Effect of fluoride mouthrinses and stannous ions on the erosion protective properties of the in situ pellicle.

The particular feature of this study is the investigation of effects of pure fluoride- or stannous ions based mouthrinses on the erosion protective properties and the ultrastructure of the in situ pellicle (12 volunteers). Experimental solutions were prepared either from 500 ppm NaF, SMFP, AmF or SnF2 or 1563 ppm SnCl2, respectively. After 1 min of in situ pellicle formation on bovine enamel slabs, rinses with one of the preparations were performed for 1 min and intraoral specimens' exposure was continued for 28 min. Native enamel slabs and rinses with bidestilled water served as controls. After oral exposure, slabs were incubated in HCl (pH 2; 2.3; 3) for 120 s and kinetics of calcium- and phosphate release were measured photometrically; representative samples were analysed by TEM and EDX. All mouthrinses reduced mineral loss compared to the native 30-min pellicle. The effect was pH-dependent and significant at all pH values only for the tin-containing mouthrinses. No significant differences were observed between the SnF2- and the SnCl2-containing solutions. TEM/EDX confirmed ultrastructural pellicle modifications. SnF2 appears to be the most effective type of fluoride to prevent erosive enamel demineralisation. The observed effects primarily have to be attributed to the stannous ions' content.

The unusual visual system of the Strepsiptera: external eye and neuropils.

Adult males of the insect order Strepsiptera are characterized by an unusual visual system that may use design principles from compound as well as simple eyes. The lenses of this eye are unusually large and focus images onto extended retinae. The light-gathering ability of the lens is sufficient to resolve multiple points of an image in each optical unit. We regard each unit as an independent image-forming eye that contributes an inverted partial image. Each partial image is re-inverted by optic chiasmata between the retinae and the lamina, where the complete image could be assembled from the neighboring units. The lamina, medulla and lobula are present, but their organization into cartridges is not clearly discernable. Fluorescent fills, whole-tissue stains, and synaptotagmin immunohistochemistry show that the optic neuropils nevertheless are densely packed, and that several parallel channels within the medulla underlie each of the lenses. The size and shape of the rhabdoms, as well as a relatively slow flicker-fusion frequency could suggest that these eyes evolved through a nocturnal life stage.

Eye stalks or no eye stalks: a structural comparison of pupal development in the stalk-eyed fly Cyrtodiopsis and in Drosophila.

After emergence from the puparium, stalk-eyed flies of the family Diopsidae rapidly expand their head capsule so that the eyes and optic lobes are displaced at the ends of stalks that extend from the central head. Because the expansion takes place in only 15 minutes, we are especially interested in ontogenetic modifications that may facilitate such a rapid and dramatic change. To examine the pupal development of the brain, we used Bodian staining in the stalk-eyed fly, Cyrtodiopsis whitei and compared it with development in the fruit fly, Drosophila melanogaster, which serves as a "typical" dipteran example without eye stalks. Early in pupal development, the neuropil organization of the two species is fairly similar. In both species, columns are present in the outer medulla and giant fibers are discernible in the lobula plate. In contrast to D. melanogaster, C. whitei shows a small, neck-like constriction between the optic lobes and the rest of the brain. By 20% of pupal development, the divergence is more apparent, and by 30%, the future eye stalk and optic nerve of C. whitei has started to form. During the remaining 70% of development, the initially thick optic nerve narrows, and becomes gradually elongated, eventually coiling and folding throughout the short eye stalk. Similarly, the cuticle of the surrounding region becomes constricted, slightly elongated, and gradually appears more and more densely corrugated, like an accordion bellows. However, except for the formation of the optic nerve, the dense aggregation of cuticle around it, and a shift in orientation of the neuropils, the developmental programs of the two species are remarkably similar. This suggests that only a few aspects of development have been modified during the course of evolution to generate the stalk-eyed phenotype. At eclosion, the imago of C. whitei goes through a pumping process to inflate the eye stalks to their full length. Measurements of the diameter of the optic nerve before and after the expansion reveal only a small decrease. We propose that the cuticular folding of the eye stalk as well as the coiling of the optic nerve prepare the pupa well for the rapid and dramatic eye-stalk inflation after eclosion.

Neurobiological constraints and fly systematics: how different types of neural characters can contribute to a higher level dipteran phylogeny.

Much uncertainty still exists regarding higher level phylogenetic relationships in the insect order Diptera, and the need for independent analyses is apparent. In this paper, I present a parsimony analysis that is based on details of the nervous system of flies. Because neural characters have received little attention in modern phylogenetic analyses and the stability of neural traits has been debated, special emphasis is given to testing the robustness of the analysis itself and to evaluating how neurobiological constraints (such as levels of neural processing) influence the phylogenetic information content. The phylogenetic study is based on 14 species in three nematoceran and nine brachyceran families. All characters used in the analysis are based on anatomical details of the neural organization of the fly visual system. For the most part they relate to uniquely identifiable neurons, which are cells or cell types that can be confidently recognized as homologues among different species and thus compared. Parsimony analysis results in a phylogenetic hypothesis that favors specific previously suggested phylogenetic relationships and suggests alternatives regarding other placements. For example, several heterodactylan families (Bombyliidae, Asilidae, and Dolichopodidae) are supported in their placement as suggested by Sinclair et al. (1993), but Tipulidae and Syrphidae are placed differently. Tipulidae are placed at a derived rather than ancestral position within the Nematocera, and Syrphidae are placed within the Schizophora. The analysis suggests that neural characters generally maintain phylogenetic information well. However, by "forcing" neural characters onto conventional phylogenetic analyses it becomes apparent that not all neural centers maintain such information equally well. For example, neurons of the second-order visual neuropil, the medulla, contain stronger phylogenetic "signal" than do characters of the deeper visual center, the lobula plate. These differences may relate to different functional constraints in the two neuropils.

