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The highly specialized eye of an airborne predator, the owlfly (Libelloides macaronius, Insecta:Neuroptera)

Gregor Belušič1*, Primož Pirih1, 2 and Doekele G. Stavenga3
  • 1 University of Ljubljana, Biotechnical Faculty, Slovenia
  • 2 University of Ljubljana, Faculty of Natural Sciences and Engineering, Slovenia
  • 3 University of Groningen, Zernike Institute for Advanced Materials, Netherlands

The owlfly Libelloides macaronius (Scopoli, 1763) (formerly: Ascalaphus macaronius) (Insecta: Neuroptera: Ascalaphidae) is a remarkable airborne predator of warm meadows of the Pontomediterranean. Its success as a hunter depends on its visual sense, which is mediated by a pair of large, bipartite compound eyes with refractive superposition optics. The dorso-frontal (DF) part of the eyes is exclusively UV sensitive, and the ventro-lateral (VL) part has a small additional sensitivity peak in the blue-green wavelength range. This eye design is highly conserved in the Ascalaphidae family (virtually identical in Libelloides ictericus and Deleproctophylla dusmeti). We demonstrate that an achromatic eye sensitive in the UV (λmax = 350 nm) brings numerous advantages for an animal hunting against the sky. The clear skies are very uniformly illuminated, the clouds are barely visible and the sun glow is very small in the ultraviolet part of the spectrum. The decreased photon flux in the UV is compensated by a large aperture of the superposition eye, which is estimated to be ca. 300 facets in the DF part both in the dark and light adapted state. We show that the spatial resolving power in the owlfly is not compromised by the superposition optical design since the acceptance angle (minimal Δρ = 1.4°) is only slightly broader than the interommatidial angle (minimal Δφ = 1.1°). The resolution of the dioptrical apparatus is supported by a tracheolar sheath surrounding each ommatidium at the level of the rhabdom, which creates a mirrored box and increases the photon catch. A bright diurnal superposition eye allows for a low voltage gain (quantum bumps are always indistinguishable) and a steep intensity-response relation (Hill coefficient n = 1.2) of the photoreceptors and consequently a high signal to noise ratio (receptor potential noise, RMS < 0.5% Vmax). The visual environment of the owlfly is simplified and sampled with extreme contrast, so that an insect prey is observed as a contrastful dark spot against the bright, uncluttered skies. The temporal resolving power of the eye is optimal above 35°C since the phototransduction machinery is adapted to the high body temperatures resulting from the active flight under the summer sun.

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References

Belušič, G., Pirih, P., Stavenga, D.G. (2013) A cute and highly contrast-sensitive superposition eye - the diurnal owlfly Libelloides macaronius. J. Exp. Biol., approved for publication

Keywords: superposition eye, diurnal superposition, acceptance angle, owlfly, Ultraviolet

Conference: International Conference on Invertebrate Vision, Fjälkinge, Sweden, 1 Aug - 8 Aug, 2013.

Presentation Type: Oral presentation preferred

Topic: Eye design, optics and spatial vision

Citation: Belušič G, Pirih P and Stavenga DG (2019). The highly specialized eye of an airborne predator, the owlfly (Libelloides macaronius, Insecta:Neuroptera). Front. Physiol. Conference Abstract: International Conference on Invertebrate Vision. doi: 10.3389/conf.fphys.2013.25.00027

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Received: 22 Feb 2013; Published Online: 09 Dec 2019.

* Correspondence: Dr. Gregor Belušič, University of Ljubljana, Biotechnical Faculty, Ljubljana, Slovenia, gregor.belusic@bf.uni-lj.si