Visual detection of diminutive floral guides in the bumblebee Bombus terrestris and in the honeybee Apis mellifera

Many flowers display colour patterns comprising a large peripheral colour area that serves to attract flower visitors from some distance, and a small central, contrastingly coloured area made up by stamens or floral guides. In this study, we scaled down the size of floral guides to detect the minimal size bumblebees (Bombus terrestris) and honeybees (Apis mellifera) require for guidance. We analyzed the approach and the precise contact of the antennal tips with the floral guide of artificial flowers which precedes landing and inspection. Both bumblebees and honeybees were able to make antennal contact with circular floral guides which were 2 mm in diameter; bumblebees performed better than honeybees and antennated also at floral guides smaller than 2 mm. In discrimination experiments with bumblebees, a minimum floral guide size of 2 mm was required for discrimination between artificial flowers with and without floral guides. With increasing experience bumblebees targeted close to the site of reward instead of making antennal contact with the floral guide, whereas honeybees did not alter their initial behaviour with growing experience. Bumblebees and honeybees spontaneously target diminutive floral guides to achieve physical contact with flowers by means of their antennae which helps them to inspect flowers.