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Harvestmen have a pair of scent glands that open through ozopores. The literature suggests a link between the morphology of the ozopore area and the emission of a defensive secretion. A previous study on a species that aggregates in open areas, where individuals are probably more easily spotted by predators, showed that this defensive secretion causes conspecifics to flee. However, it is unknown whether this behavior occurs in species that aggregate in sheltered areas, where prey are harder to find. Herein, we describe the morphology of the ozopore area, the mode of emission of the defensive secretion, and its chemical composition in the harvestman Discocyrtus pectinifemur. We also tested if the defensive secretion is used as an alarm pheromone. We found that D. pectinifemur releases the defensive secretion in different ways, one of them being as a jet. Emission as a jet contrasts with that known for all congeners previously studied, and is in accord with the expected morphology of the ozopore. We found that the defensive secretion of D. pectinifemur does not function as an alarm pheromone. The composition of the defensive secretion, a mixture of quinones, is congruent with those already described for the clade that includes Discocyrtus. Our results support the link between the morphology of the scent glands area and the emission behavior of the defensive secretion, and they suggest that the alarm pheromone function in harvestmen may be dependent on ecological factors.

The threat sensitive hypothesis predicts that animals modulate the defensive behaviour with the level of threat. Therefore, responses to predator cues may differ from responses to the actual predator in close range. Also, in high threat situations, prey would be expected to use their most dangerous defences. The recluse spider Loxosceles gaucho (Araneae, Sicariidae) is known to prey upon well defended harvestmen such as the laniatorid Mischonyx cuspidatus (Opiliones, Gonyleptidae), which has been reported to use tanathosis, chemical defences, pinching with sharp apophyses on legs, chelicerae and pedipalps. Because of harvestmen’s dependence on chemical stimuli, we tested if M. cuspidatus would change its locomotory behaviour in the presence of chemicals of the recluse spider (low threat situation: spider vs blank vs chemical control; one at a time). Subsequently, we tested harvestmen behaviour in the presence of the spider in close range, a high-threat situation. Finally, we looked at the survival rate of spiders after being pierced by sharp apophyses that M. cuspidatus have on legs IV. The harvestmen only showed defensive behaviours in the high threat situation. Surprisingly, their mostly known defensive behaviours (chemical defence, tanathosis, pinching with chelicerae and pedipalps) were not seen even in the high threat situation. This is the first evidence that these behaviours are not used against a natural predator that has an almost 80% predation success when attacking harvestmen. Pinching with the sharp legs IV apophyses may perforate but do not kill the spiders. We highlight the importance of the traditional descriptive approach with natural predators to understand the specificities of defensive behaviours against different types of predator.

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