Observations were carried out at the Biological Station Lunz in Lower Austria.If chemical and temperature conditions are kept constant, the subimagines of B. alpinus emerge under natural illumination only in light, mainly between 12.00 and sunset. In permanent light the rhythmicity of emergence seems to be disturbed, in permanent darkness emergence is suppressed.Under natural conditions the duration of the subimago stage depends only on temperature and is not fixed to a particular time of day.