Defences of a Neotropical harvestman against different levels of threat by the recluse spider

摘要

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.