Interação predador-presa entre a aranha Loxosceles gaucho (sicariidae) e o opilião Mischonyx cuspidatus (gonyleptidae)

Abstract

CHAPTER I: Consuming prey involves finding an adequate site to forage, detecting and manipulating it. We investigated these three steps in interactions between the delicate recluse spider Loxosceles gaucho (Araneae) and a heavy bodied and armored harvestman, here represented by the harvestman Mischonyx cuspidatus (Opiliones). The hard integument of such harvestmen had been previously reported to protect them from spiders larger than Loxosceles. We tested the following hypotheses: (1) spiders prefer areas with cues from their prey, (2) vibratory cues are essential information in the predatory process and (3) the web sheet allows handling prey adequately so that the vulnerable spots of the prey can be bitten. To understand exactly how a delicate predator can overcome the defenses of a heavy bodied and well-defended prey, we also quantitatively described the spider´s behavior. To test hypothesis (1), we compared the time spent in areas with harvestmen, crickets and no cues. For hypothesis (2), we compared latency to bite and number of bites in the presence or absence of vibratory cues, and for hypothesis (3) we compared latency to detect, to capture and predation success. All hypotheses were rejected. Loxosceles gaucho seems to be exceptional among spiders by not needing its web, indirect prey chemical cues, or prey´s substrate borne vibrations to hunt it. What seems to enable Loxosceles to prey upon M. cuspidatus is its totally different hunting strategy, compared to previously studied spiders attempting to eat these armored harvestmen. The spider quickly touches the prey with its tarsi, finding and biting several times the weak parts of the prey body, such as joints and distal parts of the legs. With several previous papers reporting spiders rejecting these harvestmen, this is the first that describes how a spider overcomes the defenses of an armored harvestman. CHAPTER II: 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 (medium 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 M. cuspidatus that have on the legs. 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.