Physiological optics in the hummingbird hawkmoth: a compound eye without ommatidia

The fast-flying day-active hawkmoth Macroglossum stellatarum (Lepidoptera: Sphingidae) has a remarkable refracting superposition eye that departs radically from the classical principles of Exnerian superposition optics. Unlike its classical counterparts, this superposition eye is highly aspherical and contains extensive gradients of resolution and sensitivity. While such features are well known in apposition eyes, they were thought to be impossible in superposition eyes because of the imaging principle inherent in this design. We provide the first account of a superposition eye where these gradients are not only possible, but also produce superposition eyes of unsurpassed quality. Using goniometry and ophthalmoscopy, we find that superposition images formed in the eye are close to the diffraction limit. Moreover, the photoreceptors of the superposition eyes of M. stellatarum are organised to form local acute zones, one of which is frontal and slightly ventral, and another of which provides improved resolution along the equator of the eye. This angular packing of rhabdoms bears no resemblance to the angular packing of the overlying corneal facets. In fact, this eye has many more rhabdoms than facets, with up to four rhabdoms per facet in the frontal eye, a situation which means that M. stellatarum does not possess ommatidia in the accepted sense. The size of the facets and the area of the superposition aperture are both maximal at the frontal retinal acute zone. By having larger facets, a wider aperture and denser rhabdom packing, the frontal acute zone of M. stellatarum provides the eye with its sharpest and brightest image and samples the image with the densest photoreceptor matrix. It is this eye region that M. stellatarum uses to fixate flower entrances during hovering and feeding. This radical departure from classical Exnerian principles has resulted in a superposition eye which has not only high sensitivity but also outstanding spatial resolution.

Investigation of the Screening Pigment System in the Compound Eye of the Moth Agrotis segetum (fam. Noctuidae) by Visible Reflectometry

The functional properties of the light adaptation system in the superposition eye of the moth Agrotis segetum have been investigated by reflection spectroscopy. The spectrum of the reflected light from the tapetum of dark adapted eyes had a peak at about 580 nm corresponding to a spacing between the reflecting layers in the tapetum of 145 nm. During light adaptation of the eye the observed reflectance changes could be explained by light extinction in one screening pigment. The shape of the extinction spectrum of the screening was constant throughout the adaptation process and after a lag phase the optical density of the pigment in the light path increased linearly with time. The screening pigment caused light extinction both by absorption and to some degree also by scattering. The absorption spectrum of the screening pigment had a broad maximum about 590 nm and the scattering efficiency of the pigment particles seemed to be nearly independent of the wavelength.