The dynamics of light adaptation in Ascalaphus (Libelloides macaronius; Neuroptera)

Abstract

The owl-fly or Ascalaphus (Libelloides macaronius; Neuroptera) is an insect with a UV-sensitive superposition eye. Although optical superposition is mainly a feature of dusk/dark active animals, this is a predator living and hunting in bright sunlight. In such conditions the process of light adaptation is believed to be very important, yet it has so far only been partially explored in the owl-fly. Here we present physiological evidence for the migration of the screening pigment, which functions as a light control mechanism. The process of light adaptation was studied optically by dynamic imaging and optical reflection spectroscopy of the eye-glow. We established that the eye-glow is reduced uniformly upon illumination and that its diameter doesn’t get smaller, which is indicative of pigment migration in the primary pigment cells. The change in spectral absorbance of the dorso-frontal eye is very similar to the absorbance spectrum of the primary pigment cell screening pigment. We found that the change in the light screening due to adaptation is rather small – no more than 10 fold for a 10000 fold change in light intensity. We also found that the rate of adaptation is light-sensitive. We propose that a significant part of this light sensitivity is due to indirect heating of the eye and to the very steep temperature dependency of the rate of adaptation between 30 and 35°C.