Columnar self-assembly, gelation and electrochemical behavior of cone-shaped luminescent supramolecular calix[4]arene LCs based on oxadiazole and thiadiazole derivatives

A new class of blue light-emitting supramolecular liquid crystalline cone or bowl-shaped compounds were synthesized from substituted 1,3,4-oxadiazoles and 1,3,4-thiadiazoles with calix[4]arene derivatives.

The compound eye as an indicator of age and shrinkage in Antarctic krill

Laboratory studies have shown that Antarctic krill (Euphausia superba) shrink if maintained in conditions of low food availability. Recent studies have also demonstrated that E. superba individuals may be shrinking in the field during winter. If krill shrink during the winter, conclusions reached by length-frequency analysis may be unreliable because smaller animals may not necessarily be younger animals. In this study, the correlation between the body-length and the crystalline cone number of the compound eye was examined. Samples collected in the late summer show an apparent linear relationship between crystalline cone number and body-length. From a laboratory population, it appears that when krill shrink the crystalline cone number remains relatively unchanged. If crystalline cone number is little affected by shrinking, then the crystalline cone number may be a more reliable indicator of age than body-length alone. The ratio of crystalline cone number to body-length offers a method for detecting the effect of shrinking in natural populations of krill. On the basis of the crystalline cone number count, it appears from a field collection in early spring that E. superba do shrink during winter.

Eye structure and optics in the pelagic shrimp Acetes sibogae (Decapoda, Natantia, Sergestidae) in relation to light-dark adaptation and natural history

The structure and optics of the compound eyes of the neritic sergestid shrimp, Acetes sibogae , are described. The eyes are nearly spherical and heavily pigmented. The facets are square, indicating that the eye operates by the recently recognized mechanism of reflecting superposition. The most distal portion of each ommatidium is the corneal lens, which is secreted by two underlying corneagenous cells. These two cells surround the crystalline cone and cone stalk and the four cells of which they are composed and extend proximally at least as far as the distal rhabdom. Near the base of the cone stalk the extensions of the corneagenous cells swell and enclose spheres which bear on their surfaces small particles similar to ribosomes in appearance. Beneath the corneagenous cells lie four crystalline cone cells, parts of which differentiate to form the crystalline cone and cone stalk. The latter structures are compound, one quarter of each being contributed by each crystalline cone cell. Distally the crystalline cone cells send a small projection, which is surrounded by the corneagenous cells, to the cornea. Proximal extensions of each of the four parts of the cone stalk extend between the retinula cells and meet within the basement membrane. Between the base of the cone stalk and the regularly layered rhabdom lies the distal rhabdom. It is surrounded by a cell that we have termed retinula cell eight (R8), by analogy with other crustacean systems, and consists of unordered microvilli projecting from the cell membrane into the extracellular space above the layered rhabdom. In addition to R 8, which contributes only to the distal rhabdom, seven other retinula cells contribute to the proximal rhabdom, which consists of alternating ordered layers of orthogonally arranged microvilli. Four of these retinula cells are arranged orthogonally and extend far distally along the crystalline tract. The other three do not extend as far distally and alternate with the first four in their position around the axis of the ommatidium. R8 is located still further proximally at the level of the distal rhabdom. All seven of the retinula cells which contribute to the proximal rhabdom contain proximal pigment and extend through the basement membrane. The basement membrane consists of a meshwork grid with each intersection supporting a rhabdom so at this point the retinula cell axons project into different squares of the meshwork. Tapetal pigment cells are present in the vicinity of the basement membrane and extend downward to the lamina. The granules of tapetal pigment are covered or exposed by movements of the proximal pigment and also change their intracellular distribution depending on illumination. In addition to the proximal (retinula cell) pigment and the tapetal pigment the eye contains four types of distal pigment. Moving inward from the cornea these are the distal yellow pigment (DYP) which surrounds the entire eye; the distal reflecting pigment (DRP), which forms a thin layer and is continuous with the tapetal pigment at the edge of the eye; and the black distal pigment and the mirror pigment (MP) both contained within distal pigment cells (DPC). In the light-adapted state the proximal pigment moves distally, surrounding the rhabdoms, and the tapetal pigment granules move proximally so that they are mainly found beneath the basement membrane. Movements of the distal pigments are less clearcut, but they all appear to move somewhat proximally in the light-adapted state. Multivesicular bodies are more abundant in the retinula cells shortly after dawn, and are possibly related to membrane turnover. Interommatidial angle, as measured on both fixed and fresh material, varied from 2.8 to 3.8° in different parts of the eye. The crystalline cones were found to have a uniform refractive index radially, which, combined with their square shape, indicates that they function by reflecting superposition. Total internal reflection from the sides of the cones is adequate to explain the maximum diameter of the eyeshine from the dark-adapted eye at night without the need for additional mirrors. Nevertheless, from its organization and appearance the mirror pigment could act as a reflector in the dark-adapted eye. Also, the size of the glow patch indicates that there would be a gain of nearly two log units in image brightness in going from the light-adapted to the dark-adapted state. Each corneal facet was found to act as a weak converging lens, with a focal length of approximately 300 μm. The eye structure of Acetes is discussed in relation to that of other shrimp and to the natural history of Acetes .

