The eye of the dragonfly is specialized for tracking and capturing fast flying insect prey. Tracking requires a rapid rate of signaling. A notable feature of the dragonfly eye is the yellow pigment in the upper half of the eye that is absent in the lower half of the eye. What does the yellow pigment do?
Insect photoreceptors have proteins called rhodopsins that exist in two states, rhodopsin and metarhodopsin. A signal is created when a photon of light hits a rhodopsin and converts it to metarhodopsin. The metarhodopsin must be reconverted to rhodopsin by accepting another photon before a new signal can be generated. This would seem to impair vision as only half the photons could be used to create a signal, the other half required for regeneration.
The photorecptors in the top of the dragonfly compound eye are sensitive to short wavelength light and only accept short wavelength, high energy photons. The converted metarhodopsin accepts longer wavelength lower energy photons to reconvert to rhodopsin. Thus, short wavelength “blue” light generates the signal and the yellow light regenerates the rhodopsin.
The short wavelength light is focused by the lens of the dragonfly eye on the receptors. The yellow pigment in the top of the eye scatters the yellow light making yellow photons widely available for rhodopsin regeneration. This leaves the dragonfly “blind” in the yellow part of the light spectrum but increases sensitivity and response time in the important blue range of the light spectrum.
*Mathias F. Wernet, Michael W. Perry & Claude Desplan. 2015. The evolutionary diversity of insect retinal mosaics: common design principles and emerging molecular logic. Trends in Genetics, June 2015, Vol. 31, No. 6.