Spatial Vision in Bees : Behavioural investigation of spatial resolution and contrast sensitivity

Bees use multiple sensory modalities to perform day-to-day tasks. Light allows them to perceive objects and the general features of their surroundings, enabling them to find food and to navigate safely across the landscape. Although both these tasks involve vision, the requirements of the visual system differ depending upon what the bee wants to achieve; locate a flower to feed from or adjust its position in space to support a safe flight. This thesis addresses to what detail and contrast bees can perceive their surroundings under these two different behavioral contexts.The thesis starts with a brief introduction to the fundamental aspects of spatial vision and a review of the methods used to study it in bees. In Paper I, I estimate the behavioral limits of spatial resolution and contrast sensitivity of the object discrimination system of the buff-tailed bumblebee Bombus terrestris. To do this, I trained the bees to discriminate between a rewarded and an unrewarded stimulus grating based on its orientation (horizontal or vertical). Once the bees had learnt this task, I changed grating frequency and contrast up until the point when the bees could no longer tell the differently oriented gratings apart. This defined the limits of spatial resolution and contrast sensitivity for this species for this visual task. The results suggest that buff-tailed bumblebees have a resolution threshold of about a quarter of a grating cycle per degree of visual angle. This is similar to the resolution threshold previously determined for honeybees. The contrast sensitivity of the bumblebees, however, was several times lower than that reported for honeybees. To determine if this difference in contrast sensitivity is species dependent, or possibly context dependent, I performed another three studies on two more bee species in a different behavioral context.In Paper II, III, IV I estimated the spatial resolution and contrast sensitivity underlying the motion detection system in Asian honeybees, Apis cerana, buff-tailed bumblebees and European honeybees, Apis mellifera. To do this, I trained the bees to fly through a tunnel dressed with gratings of different spatial frequency and contrast and analyzed their flight trajectories as a function of the gratings: a bee that could resolve the grating shifted its lateral position away from it, while a bee that could not resolve it flew along the midline of the tunnel. From these results, it became obvious that the motion detection system of honeybees seems to have a twofold decrease in the spatial resolution compared to their object discrimination system. In addition, all three bee species tested have a very high contrast sensitivity underlying their motion detecting system.In summary, the results presented in thesis indicate that the limits of spatial vision in bees depend largely on the behavioral context