Factors affecting the microdistribution of Gammarus pulex (Amphipoda): an experimental study

SUMMARY. 1. The tnierodistribution of Gammarus species is size assortative: larger animals are associated with larger substrate particles. Using an artificial stream with scrubbed substrate we investigate the hypothesis that the assortative mechanism is stimulated by current avoidance rather than food searching.
2. In uniformly mixed substrate, G. pulex tend to move upstream, but in patchy substrate they are found in a predictable substrate grade. There is no discrimination between patches by different size classes of G. pulex in static water. The size assortative field pattern can only be reproduced under flow conditions in the laboratory. It occurs in the absence of food and irrespective of the sequence or location of substrate patches.
3. As density increases, G. pulex are displaced from the optimal patches but move into the open stream rather than into the cover of alternative patch types.
4. We discuss the significance of these results and suggest that some earlier models explaining mechanisms for microdistribution patterns tnay have been unnecessarily complex. Contrasts between natural and simulated stream situations and their experimental advantages are noted.     

Permethrin-impregnated bednets reduce nuisance arthropods in Gambian houses

The prevalence of bedbugs (Cimex hemipterus L.), chicken ticks (Argas persicus Oken) and headlice (Pediculus capitis De Geer) was surveyed in a rural area of The Gambia. At the beginning of the study 37.5% of children's beds were infested with bedbugs and 3.9% with chicken ticks, whilst the prevalence rate of pediculosis in children under 10 years old was 28.8%. Both bedbugs and headlice were clustered within compounds. Headlice prevalence increased with hair length and they were more common on girls than boys. Following this cross-sectional survey all bednets in the sixteen hamlets were either dipped in permethrin or a placebo. About 4 months later it was found that bedbugs and chicken ticks had disappeared from homes in which the bednets had been impregnated with permethrin. There was no reduction in hamlets with placebo-treated bednets. The rate of acquiring headlice between the two surveys was reduced by 91.1% in children who slept under insecticide-treated bednets compared with children with placebo-treated bednets. There were also significantly fewer day-flying and crawling insects, except earwigs, in homes of children who slept under insecticide-treated bednets compared with those with placebo-treated nets. These additional benefits of permethrin-treated bednets should contribute to their widespread acceptance and utilization by the community for personal protection.     

Epiphytic diatoms in two freshwater maritime Antarctic lakes

SUMMARY.

• 1 An ecological study of two small maritime Antarctic lakes on Signy Island, South Orkney Islands, was undertaken from January 1986 to March 1987. Analysis of diatom counts from the lakes provided examples of oligotrophic and mesotrophic ecosystems.
• 2 A diverse community of 104 epiphytic taxa was identified. Twenty-eight taxa had a percentage abundance greater than 1% in both lakes. Distinctive dominant taxa were identified from each lake.
• 3 A variety of ordination techniques was performed on the abundance data and a principal components analysis demonstrated differences in the diatom assemblages between the two lakes. Clear separations of sites and species were evident between the lakes, and accounted for the greatest percentage variance.
• 4 Species composition varied with depth within each lake and was also important in influencing changes in assemblage composition between sites.
• 5 A redundancy analysis indicated that species composition was correlated to concentrations of nitrogen and phosphorus but the total variance accounted for by the four physical and chemical factors measured was low (24%).

     

Interspecific relations of African crocidurine shrews (Mammalia: Soricidae) based on multivariate analysis of mandibular data

Eighteen graded non-metric characters and 30 measurements of the mandible and lower dentition were used to investigate the grouping of African species of the genera Crocidura, Suncus and Sylvisorex. Seventy-seven taxa (species, subspecies and synonyms), including four European species, were represented by one to nine mandibles (147 mandibles in all), and the means of four Pleistocene African species were also included. The metric data were subjected to principal component and canonical variate analyses, and the non-metric data to principal component analysis. Grouping was detected by comparing the results of the three analyses. Six groups are recognized within the genus Crocidura , typified by C. fumosa, C. turba, C. russula, C. fuscomurina, C. hirta and C. flavescens. Sylvisorex is divisible into two groups, typified by S. lunaris and S. granti. The African species of Suncus (but not the introduced species, S. murinus ) form a single group, which is linked to the Sylvisorex granti group. Suncus is closely related to, or convergent with, the Crocidura fuscomurina group. The first non-metric principal component, the second metric principal component and the second canonical variate are significantly correlated. It is considered that these together represent a general trend of evolutionary advance. Phylogenetic relations are discussed in the light of this; Crocidura may be diphylefic.     

