Zur Morphologie des Dorsal- und Ventralauges des Taumelk�fersGyrinus substriatus (Steph.) (Coleoptera, Gyrinidae)

Functional Anatomy of Split Compound Eyes of the Whirligig Beetles Dineutus mellyi (Coleoptera: Gyrinidae)

The functional anatomy of the split compound eyes of whirligig beetles Dineutus mellyi (Coleoptera: Gyrinidae) was examined by advanced microscopy and microcomputed tomography. We report the first 3D visualization and analysis of the split compound eyes. On average, the dorsal and ventral eyes contain 1913 ± 44.5 facets and 3099 ± 86.2 facets, respectively. The larger area of ventral eyes ensures a higher field of vision underwater. The ommatidium of the split compound eyes is made up of laminated cornea lenses that offer protection against mechanical injuries, bullet-shaped crystalline cones that guide light to the photoreceptive regions, and screening pigments that ensure directional light passage. The photoreceptive elements, made up of eight retinular cells, exhibit a tri-tiered rhabdom structure, including the upper distal rhabdom, a clear zone that ensures maximum light passage, and an enlarged lower distal rhabdom that ensures optimal photon capture.

Stark trade-offs and elegant solutions in arthropod visual systems

Vision is one of the most important senses for humans and animals alike. Diverse elegant specializations have evolved among insects and other arthropods in response to specific visual challenges and ecological needs. These specializations are the subject of this Review, and they are best understood in light of the physical limitations of vision. For example, to achieve high spatial resolution, fine sampling in different directions is necessary, as demonstrated by the well-studied large eyes of dragonflies. However, it has recently been shown that a comparatively tiny robber fly (Holcocephala) has similarly high visual resolution in the frontal visual field, despite their eyes being a fraction of the size of those of dragonflies. Other visual specializations in arthropods include the ability to discern colors, which relies on parallel inputs that are tuned to spectral content. Color vision is important for detection of objects such as mates, flowers and oviposition sites, and is particularly well developed in butterflies, stomatopods and jumping spiders. Analogous to color vision, the visual systems of many arthropods are specialized for the detection of polarized light, which in addition to communication with conspecifics, can be used for orientation and navigation. For vision in low light, optical superposition compound eyes perform particularly well. Other modifications to maximize photon capture involve large lenses, stout photoreceptors and, as has been suggested for nocturnal bees, the neural pooling of information. Extreme adaptations even allow insects to see colors at very low light levels or to navigate using the Milky Way.

Under- and over-water halves of Gyrinidae beetle eyes harbor different corneal nanocoatings providing adaptation to the water and air environments

Whirligig beetles (Gyrinidae) inhabit water surfaces and possess unique eyes which are split into the overwater and underwater parts. In this study we analyze the micro- and nanostructure of the split eyes of two Gyrinidae beetles genera, Gyrinus and Orectochilus. We find that corneae of the overwater ommatidia are covered with maze-like nanostructures, while the corneal surface of the underwater eyes is smooth. We further show that the overwater nanostructures possess no anti-wetting, but the anti-reflective properties with the spectral preference in the range of 450-600 nm. These findings illustrate the adaptation of the corneal nanocoating of the two halves of an insect's eye to two different environments. The novel natural anti-reflective nanocoating we describe may find future technological applications.

The divided eye of the isopod Glyptonotus antarcticus : effects of unilateral dark adaptation and temperature elevation

The literature on the structure and function of isopod compound eyes is briefly reviewed. Unlike other isopods studied, Glyptonotus antarcticus possesses physi­cally separated large dorsal compound eyes and small ventral compound eyes. G. antarcticus turns upside down when it swims, and it seems that this is when the ventrally located eyes become useful. Structurally, the two types of eye are very similar : both consist of individual ommatidia, which in an adult specimen can be 80–100 μm wide and 300 μm long. Each ommatidium contains a bipartite crystalline cone and a long rhabdom, approximately 100 μm long, which is characteristically star-shaped when sectioned transversely. Sometimes five and sometimes six retinula cells contribute to the formation of the centrally located, fused and unbanded rhabdom. Dark-light adaptational changes were difficult to demonstrate and did not occur until one eye was kept covered and shielded from light for one week, while the other one remained uncovered. In eye pairs of five animals treated in this way, it was obvious that prolonged darkness leads to an outward migration of retinula screening pigment granules, to the formation of multivesicular bodies in the retinula cells, and to an increase in size and abundance of spherical organelles in the interstitial cells. Exposure to light, on the other hand, results in an inward (towards the rhabdom) migration of retinula cell screening pigment granules, the formation of multilamellar bodies through pinocytotic processes at the rhabdom edge, and a swelling of interstitial cells. Temperature elevation alone mimics the effects of bright light with regard to pigment granule migration. It is suggested that, when pigment granules absorb radiation during exposure to light under normal environ­mental temperature conditions, they may heat up their immediate surroundings sufficiently to contribute to membrane damage.

Daily changes in the compound eye of a beetle ( Macrogyrus )

(i) Graded index lenses in the cornea and the crystalline cone form the optical system in each ommatidium. (ii) By night the crystalline cone has a blunt ellipsoidal proximal end which contributes to the formation of a superposition image across the clear zone. By day the cone is a tapering point that is extended as a light guide through a dense layer of pigment. (iii) The action of extending the cone and moving the pigment towards the clear zone from between the cones occurs as the retinula-cell column contracts. (iv) Modelling of the ommatidial lens system shows how the superposition image is formed in the night eye, and suggests that axial rays are not well focused on the crystalline tract in the day eye. (v) All cells had peak sensitivity in the green near 552 nm. (vi) In the dark-adapted day eye, fields are ∆ ρ (acceptance angle) = 3.4–6.6°, narrowing to 2.8° minimum upon light adaptation. Sensitivity to a point source on axis is reduced during the day: the dark-adapted day eye requires 200 times more light to give the same response as the dark-adapted night eye. There is a further attenuation of about 100 upon light adaptation of the day eye. (vii) The superposition image of the night eye produces fields of width ∆ ρ = 12-15° at 50% sensitivity as recorded electrophysiologically, and therefore the image of a point source covers several rhabdoms. (viii) In recordings from single units in the night eye two bumps (effective photon captures) are counted when the intensity is such that one photon falls on the area of one facet, with parallel axial illumination at the peak of the spectral sensitivity, 552 nm. (ix) Marking of cells with Lucifer Yellow suggests that about four to six receptor units per ommatidium are involved, giving a sensitivity of eight to twelve bumps for the ommatidium at this intensity. (x) Locust apposition eyes, with facets twice the area of those in Macrogyrus eyes, give at best 0.5 bumps with the same intensity, so that the actual superposition gain is 32–48. (xi) All marked cells were of the proximal rhabdom layer; cells 1 and 8 have not been investigated.