Abstract
The sensitivity of animal photoreceptors to different wavelengths of light strongly influence the perceived visual contrast of objects in the environment. Outside of the human visual wavelength range, ultraviolet sensitivity in many species provides important and behaviourally relevant visual contrast between objects. However, at the opposite end of the spectrum, the potential advantage of red sensitivity remains unclear. We investigated the potential benefit of long wavelength sensitivity by modelling the visual contrast of a wide range of jewel beetle colours against flowers and leaves of their host plants to hypothetical insect visual systems. We find that the presence of a long wavelength sensitive photoreceptor increases estimated colour contrast, particularly of beetles against leaves. Moreover, under our model parameters, a trichromatic visual system with ultraviolet (λmax = 355 nm), short (λmax = 445 nm) and long (λmax = 600 nm) wavelength photoreceptors performed as well as a tetrachromatic visual system, which had an additional medium wavelength photoreceptor (λmax = 530 nm). When we varied λmax for the long wavelength sensitive receptor in a tetrachromatic system, contrast values between beetles, flowers and leaves were all enhanced with increasing λmax from 580 nm to at least 640 nm. These results suggest a potential advantage of red sensitivity in visual discrimination of insect colours against vegetation and highlight the potential adaptive value of long wavelength sensitivity in insects.
Highlights
The sensitivity of animal photoreceptors to different wavelengths of light strongly influence the perceived visual contrast of objects in the environment
To test whether the presence of an long wavelength sensitive (LWS) photoreceptor enhances visual contrast, we created three trichromatic visual systems that varied in their photoreceptor composition, based on common insect photoreceptor combinations [Fig. 1c; USM (UVS, short wavelength sensitive (SWS), medium wavelength sensitive (MWS)); UML (UVS, MWS, LWS); USL (UVS, SWS, LWS)] we identified from van der Kooi et al.[9]
Behavioural experiments confirmed red sensitivity in both species of jewel beetle and possibly far-red sensitivity in C. dimidiata. 96% of jewel beetles were attracted by light at 590 nm and ca. 87% of them still responded to light at 645 nm (C. dimidiata: 100% and 96%; C. flavopicta, 93% and 78%; at 590 nm and 645 nm) (Fig. S1)
Summary
The sensitivity of animal photoreceptors to different wavelengths of light strongly influence the perceived visual contrast of objects in the environment. Use of an A2 rather than A1 chromophore, is one of the main mechanisms of increasing long wavelength sensitivity in vertebrates[23,24], but has not been found in insects The latter two mechanisms shift the absorption spectra of visual pigments to longer wavelengths and do not decrease absolute sensitivity. We modelled the effect of light environment by running the same models using both daylight and civil twilight (when the sun is 0°–6° below the horizon) illumination spectra because these periods differ markedly in the spectral distribution and represent the lowest light level for colour discrimination of most diurnal insects[32] These modelled contrasts provide an indication of how variation in visual sensitivities may influence visual discrimination, actual discrimination for any given organism depends on colour opponent mechanisms and neural processing and can be context d ependent[33]
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