Innate Flower Recognition in Bumblebees (Bombus terrestris,B. lucorum; Apidae): Optical Signals from Stamens as Landing Reaction Releasers

Innate behavioural reactions in bumblebees were observed in flower dummy experiments. In a previous paper, it was shown that the approach of flower-naive, untrained bumblebees toward flower dummies is released by colour signals, i.e. a gradient of spectral purity. The bumblebees are directed toward that part of a flower dummy which exhibits the highest spectral purity (Lunau 1990). In the present work it is shown that the bumblebees, while still in flight, make contact with this part using their antennal tips. The antennae reaction is followed by a landing reaction which is released by olfactory and optical signals. The specific optical signals of the guide area which release the landing reaction were investigated. Since in the flower dummies which were used, the guide had the highest spectral purity, most of the antennae reactions were executed at the guide. The antennae reaction allows for a precise orientation and may involve the stimulation of a specialized area of ommatidia. Minute changes in optical guide parameters could thus be detected. The following features of stamens which most effectively released the landing reaction were: a minimum of 2 theca dummies; an oval shape of theca dummies which face each other along the longitudinal axis (both these characteristics are found in genuine stamens); 4 mm distance between theca dummies (this coincides with the distance between the antennal tips of bumblebees); two equally sized theca dummies each with an area of 7 mm² (this size of theca dummies was preferred over larger or smaller ones; the latter parameter was not changed when the corolla size was varied). Colour parameters are not involved in releasing the landing reaction. Clearly, bumblebees possess an innate releasing mechanism (IRM) that responds to optical signals of anthers which release the landing reaction. The experimental results are in accordance with conclusions drawn from the comparative studies of Osche (1979, 1983a, 1983b) that many flower guides can be interpreted as stamen mimics. Therefore stamens as well as stamen mimics represent a highly standardized signal for the insect pollinators and they are well suited for innate orientation at flowers.