J Comp Physiol A (1994) 175:363-369 9 Springer-Verlag 1994 U. Brtinnert 9 A. Kelber 9 J. Zeil Ground-nesting bees determine the location of their nest relative to a landmark by other than angular size cues Accepted: 11 March 1994 Abstract Bees and wasps acquire a visual representa- tion of their nest's environment and use it to locate their nest when they return from foraging trips. This represen- tation contains among other features cues to the distance of near-by landmarks, We worked with two species of ground-nesting bees, Lasioglossum malachurum (Hymen- optera: Halictidae), Dasypoda hirtipes (Hymenoptera: Melittidae) and asked which cues to landmark distance they use during homing. Bees learned to associate a single cylindrical landmark with their nest's location. We subsequently tested returning bees with landmarks of different sizes and thus introduced large discrepancies between the angular size of the landmark as seen from the nest during training and its distance from the nest. The bees' search behaviour and their choice of dummy nest entrances show that both species of ground-nesting bees consistently search for their nest at the learned di- stance from landmarks. The influence of the apparent size of landmarks on the bees' search and choice behav- iour is comparatively weak. We suggest that the bees ex- ploit cues derived from the apparent speed of the land- mark's image at their retina for distance evaluation. Key words Hymenoptera 9 Bees 9 Homing Landmark guidance - Distance perception Introduction Bees and wasps use a visual representation of a goal's environment, be it a nest or a food place, to guide their approach to the goal. This representation contains picto- rial cues such as the apparent size, the shape, the texture and the colour of landmarks (for a review see Collett 1992). Shape, texture and colour can serve to identify a U. Br0nnert - A. Kelber ( ~ ) 9 J. Zeil ~ Lehrstuhl for Biokybernetik, Universit~itTiJbingen, Auf der Morgenstelle 28, D-72076 T0bingen, Germany Present address: Kuwait University, Faculty of Science, Department of Zoology, P.O. Box 5969 Safat, 13060 Kuwait landmark while in a world of rigid objects angular size is a robust cue to spatial relations like the distance of a landmark relative to the goal. There is growing evidence, however, that honeybees and solitary wasps acquire and use in addition size-independent information about the distance of landmarks at least under certain circum- stances and possibly only transiently (Lehrer and Collett 1994; Zeil 1993b). The acquisition of size-independent distance information seems to be closely linked to the orientation flights animals perform on departure from the nest or a newly discovered food place (Lehrer 1993; Lehrer and Collett 1994; Zeil 1993 a,b). We studied landmark guidance during homing in ground-nesting bees by confronting them with large discre- pancies of the apparent landmark size seen during train- ing and that encountered during tests (see also Cart- wright and Collett 1979, 1983; Lehrer and Collett 1994; Wehner and Raber 1979; Zeil 1993b). Our aim was to test whether ground-nesting bees are guided by the appa- rent size of the landmark or by other cues indicating the distance between landmark and nest. If bees learn to lo- cate their nest by moving to a position where the appa- rent size of a landmark matches that seen during train- ing, returning bees when confronted with landmarks of different sizes should search for the nest closer to a smal- ler and further away from a larger landmark. Methods Lasioglossum malachurum (Hymenoptera: Halictidae)is a quite common primitively eusocial species. In March females dig indi- vidual nests in large aggregations in clay soils and provision the brood with pollen and honey, About 3-10 worker females of the summer generation emerge in June and collect food for the next generation of fertile females and males which emerges in Septem- ber. The bees perform orientation and re-orientation flights (Zeil and Kelber 1991; see also Wcislo 1992) that are organised similar- ly to those of sphecid wasps (Zeil 1993a). The experiments described in this study were carried out on bees from a large nest aggregation along a clay path in the vicinity of T0bingen in March and June to July. The summer generation bees were marked individually by a spot of colour on the thorax. The ground around the nest was covered by a light brown sheet of