Proximate factors and potential benefits influencing selection of Psychotria suterella for shelter by the harvestman Jussara spec

Harvestmen (Arachnida, Opiliones) are arachnids that rely on chemicals for communication and are particularly dependent on high humidity. The harvestman Jussara spec. (Sclerosomatidae) clearly prefers to rest on the stem and leaves of Psychotria suterella Muell. Arg. (Rubiaceae), a plant having a complex architecture of overlapping branches. So far, few studies have focused on understanding how harvestmen find their host plant and the benefits associated with the selected plant. Here, we investigated cues harvestmen may exploit to find P. suterella and the potential benefits for harvestmen of this interaction. To address how harvestmen find the plant, we tested two non‐exclusive hypotheses: harvestmen use chemicals from conspecifics, and/or harvestmen use chemicals from plants. For the first hypothesis, we assessed the number of harvestmen choosing P. suterella with or without chemicals of Jussara spec. on the main stem. We did not find evidence that Jussara spec. uses chemicals from conspecifics to select P. suterella, at least in isolation, without any mechanical stimuli such as stem texture or size. For the second hypothesis, we tested harvestman behavior exposed to volatiles from P. suterella, a non‐preferred host plant – Impatiens walleriana Hook.f. (Balsaminaceae) – and a blank treatment, both in a triangular arena and in a Y‐tube olfactometer. We also found no evidence that the harvestmen use plant volatiles to find it. We also tested two non‐exclusive hypotheses regarding the benefits of selecting P. suterella for harvestmen. The first hypothesis is that P. suterella offers a higher humidity than other plants in the micro‐environment surrounding the leaves because of a high leaf transpiration rate. Harvestmen could benefit from this because they rest with the body in contact with the leaf or at less than 5 mm from it. We did not find evidence that leaf transpiration rate is important for Jussara spec., as P. suterella did not present high rates compared to other local species. The second hypothesis is that the leaves of P. suterella provide a better shadow than other plants, acting as a sun shade due to its complex architecture with overlapping leaves. We measured light passage through the leaves of P. suterella and three other local species and found that less light passes through the leaves of P. suterella, which therefore provides darker shadow. This potentially provides a favorable micro‐climate for harvestmen, which might help to explain the micro‐habitat preference of Jussara spec.