It's easy to measure the distance you have travelled if you walk, but how do flying insects gauge their progress without the aerodynamic equivalent of a stride? According to Megan Eckles from the University of California San Diego, USA, ‘the honeybee odometer relies on optic flow, the perceived movement of images across the retina.’ In other words, they keep track of time and the speed of images moving across the retina to tell them how far they have gone and to control manoeuvres such as landing. However, Eckles explains that optic flow appears to offer honeybees less precise control during ascent and descent, which might pose a challenge for the honeybee's distant relatives, the so-called ‘stingless bees’ that frequent tropical rainforests. ‘These bees live in a complicated forest environment, and routinely search for food at various heights in the canopy’, says Eckles. Teaming up with David Roubik and James Nieh, Eckles wondered whether stingless bees might share the honeybee's navigational strategy for judging how far and high they fly (p. 3155).
Travelling to the Smithsonian Tropical Research Institute's Barro Colorado Island field station in the Republic of Panama, the trio trained Melipona panamica stingless bees to forage from a well-stocked feeder located 1.25 m along a 2-m-long horizontal tunnel lined with 8-cm-wide black and white stripes. Eckels admits that getting the bees to cooperate was the hardest part of this study. ‘There were plenty of days when the rains came early and stayed late, and the bees were understandably reticent about leaving the warmth of their nest to come explore one of our tunnels’, she recalls.
However, once the bees were content to forage, the team took away the feeder and watched the returning insects make repeated 180 deg turns as they tried to locate the missing feeder. Converting the average distance travelled by each insect into the amount that the tunnel image moved across the insect's eye, Eckles and her colleagues found that on average the bees began searching for the feeder roughly 1.29 m along the tunnel. Next the team began playing with the insect's view of the tunnel by making it narrower – which would make the image of the tunnel move faster across the bee's eye, convincing it that it had arrived at its destination earlier – and wider – slowing the image's movement and duping the bee into flying further if it was judging distance by optic flow.
As the team predicted, the bees in the narrower tube stopped short at approximately 0.9 m, whereas the foragers in the wider tube continued on to approximately 1.75 m. The stingless bees, just like their temperate cousins, were using optic flow to judge the horizontal distance that they had travelled, but how were they judging vertical distance?
This time the team turned the tunnel on its end, suspended the feeder 0.75 m from the top and encouraged the bees to ascend to the feeder from an entrance at the bottom. However, when the team removed the feeder on this occasion, they were amazed by the precision of the insects' vertical odometer. The bees repeatedly began searching at a height of 1.2 m in the standard width tunnel, while cutting their search distance by a half in the narrow tunnel and extending it to over 160 cm in the wide tunnel.
Eckles admits that she was surprised by the improvement in the bees' vertical accuracy, saying, ‘We did not expect there to be much difference in the precision of distance measurement versus height’, adding that she is keen to understand more about how stingless bees cope with their visually complex tropical environment.