Summary
The owlfly Libelloides macaronius (Insecta: Neuroptera) has large bipartite eyes of the superposition type. The spatial resolution and sensitivity of the photoreceptor array in the dorsofrontal eye part was studied with optical and electrophysiological methods. Using structured illumination microscopy, the interommatidial angle in the central part of the dorsofrontal eye was determined to be Δφ = 1.1°. Eye shine measurements with an epi-illumination microscope yielded an effective superposition pupil size of about 300 facets. Intracellular recordings confirmed that all photoreceptors were UV-receptors (λmax = 350 nm). The average photoreceptor acceptance angle was 1.8°, with a minimum of 1.4°. The receptor dynamic range was two log units, and the Hill coefficient of the intensity-response function was n = 1.2. The signal-to-noise ratio of the receptor potential was remarkably high and constant across the whole dynamic range (RMS noise = 0.5% Vmax). Quantum bumps could not be observed at any light intensity, indicating low voltage gain. Presumably, the combination of large aperture superposition optics feeding an achromatic array of relatively insensitive receptors with a steep intensity-response function creates a low-noise, high-spatial-acuity instrument. The sensitivity shift to the UV range reduces the contrasts within the sky image. These properties of the visual system are optimal for detecting small insect prey as contrasting spots against both clear and cloudy skies.
