From keel to inverted keel flowers: functional morphology of “upside down” papilionoid flowers and the behavior of their bee visitors

Abstract

Most Papilionoideae species exhibit specialized flowers with the standard petal placed in the upper position and the keel and surrounding wings acting as the landing platform for bee visitors. For some groups, however, flowers became resupinate so that bees land at the standard petal. In this upside-down morphology, the pollen is deposited onto the dorsal side of visitors, instead of the ventral side as in most keel flowers. To understand how these inverted keel flowers work and their differences from normal keel flowers, we studied the resupinate species Canavalia brasiliensis, Centrosema pascuorum and Clitoria fairchildiana, and compared with the normal keel Lathyrus pubescens, Crotalaria micans and Dioclea violacea. Both the behavior and morphology of bee visitors were observed and compared to the flower morphology. We observed that most features of the mechanism exhibited by inverted keel flower are found at keel flowers, except for three key innovations: a thickening at wings’ margin in contact with the visitors’ body, tighter interlocking between petals and strengthening of the vexillum structure. Most visitors are adapted to flower characteristics in their morphology and behavior, although some act as thieves due to some of their morphological features, such as mouthpart length. In spite of their differences, bees behave almost in the same way when visiting both normal and inverted keel morphologies. Compared to keel flowers, inverted keel flowers evolved toward more restrictive structures that require large-bodied bees to operate them. Finally, we discuss possible selective pressures that might have led to the evolution of inverted keel flowers.