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 bioluminescent glow at night to mate. In addition to screening
pigments, males possess a differentially pigmented pronotal shield (PS)
beneath which they can retract their compound eyes. The possible role of
the PS in behavioural regulation of the light reaching their eyes has not,
to our knowledge, been investigated. Here, we characterise the spectral
filter properties of the PS and combine this with behavioural assays of
males’ response to light exposure. We show that the PS 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 PS when exposed to white and UV light
thereby shielding their compound eyes. Thus, our findings show the PS is a
novel, behaviourally implemented extraretinal optical filter for the
compound eye.