Achromatic Cues Are Important for Flower Visibility to Hawkmoths and Other Insects

Casper J Van Der Kooi,Almut Kelber

doi:10.3389/fevo.2022.819436

Casper J Van Der Kooi, Almut Kelber

Open Access

https://doi.org/10.3389/fevo.2022.819436

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Abstract

Studies on animal colour vision typically focus on the chromatic aspect of colour, which is related to the spectral distribution, and disregard the achromatic aspect, which is related to the intensity (“brightness”) of a stimulus. Although the chromatic component of vision is often most reliable for object recognition because it is fairly context independent, the achromatic component may provide a reliable signal under specific conditions, for example at night when light intensity is low. Here we make a case for the importance of achromatic cues in plant-pollinator signalling, based on experimental data on naïve Deilephila elpenor and Macroglossum stellatarum hawkmoths, optical modelling and synthesising published experiments on bees, flies, butterflies and moths. Our experiments show that in ecologically relevant light levels hawkmoths express a strong preference for brighter stimuli. Published experiments suggest that for flower-visiting bees, butterflies, moths and flies, achromatic cues may be more important for object detection than often considered. Our optical modelling enabled disentangling the contribution of pigments and scattering structures to the flower’s achromatic contrast, and illustrates how flower anatomy and background are important mediating factors. We discuss our findings in the context of the often-assumed dichotomy between detection and discrimination, chromatic versus achromatic vision, and the evolution of floral visual signals.

Highlights

Many animals rely on colour information for numerous tasks, such as finding mates and food or avoiding predators
Colour has a chromatic aspect, which is related to the spectral distribution of the reflected or emitted light, and an achromatic aspect, which is related to the intensity or total reflectance of a stimulus (Kelber et al, 2003; Osorio and Vorobyev, 2005; Kemp et al, 2015)
When flowers are high up a tree or stalk and presented against the sky background, the visual signal for pollinators may be determined by the transmission instead of the reflection, or a combination of both, which may differ in spectral composition and intensity

Summary

INTRODUCTION

Many animals rely on colour information for numerous tasks, such as finding mates and food or avoiding predators (reviewed by Kelber et al, 2003; Cronin et al, 2014; van der Kooi et al, 2021). In behavioural terms, honeybees use achromatic or chromatic contrast depending on the size of and distance to a flower In other insects, this separation may not strictly be the case (e.g., in butterflies, Stewart et al, 2015). Fly eyes have four spectrally different types of photoreceptors that are used for colour vision (Hardie, 1986) This general scheme seems to be universal among Diptera, at least for common flower visitors such as Syrphids (reviewed by van der Kooi et al, 2021). The same may apply to other insects such as moths and bees These three historical facets of studies on insect colour vision have contributed to our lack of understanding of the use of achromatic cues by flower visitors.

CONCLUDING REMARKS

Findings

DATA AVAILABILITY STATEMENT