Use of a pronotal shield as an extraretinal short-wavelength filter for the dorsal compound eye of an insect.

Adaptations of nocturnal insects' visual systems to dim light environments include increased photon capture and sensitivity. Yet this risks saturation, which is countered by screening pigments limiting the photons reaching the photoreceptors. Male glow-worms must detect the females' green bioluminescence at night to mate. In addition to screening pigments, males possess a differentially pigmented pronotal shield beneath which they can retract their compound eyes. The possible role of the pronotal shield in behavioural regulation of the light reaching their eyes has not, to our knowledge, been investigated. Here, we characterize the spectral filter properties of the pronotal shield and combine this with behavioural assays of males' response to light exposure. We show that the pronotal shield acts as a differential filter transmitting longer wavelengths of light to the compound eyes beneath while blocking shorter wavelengths. Male glow-worms in the dark or in the presence of green light extend their head but rapidly retract it beneath the pronotal shield when exposed to white or UV light, thereby shielding their compound eyes. Thus, our findings suggest the pronotal shield acts as a behaviourally implemented extraretinal optical filter for the compound eye.