Chunk versus point sampling: visual imaging in a small insect.

The eyes of strepsipteran insects are very unusual among living insects. In their anatomical organization they may form a modern counterpart to the structural plan proposed for the eyes of some trilobites. Externally they differ from the usual "insect plan" by presenting far fewer but much larger lenses. Beneath each lens is its own independent retina. Anatomical and optical measurements indicate that each of these units is image-forming, so that the visual field is subdivided into and represented by "chunks," unlike the conventional insect compound eye that decomposes the visual image in a pointwise manner. This results in profound changes in the neural centers for vision and implies major evolutionary changes.

Visual system of the stalk-eyed fly, Cyrtodiopsis quinqueguttata (Diopsidae, Diptera): an anatomical investigation of unusual eyes.

Diopsid flies have eye stalks up to a centimeter in length, displacing the retina laterally from the rest of the head. This bizarre condition, called hypercephaly, is rare, but has evolved independently among several insect orders and is most common in flies (Diptera). Earlier studies of geometrical optics and behavior have led to various hypotheses about possible adaptive advantages of eye stalks, such as enhanced stereoscopic vision while other hypothesis suggest that eye stalks are an outcome of sexual selection. Here, we focus on how these curious distortions of head/eye morphology are accompanied by changes in the neural organization of the visual system of Cyrtodiopsis quinqueguttata. Histological examinations reveal that the optic lobes, lamina (La), medulla (Me), lobula (Lo), and lobula plate (LP) are contained entirely within the fly's eye bulbs, which are located at the distal ends of the eye stalks. We report that the organization of the peripheral visual system (La and Me) is similar to that of other Diptera (e.g., Musca and Drosophila), but deeper visual areas (Lo and LP) have been more strongly modified. For example, in both the lobula and lobula plate, fewer but larger giant collector neurons are found. The most pronounced difference is the reduction in the number of wide-field vertical cells of the lobula plate, where there are only four relatively large fibers, as opposed to 11 in Musca. The "fewer but larger" neural organization may enhance the conduction velocities of these cells, but may result in a loss of spatial resolution. At the base of the eye bulb, axon bundles collect and form a long optic nerve that extends the length of the eye stalk. We suggest that this organization of the diopsid visual system provides evidence for the costs of possessing long eye stalks.

The relevance of neural architecture to visual performance: phylogenetic conservation and variation in Dipteran visual systems.

In cyclorrhaphan flies, giant tangential neurons in the lobula plate are supplied by isomorphic arrays of evolutionarily conserved achromatic elementary motion detecting circuits originating in the retina. The arrangements among giant tangential neurons is characteristic of a taxon and can differ between taxa having different visual performances. Observations of 12 brachyceran and 4 nematoceran species have identified different behaviors associated with visually stabilized flight. Neuroanatomical comparisons between closely related species having different behaviors and phylogenetically distant species that have similar behaviors suggest that such differences relate to differences of giant tangential cell architecture in the lobula plate. These functionally related differences contrast to anatomical features that reflect phylogenetic affinities. For example, the lobula plates of robber flies, typified by ballistic flight behavior, all differ from other taxa in lacking cyclorrhaphan-type vertical motion-sensitive neurons; instead, they possess an extra complement of horizontal cells in their place. The results suggest that, although circuits that compute elementary motion are conserved across the Diptera, selective pressure has resulted in modifications of their target neurons, thus contributing to the wide variety of visual behaviors observed within this group of insects.

Visual motion-detection circuits in flies: small-field retinotopic elements responding to motion are evolutionarily conserved across taxa.

The Hassenstein-Reichardt autocorrelation model for motion computation was derived originally from studies of optomotor turning reactions of beetles and further refined from studies of houseflies. Its applicaton for explaining a variety of optokinetic behaviors in other insects assumes that neural correlates to the model are principally similar across taxa. This account examines whether this assumption is warranted. The results demonstrate that an evolutionarily conserved subset of neurons corresponds to small retinotopic neurons implicated in motion-detecting circuits that link the retina to motion-sensitive neuropils of the lobula plate. The occurrence of these neurons in basal groups suggests that they must have evolved at least 240 million years before the present time. Functional contiguity among the neurons is suggested by their having layer relationships that are independent of taxon-specific neurons, or the absence of orientation-specific motion-sensitive levels in the lobula plate.