A circle swimmer at low Reynolds number

Swimming in circles occurs in a variety of situations at low Reynolds number. Here we propose a simple model for a swimmer that undergoes circular motion, generalising the model of a linear swimmer proposed by Najafi and Golestanian (Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres arranged in a triangular configuration, joined by two links of time-dependent length. For small strokes, we discuss the motion of the swimmer as a function of the separation angle between its links. We find that swimmers describe either clockwise or anticlockwise circular motion depending on the tilting angle in a non-trivial manner. The symmetry of the swimmer leads to a quadrupolar decay of the far flow field. We discuss the potential extensions and experimental realisation of our model.Comment: 9 pages, 9 Figure

A reflection on Mg, Cd, Ca, Li and Si isotopic measurements and related reference materials

This contribution aims to report the reflections we had with the scientific community during two international workshops on reference materials for stable isotopes in Davos (2002) and Nice (2003). After evaluating the isotopic homogeneity of some existing reference materials, based on either certificates, literature data or specific inter-laboratory rounds, we confirm these as primary reference materials or propose new ones relative to which stable isotope compositions should be reported. We propose DSM-3 for Mg, NIST SRM 915a for Ca, L-SVEC for Li and NBS28 for Si. Cadmium does not yet have a well identified delta zero material, although three commercial mono-elemental Cd solutions have yielded the same isotopic composition relative to one another. In order to scale the linearity of any mass spectrometer, some secondary reference materials are also proposed: Cambridge-1 solution for Mg, the “Münster-Cd” and JEPPIM Cd solutions for Cd and the “Big Batch” silicate for Si. The team from Nancy propose to prepare a mixed spike solution for Li isotopes. Well-characterised natural samples such as ocean or continental waters, diatoms, sponges, rocks and minerals are needed to validate the entire analytical procedure, particularly to take into account the effect of sample mineralisation and of chemical manipulations for elemental separation prior to analysis

Solvent extraction of cues in the dust and frass of Prostephanus truncatus and analysis of behavioural mechanisms leading to arrestment of the predator Teretrius nigrescens

Teretrius nigrescens is a predatory beetle released into Africa for classical biological control of Prostephanus truncatus, currently the most serious insect pest of stored maize on the continent. T. nigrescens is arrested by the dust and frass produced by P. truncatus boring into maize and it has been concluded that this effect is most likely to be caused by chemicals in the dust/frass. A bioassay is presented in which EthoVision software (Noldus Information Technology, the Netherlands) captures and analyses the movement of individual insects within arenas containing zones to which solvent extracts are applied. The results demonstrate that cues enabling adult T. nigrescens to discriminate between maize flour and dust/frass are extractable in hexane, methanol and chloroform, but not water. Such discrimination is manifested by an increase in residence time, distance covered and path sinuosity in the dust/frass zone. Analysis of behaviour demonstrates cues that trigger inverse orthokinetic mechanisms contributing to arrestment in adult T. nigrescens. An increase in angular velocity would also suggest a direct klinokinetic component, although this parameter could be affected by postulated tropotactic and/or transverse klinotactic and/or longitudinal klinotactic mechanisms occurring at the edge of the application zone. Development of this bioassay, for the first time, permits responses of larval T. nigrescens to prey cues to be assessed. The results show that larvae apparently respond to cues extracted from dust/frass in a stronger manner than adults, with significant increases in the same descriptive parameters of time, distance and path sinuosity. However, unlike in the adults, there is no orthokinesis. It is proposed that strong larval response is attributable to either direct klinokinesis and/or tropotaxis interacting with transverse klinotaxis. Observations of larval backtracking at the zone boundary may also indicate longitudinal klinotaxis. The reasons for the differences in the adult and larval response to prey cues are considered, and how the described behavioural mechanisms combine to improve prey foraging is discusse