No description available.

Investigations on the relationship between the moth, Coleophora caespititiella Zell., and the rush, Juncus squarrosus L., were carried out on the Moor House Nature Reserve, in the northern Pennines, from 1952 to 1954.Details of the life-history of G. caespititiella. which in the larval stage feeds on the seeds of various rush species, are given. Behaviour studies were carried/out on the final instar larvae (marking experiments showed that these visited a mean of 2.28 rush capsules during their feeding period in 1953) and also on the imagines. J. squarrosus is shown to 'be broadly distributed over the study area. The capacity of the rush to produce ripe seeds diminished with increasing altitude, and fruiting was less extensive in the cool summer of 1954 than in the two preceeding seasons. The distribution of Coleophora closely corresponded to the degree of ripe seed production by the rush in each season. In all years ripe seed production and Coleophora infestation of the rush to any given extent, occurred about 300’ higher on the eastern Pennine slopes than on the western. Hymenopterous parasites of the Coleophora. Larvae were recorded only in populations below 1050’ on the western and 1525’ on the eastern Pennine slopes. Seasonal surveys of two Coleothora populations are described. Heavy mortality was demonstrated in the egg and early larval stages and also in the overwintering larval population in the ground litter. Some evidence for competition between early instar Coleophora larvae inside mature rush capsules is presented. It is suggested that competition at this stage would result in a degree of mortality dependent on the density of the initial larval population. The various mortality factors affecting different Coleophora populations are discussed.

Work on the biology of Collembola (Springtails) was carried out betweenn Ootober 1959 and October I962, on the Moor House National Nature Reserve, in Westmorland. This is an area of high Pennine moorland (l840ft. O.D.) which experiences a sub-Arctic olimate. Population studies wars made on Limestone grassland, Alluvial grassland and Heather moor, by means of a statistical sampling method. Juncus squarrosus grassland and the erosion and recolonisation of blanket bog were also studied from the points of view of population densities and species differences. In an attempt to explain the fluctuations in numbers rocorded, biological data was also obtained from laboratory cultures of selected species. Observations on reproductive behaviour, fecundity, egg development, frequency of moulting, sex ratios and age distributions were made. Breeding experiments on members of the Onychiurus arnatus species group were carried out, and these revealed what appears to be an unusual form of parthenogenesis; these experiments also showed that in some species, at least, the criteria for the division of the 0. aimatus species group, which have been questioned by some continental workers, are valid. Regular sampling of the selected vegetation types provided data on horizontal distribution (aggregations), vertical distribution and seasonal variations in the numbers and biomass of Collembola. Limestone grassland carried the highest mean anual population density (52.92 x 10(^3) per m(^2)) and Juncus squarrosus grassland the lowest (20.93 x 10(^3) per m(^2)). A flotation extractor, to remove Collembola from organic soils, was designed and built, and this may prove an important step forward in the technique of studying the moorland fauna. The work forms a contribution to the study of moorland ecology, but it is clear that to obtain a comprehensive picture of the ecological importance of Collembola on moorland, a great deal of work will be necessary in the future.

Notes on the autecology of 32 species of Auchenorrhyncha from the Moore House National Nature Reserve are given. Studies were made of the microclimates of the common vegetation types on which these occur and it is shown that the size and function as a temperature regulator of the spittle (produced by nymphs of the cercopidae) is associated with these gradients. Cercopid nymohs are shown to change feeding sites after moulting. The need to change host plants and thus leave protective spittle is thought to result in an increased mortality of nymphs (up to 3 per cent per day). Overall nymphal mortality is not dependent on population density. In two cercopids (Neophilaenus lineatus and Neophilaenus exclamationis), there are marked changes in population density in each year (1961 to 1963) and these are partly attributed to climatic factors. The hatch of cercopidae is delayed, and development is retarded at high altitides. Nymphal mortality rates in a cicadellid (macrosteles sexnotatus) and a delphacid (conomelus anceps) did not differ from those of the ceropidae studied, although they have no protective spittle. It is concluded that cercopid spittke is of some survival value at moore House by enabling the nymph to avoid parasites, if not predators, and unfavourable climatic conditions encountered whilst remaining stationary on the host plant during feeding. These advantages are not sufficient to give the Cercopidae a significantly higher survival rate than other Auchenorrhyncha at Moor House. Non-density dependent factors are thought to be responsible for a major part of the changes in population density. A compensatory mechanism at other stages of the life cycle has not been demonstrated. Local extinction at the edge of the range is a result of climatic factors. The population studied is probably below the density level at which regulating mechanisms may occur.

No description available.

The ability of a nervous system to correctly process sensory information depends on the precise wiring of axonal and dendritic projections. The visual system of vertebrates and invertebrates consists of many neuron subtypes, whose neurites are organized into columns and layers. Layered pathways are pivotal to enable parallel processing of several visual features within a network, such as motion and color detection. Despite their functional relevance, the molecular mechanisms controlling their formation are still poorly understood. In the visual system of Drosophila, photoreceptor neurons (R-cells, R1-R8) extend axons into the optic lobe. R1-R6 axons target to the lamina, while R8 and R7 axons terminate in two distinct neuropil layers in the medulla, called M3 and M6. This thesis investigated the roles of Netrin ligands and the attractive receptor Frazzled and repellent receptor Unc-5 in mediating layer-specific targeting of R8 axons during development. Immunohistological and genetic analyses showed that Frazzled is expressed and cell-autonomously required in R8 photoreceptors for targeting their axons to layer M3. Netrin-B is specifically localized in this layer due to axonal release primarily by lamina neurons L3 and ligand capture by Frazzled expressed in target neurons. Loss of Netrins in the target area causes similar defects as loss of Netrins in R-cells. Netrin-B expression in L3 is sufficient to substantially rescue these R8-axon targeting defects. R8 axons target normally despite replacement of diffusible Netrin-B by membrane-tethered ligands, indicating that they act at short-range. Netrin localization is instructive, as expression of a tethered ligand-variant in ectopic layers can retarget R8 axons. Finally, R8 axons also express and require Unc-5 for layer-specific targeting suggesting a modulatory role of repellent input in this process. Together, this provides evidence for a distinct mechanism that uses the release of localized chemotropic guidance cues to precisely direct axons to a positionally defined layer.

No description available.

No description available.

No description available.

No description available.