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

Measurement properties of self-report questionnaires on health-related quality of life and functional health status in dysphonia: a systematic review using the COSMIN taxonomy.

PURPOSE: The current review was conducted to identify all self-report questionnaires on functional health status (FHS) and/or health-related quality-of-life (HR-QoL) in adult populations with dysphonia (voice problems), and to evaluate the psychometric properties of the retrieved questionnaires. METHODS: A systematic review was performed in the electronic literature databases PubMed and Embase. The psychometric properties of the questionnaires were determined using the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) taxonomy and checklist. Responsiveness was outside the scope of this review and as no agreed 'gold standard' measures are available in the field of FHS and HR-QoL in dysphonia, criterion validity was not assessed. Only questionnaires developed and published in English were included. RESULTS: Forty-eight studies reported on the psychometric properties of 15 identified questionnaires. As many psychometric data were missing or resulted from biased study designs or statistical analyses, only preliminary conclusions can be drawn. Based on the current available psychometric evidence in the literature, the Voice Handicap Index seems to be the most promising questionnaire, followed by the Vocal Performance Questionnaire. CONCLUSIONS: More research is needed to complete missing data on psychometric properties of existing questionnaires in FHS and/or HR-QoL. Further, when developing new questionnaires, the use of item response theory is preferred above classical testing theory, as well as international consensus-based psychometric definitions and criteria to avoid bias in outcome data on measurement properties