Habituation to a predatory stimulus in a harvester (Arachnida, Opiliones).

Several studies have investigated habituation in a defensive context, but few have addressed responses to dangerous stimuli. In such cases, animals should not habituate since this could cost their lives. Here we have stimulated individuals of the harvester Mischonyx squalidus with a predatory stimulus (squeezing with tweezers) in repeated trials within and between days, and measured the occurrence and magnitude of nipping, a defensive behavior. Contrary to our expectations, they did habituate to this stimulus. The probability and magnitude of response declined over trials during each of three days of testing in a typical habituation pattern. During the trials we also observed other defensive behaviors. We discuss our results mainly considering alternative defensive responses. Our data show that we lack information on (1) the role played by the ambiguity of stimuli, (2) the role played by subsequent stimuli and (3) the importance of the array of defensive behaviors of a species in understanding habituation. Although ubiquitous across animals and therefore expected, habituation is described for the first time in the order Opiliones.

A framework for the study of behavior.

Behavior is defined as the expression of the activity of the nervous system. The basic units of behavior are perceptual mechanisms, central mechanisms, and motor mechanisms. These units can be organized into more complex units called behavior systems such as hunger, sex, aggression, fear, etc. Perceptual and central mechanisms include cognitive mechanisms such as ideas, beliefs, memories, intentions, and cognitive modules. Behavior can be analyzed at genetic, physiological, whole organism, and population levels, and the concepts used to analyze behavior should be appropriate to each level. One can ask causal, structural, and functional questions about current behavior, ontogeny, and phylogeny. Causal and functional questions are independent of each other and should not be confused. There has been much confusion and disagreement about the relation between cause and function, and several examples are analyzed. This article is part of a Special Issue entitled: In Honor of Jerry Hogan.

Associative learning in a harvestman (Arachnida, Opiliones).

Associative learning has been demonstrated in many species of invertebrates, but has not been studied in arachnids, except for some spiders and a whip-spider. Herein, we tested the ability of a Neotropical harvestman, Discocyrtus invalidus (Arachnida, Opiliones) to associate a shelter with a chemical stimulus. We used an arena with a white light at the top and two openings on the floor, one giving access to a dark shelter and the other one closed with a mesh. Filter paper with different chemicals (mate or green tea) surrounded both openings. A harvestman (n=37) was released in the arena and its behavior recorded. The procedure was repeated for 14 consecutive days with each individual. We found that harvestmen got faster at finding the refuge, became less exploratory and tended to move toward the open shelter as the days passed. We conclude that the animals learned to associate the chemical stimulus with the shelter.