POPULATION DYNAMICS OF THE FRESHWATER SNAIL BIOMPHALARIA PFEIFFERI IN EASTERN ZAIRE

The population dynamics of the freshwater snail Biomphalariapfeifferi, an intermediate host of the parasitic worm Schistosomamansoni, was studied in the Virunga stream in Eastern Zaïre. Methods are developed to estimate age-specific survivorshipfrom regular quadrat-sampling data without assuming a constantmortality rate, and to approximate age-specific fecundity whenthe fecundity function is known in the laboratory. The population dynamics in the field was found to be very differentfrom that in the laboratory: it is basically discontinuous,with one main generation per year; a massive mortality occursafter hatching; fecundity is reduced considerably; and thereare marked seasonal variations in both survivorship and fecundity.Time-dependent demographic parameters were estimated by monthinstead of by cohort. They show that the environmental conditionsare favourable to an increase of the population only duringa short period of the year. The intrinsic rate of natural increase,r, was found to be well correlated with the changes in egg density. The environmental factors determining the population dynamicsin the Virunga are discussed; current speed seems critical. (Received 4 February 1987;     

A New Collection of Thermosensitive Endocytosis Mutants in the Cellular Slime Mold Dictyostelium discoideum

ABSTRACT. We used a photoactivatable fluid-phase marker to isolate a new collection of thermosensitive endocytosis mutants in the cellular slime mold Dicfyostelium discoideum. All the strains were thermosensitive for growth on bacteria or axenic medium at 27° C. Initial rates of endocytosis rapidly decreased upon incubation at the restrictive temperature, but surprisingly most of the strains showed a transient recovery of activity with prolonged exposure to 27° C. Endocytosis and exocytosis activities were uncoupled for some of the cell lines at 27° C whereas the others had to be shifted to 29° C. Further molecular analysis of these mutants could lead to the discovery of new proteins involved in endocytosis and its regulation.     

The visual perception of relative distances in the wood-cricket, Nemobius sylvestris

ABSTRACT. In post-embryonic development, the visual system of the cricket Nemobius sylvestris (Bosc) shows a regular increase in the length and number of the ommatidia and a decrease of inter-ommatidial angle so that the adult's is a third of the value in the first larval instar. Further, a 20° widening of the binocular visual field, in the horizontal plane at least, and a three-fold increase of the inter-ocular distance improve the potential for binocular vision. Behavioural experiments showed that the insect orientates with differing precision depending on the distances to targets of constant angular size. Further, in a choice situation between two such vertical targets, the cricket orientates most strongly towards the closer of the two, even at target distances of 52 and 130 cm from its point of decision. In fixed tethered animals, discrimination between a close and a distant target is still possible, but disappears when the head is waxed to the thorax, so that any relative movement between the animal and the object is prevented. As these capabilities exceed the possibilities of binocular triangulation, the possible role of other mechanisms is discussed, particularly that involving movement parallax using both eyes.     

Functional exploration of the visual field of the wood-cricket, Nemobius sylvestris

ABSTRACT. Orientation towards a rectangular black visual target was measured with tethered crickets, Nemobius sylvestris (Bosc). The orientation reaction was induced only if the target was located in a visual field within 100° of the sagittal plane, 60° above and 40° below the horizontal plane. The motor response was maximum when the target was presented between 30° and 40° of the sagittal plane, and between 0° and 20° above the horizontal one. The orientation reaction was induced optimally by an area of the eye which is lateral and slightly dorsal, whose ommatidia are not included in the binocular field. The ability to move the head enlarged the visual field by about 20° laterally, improving the exploration of the visual space.     

Coupling of the Deformation of a Model Endothelial Cell and a Steady Laminar Flow

Abstract

Mechanical forces influence endothelial cell's (EC) morphology and functions. In this work it was proposed a numerical analysis of steady laminar flows near a modelled monolayer of elastic ECs in order to determine the local distributions of mechanical forces on the surface and inside the cell.

Numerical results showed that the flow induced non uniform mechanical stresses on cell surface and led to a cell deformation. These numerical results were compared with experimental measurements of the deformation of cultured human aortic endothelial cells under flow. It will be interesting to study eventual correlations between the distributions of biological receptors (cytoskeleton, adhesion molecules, etc.) and that of the non-uniform mechanical forces.