The Formation of Collective Silk Balls in the Spider Mite Tetranychus urticae Koch

Tetranychus urticae is a phytophagous mite that forms colonies of several thousand individuals. These mites construct a common web to protect the colony. When plants become overcrowded and food resources become scarce, individuals gather at the plant apex to form a ball composed of mites and their silk threads. This ball is a structure facilitating group dispersal by wind or animal transport. Until now, no quantitative study had been done on this collective form of migration. This is the first attempt to understand the mechanisms that underlie the emergence and growth of the ball. We studied this collective behaviour under laboratory conditions on standardized infested plants. Our results show that the collective displacement and the formation of balls result from a recruitment process: by depositing silk threads on their way up to the plant apex, mites favour and amplify the recruitment toward the balls. A critical threshold (quorum response) in the cumulative flow of mites must be reached to observe the emergence of a ball. At the beginning of the balls formation, mites form an aggregate. After 24 hours, the aggregated mites are trapped inside the silk balls by the complex network of silk threads and finally die, except for recently arrived individuals. The balls are mainly composed of immature stages. Our study reconstructs the key events that lead to the formation of silk balls. They suggest that the interplay between mites' density, plant morphology and plant density lead to different modes of dispersions (individual or collective) and under what conditions populations might adopt a collective strategy rather than one that is individually oriented. Moreover, our results lead to discuss two aspects of the cooperation and altruism: the importance of Allee effects during colonization of new plants and the importance of the size of a founding group

The History, Relevance, and Applications of the Periodic System in Geochemistry

Geochemistry is a discipline in the earth sciences concerned with understanding the chemistry of the Earth and what that chemistry tells us about the processes that control the formation and evolution of Earth materials and the planet itself. The periodic table and the periodic system, as developed by Mendeleev and others in the nineteenth century, are as important in geochemistry as in other areas of chemistry. In fact, systemisation of the myriad of observations that geochemists make is perhaps even more important in this branch of chemistry, given the huge variability in the nature of Earth materials – from the Fe-rich core, through the silicate-dominated mantle and crust, to the volatile-rich ocean and atmosphere. This systemisation started in the eighteenth century, when geochemistry did not yet exist as a separate pursuit in itself. Mineralogy, one of the disciplines that eventually became geochemistry, was central to the discovery of the elements, and nineteenth-century mineralogists played a key role in this endeavour. Early “geochemists” continued this systemisation effort into the twentieth century, particularly highlighted in the career of V.M. Goldschmidt. The focus of the modern discipline of geochemistry has moved well beyond classification, in order to invert the information held in the properties of elements across the periodic table and their distribution across Earth and planetary materials, to learn about the physicochemical processes that shaped the Earth and other planets, on all scales. We illustrate this approach with key examples, those rooted in the patterns inherent in the periodic law as well as those that exploit concepts that only became familiar after Mendeleev, such as stable and radiogenic isotopes

Varieties of living things: Life at the intersection of lineage and metabolism

publication-status: Publishedtypes: Articl

The effects of dissection-room experiences and related coping strategies among Hungarian medical students

Background: Students get their first experiences of dissecting human cadavers in the practical classes of anatomy and pathology courses, core components of medical education. These experiences form an important part of the process of becoming a doctor, but bring with them a special set of problems. Methods: Quantitative, national survey (n = 733) among medical students, measured reactions to dissection experiences and used a new measuring instrument to determine the possible factors of coping. Results: Fifty per cent of students stated that the dissection experience does not affect them . Negative effects were significantly more frequently reported by women and students in clinical training (years 3,4,5,6). The predominant factor in the various coping strategies for dissection practicals is cognitive coping (rationalisation, intellectualisation). Physical and emotional coping strategies followed, with similar mean scores. Marked gender differences also showed up in the application of coping strategies: there was a clear dominance of emotional-based coping among women. Among female students, there was a characteristic decrease in the physical repulsion factor in reactions to dissection in the later stages of study. Conclusions: The experience of dissection had an emotional impact on about half of the students. In general, students considered these experiences to be an important part of becoming a doctor. Our study found that students chiefly employed cognitive coping strategies to deal with their experiences. Dissection-room sessions are important for learning emotional as well as technical skills. Successful coping is achieved not by repressing emotions but by accepting and understanding the negative emotions caused by the experience and developing effective strategies to deal with them. Medical training could make better use of the learning potential of these experiences