Foraging desert ants, Cataglyphisfortis, continually keep track of their own positions relative to homei.e., integrate their tortuous outbound routes and return home along straight (inbound) routes. By experimentally manipulating the ants' outbound trajectories we show that the ants solve this path integration problem not by performing a true vector summation (as a human navigator does) but by employing a computationally simple approximation. This approximation is characterized by small, but systematic, navigational errors that helped us elucidate the ant's way of computing its mean home vector.

On leaving the nest [1-9] or a newly discovered food site [10-12] for the first time, bees and wasps perform elaborate flight maneuvers to learn the location of their goal and the lay of the land surrounding it. In all these orientation flights the insects turn back and look [13] at the goal, which they can visually locate by landmark cues directly defining the goal. Here we show that Namibian desert ants, Ocymyrmex, when learning new landmarks in the neighborhood of the goal, acquire this landmark information when they cannot see the goal. They do so by performing well-choreographed rotation movements integrated in spiral-like "learning walks." Within these rotations, short (about 150 ms) stopping phases occur, during which the ants orient themselves in the direction of the nest entrance. On the barren sand surface the nest entrance is invisible, so the ants can aim at it only by reading out the current state of their path integrator [14-17]. Hence, they could associate "snapshot" views [18-20] taken of the nest surroundings during the stopping phases with path integration coordinates. In bees and ants such associations have often been discussed, but evidence has not been obtained yet [15, 20-22].

As textbook knowledge has it, bees and ants use polarized skylight as a backup cue whenever the main compass cue, the sun, is obscured by clouds. Here we show, by employing a unique experimental paradigm, that the celestial compass system of desert ants, Cataglyphis, relies predominantly on polarized skylight. If ants experience only parts of the polarization pattern during training but the full pattern in a subsequent test situation, they systematically deviate from their true homeward courses, with the systematics depending on what parts of the skylight patterns have been presented during training. This ''signature'' of the polarization compass remains unaltered, even if the ants can simultaneously experience the sun, which, if presented alone, enables the ants to select their true homeward courses. Information provided by direct sunlight and polarized skylight is picked up by different parts of the ant's compound eyes and is channeled into two rather separate systems of navigation.insect vision ͉ polarization compass ͉ sun compass ͉ Cataglyphis B y performing his famous mirror experiment in 1911, Santschi (1) was the first to show that animals (ants, in his case) could use the sun as a compass cue. Moreover, he observed that the apparent position of the sun did not always deflect the ants from their homeward courses. In species of some genera of ants (e.g., in Messor and Monomorium), the mirror experiment worked almost always, but in others (e.g., in Cataglyphis), it worked only under certain conditions (for instance, if the remainder of the sky was experimentally obscured). Santschi (2) concluded from these experiments that at least some species of ants had to be able to derive compass information from some optical property of the sun-free sky. We now know that this property is the polarization of scattered skylight (ref. 3; for reviews, see refs. 4 and 5).A substantial amount of research has been done on the behavioral neurobiology of the insect's polarization compass (6, 7), but very few data are available about the relative significance and differential processing of compass information derived from either polarized skylight or direct sunlight. This question was addressed by Frisch (8) in his early work on honey bees, but the results he obtained from bees, which performed their recruitment dances with mirrored positions of the sun, remained ambiguous. Because more recent work tells us that (i) bees and ants process information about direct sunlight and polarized skylight by employing different sensory channels [the latter by using the dorsal rim area (DRA) of the eye (9-14)] and (ii) under particular experimental conditions, bees and ants exhibit systematic navigational errors if presented with restricted parts of the polarization patterns in the sky (10,13,15,16), unique experimental paradigms can be designed that allow us to approach the problem of the relative importance of the two compass cues more directly and in more detail. In the present account, we let desert ants, Cataglyphis fortis, experience ...

The main navigational mechanism used by foraging desert ants of the genus Cataglyphis is path integration (dead reckoning). Any such egocentric system of navigation is prone to cumulative navigational errors.Hence, while homing Cataglyphis might have reset its path integration system and yet not arrived at the start of its foraging excursion, the nest entrance. Then it resorts to piloting or performs a systematic search for the nest. The search pattern consists of a system of loops of everincreasing size centred about the origin, i.e. the start of the search. Here we show that underlying the system of loops is a spiral search programme that gets transformed into the observed pattern of loops by the ant's idiosyncratic path-integration algorithm. The ant starts to follow a spiral course, then breaks off this course and walks towards the centre, i.e. to what its path-integration system has computed to be the origin of the search. This reset episode is followed by another spiral course, which is terminated by the next reset, and so forth. After each reset, the spiral gets wider, so that the whole pattern expands. Futhermore, every now and then the spiral might change its sign. Computer simulations based on these simple rules lead to search patterns of the kind actually recorded in Cataglyphis ants. These patterns ensure that those parts of the area in which the target (nest entrance) is most likely to be located are searched most heavily; in other words: the search density profile is adapted to the probability density function of the target.

While integrating their foraging and homing paths, desert ants, Cataglyphis fortis, depend on external compass cues. Whereas recent research in bees and ants has focused nearly exclusively on the polarization compass, two other compass systems-the sun compass and the wind (anemo) compass-as well as the mutual interactions of all these compass systems have received little attention. In this study, we show that of the two visual compass systems, it is only the polarization compass that invariably outcompetes the wind compass, while the sun compass does so only under certain conditions. If the ants are experimentally deprived of their polarization compass system, but have access simultaneously to both their sun compass and their wind compass, they steer intermediate courses. The intermediate courses shift the more towards the wind compass course, the higher the elevation of the sun is in the sky.