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Although a wide range of vertebrates have been considered in research on numerical competence, little is known about the role of number-related decisions in the predatory strategies of invertebrates. Here, we investigate how numerical competence is expressed in a highly specialized predatory strategy adopted by the small juveniles of Portia africana when practicing communal predation, with the prey being another spider, Oecobius amboseli. Two or more P. africana juveniles sometimes settle by the same oecobiid nest and then share the meal after one individual captures the oecobiid. Experiments were designed to clarify how these predators use number-related cues in conjunction with non-numerical cues when deciding whether to settle at a nest. We used lures (dead spiders positioned in lifelike posture) arranged in a series of 24 different scenes defined by the type, configuration and especially number of lures. On the whole, our findings suggest that P. africana juveniles base settling decisions on the specific number of already settled conspecific juveniles at the nest and express a preference for settling when the number is one instead of zero, two or three. By varying the size of the already settled juveniles and their positions around the nest, we show that factors related to continuous variables and stimulus configuration are unlikely explanations for our findings. [ABSTRACT FROM AUTHOR]

The jumping spider Portia labiata can complete detours in which it must move away from a goal (i.e. prey) before approaching it. This detouring behaviour can be divided into two phases: a scanning phase, during which Portia stays roughly in one spot and examines its environment using its principal eyes, and a locomotory phase, during which Portia performs the detour. Earlier experiments showed that when Portia is initially confined to a small, elevated platform from which it has an unobstructed view of the goal, it plans the initial stage of its detour by scanning the possible route and picking out an unbroken path from start to goal and then aiming at an initial objective along the detour. In the present experiments, I examined the detouring behaviour of P. labiata in an open arena where obstacles provided either indirect access or no access to the goal. Although scanning movements and detour decisions of spiders were similar to those of spiders initially confined to an elevated platform, spiders in the open arena not only scanned at the start but also along the route of the detour. Decision making thus occurred gradually, during both the scanning and the locomotory phases. Taken together, these results and those of previous studies suggest that detouring behaviour in Portia involves a form of vicarious trial and error in which the spider inspects possible routes and selects a series of intermediate goals during both the scanning and locomotory phases of a detour. [Copyright &y& Elsevier]

Investigates the influence of optical cues on predatory behavior used by the jumping spider, Portia fimbriata in Australia. Cryptic stalking against standard virtual lure; Influence of the size of anterior median (AM) eyes on cryptic stalking; Freezing response when faced with lure with large AM eyes.

Investigates the optical cues for prey discernment by the jumping spider Portia fimbriata. Adoption of ordinary stalking by P. fimbriata when lure did not have anterior-median eyes visible; Adoption of cryptic stalking only against intact lures on which the large anterior-median eyes are visible.

Examines scanning and route selection abilities of the jumping spider Portia labiata. Use of three different kinds of detours leading to a lure; Spider's choice of the complete route after visually inspecting two routes; Fixation on one route.