Movement of Anopheles gambiae s.l. malaria vectors between villages in The Gambia

ovement of mosquitoes belonging to the Anopheles gambiae complex (mixed wild populations of An.arabiensis, An.gambiae and An.melas ) between three neighbouring rural villages in The Gambia was investigated by mark-release-recapture. A total of 12,872 mosquitoes were collected in bednets, marked with a magenta fluorescent powder and released over a 15-day period in one of the villages. A further 15,507 mosquitoes were collected in exit traps, marked with a yellow powder and released over the same period. Mosquitoes were captured daily in all three villages using pyrethrum spray catches, as well as bednet and exit trap catches. The catching period extended for 6 days after the last day of release.
Of the mosquitoes released, 372 (1.3%) were recaptured 2–21 days later. Of these recaptures, 272 were caught in the release village, and 98 were caught in other villages situated 1–1.4 km away. The 'movement index' between villages was calculated as 17.2% (12.2–22.4% confidence limits) for mosquitoes released after feeding and 20.1% (14.7–25.3%) for those released unfed.
These results suggest that movement of mosquitoes between neighbouring villages in The Gambia seriously affects the entomological evaluation of pyrethroid-impregnated bednet programmes in areas where treated and untreated villages are interspersed.     

Why change mates?

In some wading birds (Charadrii) each adult rears a brood alone. The female leaves her first clutch with her first mate before herself rearing a clutch fertilized by a second male, who is already tending its first mate's first clutch. We here develop a simple model to account for the exchange of mates between clutches, and relate it to reported field studies. We suggest that females are attempting to be polygamous, rather than simply bigamous. Provided a female has a chance of obtaining a second, unmated male and that the costs of leaving the first male before remating are not high, matechanging will be favoured over monogamy. The implications of this model to the evolution of the more usual forms of polyandry and of male-like females are discussed.     

Identification of immature aphids by chromatography

• 1 Immature apterae of five species of cereal aphid were distinguished by ultraviolet examination of chromatograms of their fluorescent pigments.
• 2 Non-expert examiners could identify Metopolophium dirhodum, Sitobion fragariae and Rhopalosiphumpadi with a mean reliability of 98, 83 and 71% respectively; Metopolophium festucae and Sitobion avenae could be separated from the rest but not from each other.
• 3 All species gave consistent chromatograms whether reared on oats or on wheat: some species produced atypical traces if reared on certain grasses.

     

紫贻贝长期暴露于亚致死剂量的林丹和阿特拉津下的组织学反应

为开发快速、敏感的生物标记物以监测海洋贝类生物中是否存在农药,我们检测了双壳动物紫贻贝长期暴露在亚致死剂量的丙体六氯环乙烷(林丹,γ-HCH)和2-氯4-乙胺基-6异丙胺基-1,3,5-三氮苯(阿特拉津)下的组织学变化。紫贻贝容易积累环境中的杀虫剂,因此,本研究旨在阐明农药的生物积累与组织病理学效应之间的关系。利用GC/MSD分析法定期对贻贝和水样中的林丹与阿特拉津含量进行测定。将贻贝在实验室中培养21天,以使其代谢适应于带有水质控制的封闭式不间断流动系统。随后,30只贝暴露于亚致死剂量的林丹(0.9 mg/L)或阿特拉津(3.583 mg/L)溶液中56天。实验期间,控制重要的参数,比如温度和盐度分别控制在18℃和34‰。在处理28天和56天后取样,检测组织学损坏及吸收的农药量。暴露的紫贻贝每克干重分别能聚集约304.8-372.0μg/g林丹和83.3-137.4μg/g阿特拉津。组织学改变高度集中在鳃的上皮和外套膜组织;上皮与相邻的组织形成分离状态。组织病理学结果显示,抗性机制的激活使紫贻贝能在亚致死压力下存活。组织病理效应范围从浸润反应到以血淋巴细胞出现间质细胞反应为特征。因此,在农药聚集部位的组织学和超微结构的改变是敏感的,并与农药的生物积累具有正相关关系,说明这些改变可能作为农药暴露的生物标记物。     

Spatial patterns of range contraction in British breeding birds

We use ornithological atlas data to assess evidence for the existence of a number of spatial patterns of range contraction in British breeding birds. For 18 of the 25 species which suffered the greatest range contractions between 1968 and 1991, there wais a greater likelihood of local extinction in areas where the species was initially less widespread, so ranges tended to contract towards their cores. However there was evidence for a number of other patterns, with some species having a greater likelihood of local extinction in the centres of their ranges and others suffering random local extinctions throughout their range. The different spatial patterns identified were largely independent of the overall range contraction suffered by each species nationally. We suggest that range contractions in British birds can generally be explained better by a general decline in habitat quality or other factors than by contagious anthropogenic effects, as might be expected in a country with a long history of human environmental modification.