Honeybees might have small brains, but their navigation skills are second to none. Marie Dacke and Mandyam Srinivasan's bees at the Australian National University, Canberra, could find a 4 cm wide feeder 500 m from their hive on many repeat visits, and like many honey bee researchers, Dacke wondered: `how can an animal with such a small brain solve such incredibly complex tasks'? Dacke and Srinivasan specifically wanted to know how bees measure the distance that they travel using their internal `step measurer', or odometer, when they fly three-dimensional trajectories (p. 845). Bees calculate the distance they have flown using `optic flow' which is how quickly, and in which direction, the image of the environment moves across their eyes. When they return to the hive they transmit this information to the other bees through the waggle dance. The duration of dance's waggle phase tells the other bees the distance they have to fly to the food, while the direction of the dance indicates their heading relative to the sun.

To find out how bees estimated distance in three dimensions, the team first trained bees to fly to a feeder inside a 11 cm by 20 cm by 6 m tunnel. They painted the inside of the tunnel with a black and white check pattern to provide optic flow to the bees as they flew along. After training, the team changed the orientation of the tunnel, and measured the bees' waggle dances back at the hive to see how far they thought they flew to reach the feeder. The team found that the bees danced to their nest mates that they had travelled 6 m to reach the feeder, regardless of whether the tunnel was horizontal, vertical, or tilted at an angle of 48°. The bees were signalling the total distance they had travelled, not the relative horizontal and vertical distances. This is unlike another great insect navigator, the desert ant, which measures the horizontal distance it travels even when on bumpy terrain, not the total distance travelled up and over the bumps.

Wondering if changing the direction of the optic flow half way through the bees' journey would change their dances, the team built an `L' shaped tunnel, with a 2 m long vertical section attached to a 4 m horizontal section. Even though the bees changed direction on the way to the feeder, they still danced that they had travelled 6 m. To confirm that optic flow was necessary to estimate distance they painted the 2 m section of the tunnel with vertical stripes, which minimised optic flow. The bees `missed out' this part of the tunnel when estimating distance, signalling that they had flown around 4 m, proving that they do need optic flow to calculate distance.

Finally, to test how reliably the bees were finding food in horizontal and vertical tunnels, the team trained bees to find food in the tunnels, before taking the food away for the experiment. Monitoring the bees' `back and forth' searching behaviour as they tried to find the food again, the team found that bees were equally good at pinpointing where the food should have been in both horizontal and vertical tunnels, showing that the 3-D orientation they have to fly is not important when it comes to calculating distance.

`The biggest implication is the way the bees treat optic flow,' Dacke explains, `the neurones in the bees' brains responsible for the distance calculation seem to be insensitive to the direction of optic flow.' `Ultimately', she says, `we want to understand the neural basis of how the odometer works'.