22,703 research outputs found
Parasitism, Adult Emergence, Sex Ratio, and Size of \u3ci\u3eAphidius Colemani\u3c/i\u3e (Hymenoptera: Aphidiidae) on Several Aphid Species
Aphidius colemani Viereck parasitizes several economically important aphid pests of small grain crops including the greenbug, Schizaphis graminum and the Russian wheat aphid, Diuraphis noxia. The ability of A. colemani to switch from S. graminum to several species of aphids common to agricultural and associated non-agricultural ecosystems in the Great Plains, and the effects of host-change on several biological parameters that influence population growth rate were determined. Female A. colemani parasitized and developed to adulthood in nine of 14 aphid species to which they were exposed in the laboratory. All small grain feeding aphids except Sipha flava were parasiÂtized. Two sunflower feeding species (Aphis nerii and A. helianthi) and two crucifer feeding species (Lipaphis erysimi and Brevicoryne brassicae) were parasitized, as was the cotton aphid. Aphis gossypii. The average percentage of aphids parasitized differed significantly among host aphid species. as did the percentage of parasitoids surviving from the mummy to the adult stage and the time required for immature development. The sex ratio of adults that enclosed from the various hosts did not differ significantly among species. Dry weights of adult parasitoids differed significantly among host species. Adults from S. graminum weighed most (0.054 mg) while those emerging from A. helianthi weighed least (0.020 mg). Results are discussed in terms of strategies for classical biological control of aphid pests of cereals
Observation of small scale structure using sextupole lensing
Weak gravitational lensing seeks to determine shear by measuring induced
quadrupole (elliptical) shapes in background galaxy images. Small impact
parameter (a few kpc) gravitational lensing by foreground core masses between 2
10^{9} and 2 10^{12} M_\odot will additionally induce a sextupole shape with
the quadrupole and sextupole minima aligned. This correlation in relative
orientation of the quadrupole and sextupole provides a sensitive method to
identify images which have been slightly curved by lensing events. A general
theoretical framework for sextupole lensing is developed which includes several
low order coefficients in a general lensing map. Tools to impute map
coefficients from the galaxy images are described and applied to the north
Hubble deep field. Instrumental PSFs, camera charge diffusion, and image
composition methods are modelled in the coefficient determination process.
Estimates of Poisson counting noise for each galaxy are used to cut galaxies
with signals too small to reliably establish curvature. Curved galaxies are
found to be spatially clumped, as would be expected if the curving were due to
small impact parameter lensing by localized ensembles of dark matter haloes.
Simulations provide an estimate of the total required lensing mass and the
acceptable mass range of the constituent haloes. The overdensities and
underdensities of visible galaxies and their locations in the Hubble foreground
is found to be consistent with our observations and their interpretation as
lensing events.Comment: 40 pages, 44 figure
ChairMaker - a parametric chair modelling program
In this report we describe a new parametric modelling program that can produce 3D models of chairs. The parameters used are easily understood physical traits such as back height and seat width. The nished ChairMaker program is capable of producing a vast range of semi-realistic 3D models from a set of 33 parameters. It is intended that these are used to explore automated chair design
Proof that the Hydrogen-antihydrogen Molecule is Unstable
In the framework of nonrelativistic quantum mechanics we derive a necessary
condition for four Coulomb charges ,
where all masses are assumed finite, to form the stable system. The obtained
stability condition is physical and is expressed through the required minimal
ratio of Jacobi masses. In particular this provides the rigorous proof that the
hydrogen-antihydrogen molecule is unstable. This is the first result of this
sort for four particles.Comment: Submitted to Phys.Rev.Let
Zero Energy Bound States in Many--Particle Systems
It is proved that the eigenvalues in the N--particle system are absorbed at
zero energy threshold, if none of the subsystems has a bound state with and none of the particle pairs has a zero energy resonance. The pair
potentials are allowed to take both signs
Stability of quantum breathers
Using two methods we show that a quantized discrete breather in a 1-D lattice
is stable. One method uses path integrals and compares correlations for a
(linear) local mode with those of the quantum breather. The other takes a local
mode as the zeroth order system relative to which numerical, cutoff-insensitive
diagonalization of the Hamiltonian is performed.Comment: 4 pages, 3 figure
Universality in the merging dynamics of parametric active contours: a study in MRI-based lung segmentation
Measurement of lung ventilation is one of the most reliable techniques of
diagnosing pulmonary diseases. The time consuming and bias prone traditional
methods using hyperpolarized HHe and H magnetic resonance
imageries have recently been improved by an automated technique based on
multiple active contour evolution. Mapping results from an equivalent
thermodynamic model, here we analyse the fundamental dynamics orchestrating the
active contour (AC) method. We show that the numerical method is inherently
connected to the universal scaling behavior of a classical nucleation-like
dynamics. The favorable comparison of the exponent values with the theoretical
model render further credentials to our claim.Comment: 4 pages, 4 figure
Thermopower of Single-Molecule Devices
We investigate the thermopower of single molecules weakly coupled to metallic
leads. We model the molecule in terms of the relevant electronic orbitals
coupled to phonons corresponding to both internal vibrations and to
oscillations of the molecule as a whole. The thermopower is computed by means
of rate equations including both sequential-tunneling and cotunneling
processes. Under certain conditions, the thermopower allows one to access the
electronic and phononic excitation spectrum of the molecule in a
linear-response measurement. In particular, we find that the phonon features
are more pronounced for weak lead-molecule coupling. This way of measuring the
excitation spectrum is less invasive than the more conventional current-voltage
characteristic, which, by contrast, probes the system far from equilibrium.Comment: 13 pages, 7 figures included; minor changes, version published in PR
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