Visual cues used in directing predatory strikes by the jumping spider Yllenus arenarius (Araneae, Salticidae)

Abstract

Salticids are known for their complex predatory behaviour, which is based on the analysis of visual information from their prey, but the role of cues used in different predatory tasks is poorly known. We investigated which cues are used to identify the preferred target on the prey's body, examining the reactions of the euryphagous salticid Yllenus arenarius to various virtual prey presented on a miniature screen. We manipulated the number of head-indicating details (ranging from prey with four details, including a head spot, antennae, legs and wings, to prey lacking any details), the position of these details in relation to motion direction (in the leading versus in the trailing part of the body), the local motion of legs and the presence of horizontal motion. When all cues pointed to the same body end the spiders identified the preferred target almost unerringly regardless of the number of details. Movement alone, movement combined with a different number of details in the leading part, local motion of legs and head spot alone on motionless prey elicited the same reactions. When the cues provided contradictory information (motion direction and details pointing to opposite body ends) the spiders struck the trailing end more often the more details were placed there, and they visually inspected body ends of their prey before attack. These results indicate that the spiders used the direction of the prey's motion and the complexity of head-indicating details when making decisions related to strike targeting. These findings elucidate the role of motion and the complexity of details forming ‘false heads’, an antipredator adaptation assumed to redirect predatory strikes on prey from various animal groups. We demonstrate that in stationary prey even very simple patterns efficiently redirect predatory strikes. We provide the first experimental evidence of the effectiveness of ‘false head’ complexity in moving prey.