit of trail following behavior needs further investigation. We hypothesize a benefit to walking on a path clear of obstacles, and efficient detection of patchy desert food sources.