Cone cells let bees get a grip on flowers
15 May 2009, by Sara Coelho
Bees hold on to flowers by attaching their feet to special petal cells that act like velcro, according to new research. The small cone-shaped cells provide grip and allow a bee to stop beating its wings and start collecting nectar.
A bumblebee bee inspects a snapdragon.
Foraging among flowers for nectar is complicated business and bees use every possible trick to make the job easier. They can smell out the best flowers and tell fellow bees where the best spots are.
It's known that bees favour flowers with conical cells on the petals, but the exact function of the cells and the reasons of this preference were still being debated.
Previous research suggested that conical cells are attractive because they focus light on the petal, raising its temperature and making the colours more intense. Plant scientist Dr Beverley Glover, who researches the co-evolution of plants and pollinators at the University of Cambridge, put forward a new hypothesis - that the conical cells are there for grip.
Glover's team set up a series of experiments using snapdragon flowers and their pollinator, the bumblebee. The first step was to establish that bees can indeed recognise different flower textures with their feet. To rule out colour and smell, the team used two types of pure white snapdragons: a normal variety with conical cells and a mutant with flat cells only.
'It's quite remarkable that we got to the 21st century without knowing the function of petal conical cells. Now that we know what they are for, the simplicity of the solution is amazing.'
Dr Beverley Glover
They added a repulsive bitter solution to the flowers with flat cells and sweet nectar to the conical-celled flowers. The bees were then allowed to visit the flowers freely, and quickly learned that the rough texture meant goodies and smooth petals hid a bitter surprise. After 20 visits the majority of the bees took off immediately after landing on the bitter flower without even attempting to drink. The results were similar when the conditions were reversed: sweet drink on flat-celled petals and bitter on flowers with conical cells.
'Since the only difference between the flowers is the shape of petal cells, we can conclude that the bees recognised the different types based on touch alone,' says Glover.
The next step was to look at the function of the cells. Bees can tell flowers apart by smell and sight, so they don't need touch for identification. Since petal texture is something that they find out only after landing, Glover thought that conical cells could provide a better grip on otherwise slippery surfaces.
To test this idea, the team used an odourless resin to mimic snapdragon petals with conical cells and flat cells. When the casts were horizontal, the bees visited the two types equally without preference. But as tilt increased, bees started going for the mould with conical cells. When the resin flowers were vertical, the bees preferred to forage at the conical casts.
Glover filmed the bees as they were approaching the casts to find out what was happening. The videos show the bees struggling to land on the vertical flat-celled petals, with their middle legs slipping continuously over the smooth surface. But when they approach the rough textured casts, the bees managed to grab the conical cells with the tiny tarsal claws of their feet.
The results, published online in Current Biology, show that conical cells provide grip and allow the bee to stop beating its wings and start feeding.
'It's quite remarkable that we got to the 21st century without knowing the function of petal conical cells,' says Glover. 'And now that we know what they are for, the simplicity of the solution is amazing.'
H.M. Whitney, L. Chittka, T.J.A. Bruce, B.J. Glover. 2009. Conical Epidermal Cells Allow Bees to Grip Flowers and Increase Foraging Efficiency. Current Biology. doi:10.1016/j.cub.2009.04.051
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