Out of the blue: the spectral sensitivity of hummingbird hawkmoths
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The European hummingbird hawkmoth Macroglossum stellatarum is a diurnal nectar forager like the honeybee, and we expect similarities in their sensory ecology. Using behavioural tests and electroretinograms (ERGs), we studied the spectral sensitivity of M. stellatarum. By measuring ERGs in the dark-adapted eye and after adaptation to green light, we determined that M. stellatarum has ultraviolet (UV), blue and green receptors maximally sensitive at 349, 440 and 521 nm, and confirmed that green receptors are most frequent in the retina. To determine the behavioural spectral sensitivity (action spectrum) of foraging moths, we trained animals to associate a disk illuminated with spectral light, with a food reward, and a dark disk with no reward. While the spectral positions of sensitivity maxima found in behavioural tests agree with model predictions based on the ERG data, the sensitivity to blue light was 30 times higher than expected. This is different from the honeybee but similar to earlier findings in the crepuscular hawkmoth Manduca sexta. It may indicate that the action spectrum of foraging hawkmoths does not represent their general sensory capacity. We suggest that the elevated sensitivity to blue light is related to the innate preference of hawkmoths for blue flowers.
KeywordsSpectral sensitivity Macroglossum stellatarum Sphingidae Insect colour vision Action spectrum
We thank Alejandro Trillo for catching wild moths and collecting eggs in Spain and Michael Pfaff and Samantha Goyret for enduring help with breeding Macroglossum stellatarum. We are grateful to Michael Pfaff for help with building the flight cage, to Eric Warrant for sharing electrophysiology equipment, to the Lund Vision Group for inspiring discussions and to two referees for constructive comments. We gratefully acknowledge financial support from the Swedish Research Council (grant 621-2009-5683) and the Wenner-Gren foundation to AK, from CSIC Studentship JAE PRE (2011: ESTCSIC-6715 and 2012: ESTCSIC-7853) to FJT and from the Spanish Ministerio de Ciencia e Innovación/FEDER (project CGL2010-16795) to MARG.
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