Conical flower cells reduce surface gloss and improve colour signal integrity for free-flying bumblebees

Abstract

Colour signals of flowers facilitate detection, spontaneous preference, discrimination and flower constancy by important bee pollinators. At short distances bees orient to floral colour patterns to find a landing platform and collect nutrition, potentially improving the plants’ reproductive success when multiple flowers are visited sequentially. In addition to pigments and backscattering structures within the petals’ internal layers, the epidermal micro-structure of the petals’ surface may also influence petal reflectance properties and thus influence overall colour patterns via optical effects. Gloss, i.e., shine caused by specular reflections of incident light from smooth surfaces, may for example alter the visual appearance of surfaces including flowers. We classify the epidermal surface properties of petals from 39 species of flowering plants from 19 families by means of a cell shape index, and measure the respective surface spectral reflectance from different angles. The spontaneous behavioural preferences of free flying bumblebees (Bombus terrestris) for surfaces with different micro-textures was then tested using specially prepared casts of selected flower petals. We specifically tested how the petal colour as function of the angle of incident light, surface structure and bee approach angle influences bumblebees’ spontaneous choices for artificial flowers. We observe that bumblebees spontaneously prefer artificial flowers with conical-papillate micro-structures under both multidirectional illumination and under spotlight conditions if approaching against the direction of spotlight, suggesting conical cells help promote constant signals by removing gloss that may confound the integrity of colour signalling.

Why are the inner and outer sides of many flower petals differently coloured?

“…When floral pigment is concentrated in the epidermal layer at the side of viewing, the visual signal is more modulated and hence the flower more conspicuous than when pigment is distributed throughout the flower or in the epidermal layer on the other side (see figure 4 in van der Kooi et al 2016). In addition, the (surface) structure of the epidermal layer determines the degree of glossiness of a flower, with very flat and smooth surfaces creating a glossy (shiny) appearance, and cone-shaped epidermal cells a more matte appearance (van der Kooi et al 2014;Stavenga et al 2020;Wilmsen et al 2021). A comparative study covering 46 species in nine plant families revealed that, in general, the adaxial side of flowers is significantly more structured than the abaxial side (Kraaij & van der Kooi 2020).…”

“…2020; Wilmsen et al . 2021). A comparative study covering 46 species in nine plant families revealed that, in general, the adaxial side of flowers is significantly more structured than the abaxial side (Kraaij & van der Kooi 2020).…”