Figure 4.

Mean hourly values of environmental cues that could act as possible Zeitgebers on the circadian clocks of bumblebees under continuous daylight conditions. (a) light intensity, (b) ambient temperature and (c) UV radiation (320 nm to 410 nm) recorded between 5 July 2008 and 13 July 2008 at the field station. Two days (7 July 2008 and 8 July 2008) have been excluded from the UB measurements because the values were not recorded for the whole 24 h of these days. Temperature data were obtained from the Finnish Meteorological Institute and UV data were kindly provided by Dr. Iris Zellmer.