The phylogenetic significance of ommatidium structure in the compound eyes of polyphagan beetles

The structure of ommatidia in adults of more than 200 beetle species from 91 polyphagan families was surveyed. Three basic types of lens system (eucone, exocone, and acone) and two types of retinal unit (fused rhabdom, open rhabdom) are represented. The eucone (crystalline cone-containing) ommatidium is ancestral and prevails in primitive Eucinetoidea, Hydrophiloidea, and Scarabaeoidea; ommatidia of the primitive beetles Cupes and Omma as well as the Adephaga are of this type. The polyphagan founders most likely had ommatidia with small crystalline cones and narrow clear zones beneath the corneal facets. Exocone and acone eyes are derived structures, and their distribution suggests that both have evolved several times. Exocone ommatidia arose early in polyphagan evolution, possibly first in dascilloid-like founders of elateriform and bostrychiform beetles, where the exocone is commonly found. An exocone eye also evolved separately in the ancestors of several primitive scarabaeoid families; possible steps in this eucone to exocone transition may be seen in the Trogidae. The clear zones of eucone and exocone eyes are not homologous. The acone ommatidium is specialized and arose through a progressive loss of either crystalline cone or exocone. In the advanced staphylinoid beetles it is a relic of crystalline cone loss in their small ancestors. In the cucujiforms it arose likely from the loss of the exocone in their bostrychiform ancestors, associated here with a shift to an open rhabdom. Although the distribution of ommatidial types coincides with major lineages in the Polyphaga, a few anomalies remain. The Eucnemidae, Buprestidae, and Dryopidae are all eucone yet are placed in the elateriform series, in which 25 of 30 families are exocone. Scarab beetles have an extraordinary variety of lens types that presumably reflects the exceptional adaptability of the eye in this superfamily.

Graded-index optics are matched to optical geometry in the superposition eyes of scarab beetles

Detailed measurements were made of the gradients of refractive index (g.r.i.) and relevant optical properties of the lens components in the ventral superposition eyes of three crepuscular species of the dung-beetle genus Onitis (Scarabaeinae). Each ommatidial lens has two components, a corneal facet and a crystalline cone; in both of these, the gradients provide a significant proportion of the refractive power. The spatial relationship between the lenses and the retina (optical geometry) was also determined. A computer ray-trace model based on these data was used to analyse the optical properties of the lenses and of the eye as a whole. Ray traces were done in two and three dimensions. The ommatidial lenses in all three species are afocal g.r.i. telescopes of low angular magnification. Parallel incident rays emerge approximately parallel for all angles of incidence up to the maximum. The superposition image of a distant point source is a small patch of light about the size of a rhabdom. There are obvious differences in the lens properties of the three species, most significantly in the shape of the refractive-index gradients in the crystalline cone, in the extent of the g.r.i. region in the two lens components and in the front-surface curvature of the corneal facet lens. These give rise to different angular magnifications M of the ommatidial lenses, the values for the three species being 1.7, 1.3, 1.0. This variation in M is matched by a variation in optical geometry, most evident in the different clear-zone widths. As a result, the level of the best superposition image lies close to the retina in the model eyes of all three species. The angular magnification also sets the maximum aperture or pupil of the eye and hence the brightness of the image on the retina. The smaller M , the larger the aperture and the brighter the image. By adopting a suitable value for M and the appropriate eye geometry, an eye can set image brightness and hence sensitivity within a certain range. Differences in the eye design are related to when the beetles fly at dusk. Flight experiments comparing two of the species show that the species with the higher value for M and corresponding lower sensitivity, initiates and terminates its flight earlier in the dusk than the other species with 2.8 times the sensitivity.

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.