View-based navigation in insects

specifying the position of their nests and feeding sites and for guiding their path along fixed routes between them (Santschi, 1913; Tinbergen, 1932; Baerends, 1941; Rosengren, 1971; Collett et al., 1992; Wehner et al., 1996). When guided by visual landmarks, insects demonstrate an impressive ability to use long-term visual memories for controlling their movements. Current evidence suggests that landmark memories are stored in the form of views of landmarks seen from defined vantage points (Wehner and Raber, 1979; Cartwright and Collett, 1983; Judd and Collett, 1998). An insect can then return to a site corresponding to such a vantage point by moving until it has regained its stored view. Route guidance, on the face of it, poses a rather different problem in which the insect is not aiming at a site, but sticks faithfully to a narrowly defined path (Santschi, 1913; Baerends, 1941; Collett et al., 1992; Wehner et al., 1996). Little is known about the way in which landmarks help specify such a path. Two recent advances have allowed us to examine the details of ants’ paths as they follow a landmark-defined route and to elucidate a possible mechanism of path guidance. The processes of following a route and finding a place turn out to have several elements in common. The first advance underpinning this study is the finding that two species of ant, Cataglyphis fortis (Collett et al., 2001) and Leptothorax albipennis (Pratt et al., 2001), will follow a path that is defined by an extended landmark, such as a wall, that can be parallel or at an oblique angle to the ant’s path. For Cataglyphis fortis, when the barrier was rotated from the training orientation, the ants’ paths rotated by approximately the same amount. This result implies that ants’ paths can be governed entirely by visual information derived from the wall independently of compass cues and makes it possible to study landmark guidance in isolation, without complications introduced by other navigational mechanisms. We examine here what visual information the wood ant (Formica rufa) obtains from a wall when using it for guidance, how the ant might gain this information, and how this information might serve to correct the ants’ path. By rotating the wall on every trial and placing the start position and a sucrose reward at constant positions relative to the wall, we have forced ants to rely exclusively on the wall for guidance. 2499 The Journal of Experimental Biology 205, 2499–2509 (2002) Printed in Great Britain © The Company of Biologists Limited JEB4229