Aeronautic behaviour in the wasp - like spider ,

In only a few decades, A. brllennichi has colonized vast areas of continental Europe. This was most likely achieved by the ballooning of young spiderlings. Its original distribution was largely restricted to the Meditenanean. This makes A. brllennichi an interesting object for studying the proximate factors, behavioural and micrometeorologicaL which lead to this rapid mode of dispersal. After eclosion fi'om their cocoons on sunny days in spring. the spiderlings climb into the top of vegetation. Two modes of take-off can be observed. The most common method is by stal1ing tl'om the top of a grass haulm. However, if many spiderlings have emerged ti'om their cocoons, they will start from a "trapeze", a diffuse construct of silken threads connecting grass blades. In both cases they initiate ballooning by dropping on its dragl ine. As no aggregations of orb webs were detected in the direct proximity of the cocoons jj'om which spiderlings eclosed, and their first orb web was only constructed after a ballooning trip, we consider aeronautic behaviour to be an obligatory dement of A. bruennichls life cycle in southern Germany. With respect to the micrometeorological conditions, it appeared that ballooning activity correlated highly significantly with weather conditions causing thennals, which A. bruennichi is apparently able to perceive. The factors which could ultimately be responsible tor the rapid invasion of large pat1S of Europe are discussed briefly.