Mandyam Srinivasan is an electrical engineer that is intrigued by bees. `I have been fascinated for a long time how a creature with a brain the size of a sesame seed can do all of the things that it does,' says Srinivasan. Over the course of his career, Srinivasan has discovered how insects negotiate the world by analysing how images of the surroundings move across the eye rather than using stereovision. Having discovered how bees use this `optic flow' information to negotiate the environment, regulate their flight speed and control their approach during landing, Srinivasan began wondering what happens in the final moments of a touchdown. Flies landing on a ceiling simply grab hold with their front legs and somersault up as they zip along, but Srinivasan knew that a bee's approach is more sedate. Curious to know more about bee landing techniques, Srinivasan teamed up with Carla Evangelista, Peter Kraft, Marie Dacke and Judith Reinhard and used a high-speed camera to film the instant of touch-down on surfaces at various inclinations (p. 262).
First, the team built a bee-landing platform that could be inclined at any angle from horizontal to inverted (like a ceiling); they then trained bees to land on it and began filming. Having collected movies of the bees landing on surfaces ranging from 0 deg. to 180 deg., and every 10 deg. inclination between, Evangelista began the painstaking task of manually analysing the bees' landing tactics and saw that the bees' approach could be broken down into three phases.
Initially, the bees approached from almost any direction and at any speed; however, as they got closer to the platforms, they slowed dramatically, almost hovering, until they were 16 mm from the platform, when they ground to a complete halt, hovering for anything ranging from 50 ms to over 140 ms. When the surface was horizontal or inclined slightly, the bees' hind legs were almost within touching distance of the surface, so it was simply a matter of the bee gently lowering itself and grabbing hold with its rear feet.
However, when the insects were landing on surfaces ranging from vertical to inverted `ceilings', their antennae were closest to the surface during the hover phase. When the antennae grazed the surface, this triggered the bees to reach up with the front legs, grasp hold of the surface and then slowly heave their middle and hind legs up too. `We had not expected the antennae to play a role, and the fact that there is a mechanical aspect of this is something that we hadn't thought about,' admits Srinivasan.
Looking at the antennae's positions, the team realised that the bees held them roughly perpendicular to the surface in the final stages when approaching inverted surfaces. `The bee is able to estimate the slope of the surface to orient correctly the antennae, so it is using its visual system,' explains Srinivasan. But this is surprising, because the insects are almost completely stationary while hovering and unable to use image movement across the eye to estimate distances. Srinivasan suspects that the bees could be using stereovision over such a short distance and is keen to test the idea.
Finally the team realised that bees are almost tailor-made to land on surfaces inclined at angles of 60 deg. to the horizontal. `When bees are flying fast their bodies are horizontal, but when they are flying slowly or hovering their abdomen tilts down so that the tips of the legs and antennae lie in a plane that makes an angle of 60 deg.' explains Srinivasan: so the legs and antennae all touch down simultaneously on surfaces inclined at that angle. `It seems like they are adapted to land on surfaces tilted to 60 deg. and we are keen to find out whether many flowers have this natural tilt,' says Srinivasan.