0001). From the amount of crop loading and the duration of the foraging stays we estimated the mean suction rate per stay (crop loading/duration of foraging stay), which increased exponentially with Ta ( Fig. 10B). This increase was much steeper in dependence on Thd. However, we also noticed that the bees did not always drink continuously during the whole foraging

stay. They made short interruptions and often showed periods of self-grooming and walking. Especially towards the end of their stays they mTOR inhibitor filled in time for pre-flight warm-up to reach a sufficient thorax temperature for an optimal take off. Unfortunately, our thermographic sequences did not allow exact identification of drinking pauses. From our own observations and earlier measurements of Schmaranzer (2000) we estimated actual duration of suction to be about 85% of the total duration of a stay on average. The curves calculated with this

assumption matches measurements of the suction Navitoclax chemical structure rate of Ressi (1989) closely (conducted at Ta and Twater = 25 °C). The suction rate increased exponential from 0.6 to 2.2 mg s−1 as Thead increased from 26 to 36 °C (Q10 = 3.7; Fig. 10B). However, correlation with the ambient temperature in this range of Ta resulted in a smaller elevation of the suction rate, from 1.6 to 2.9 mg s−1 (Q10 = 1.8). Digby (1955) investigated the factors affecting the temperature excess of dead or anesthesized insects in artificial sunlight under Racecadotril laboratory conditions and found the temperature excess to vary directly with the radiation strength, similar to our dead bees. This applies to living insects only in the ectothermic state. Foraging honeybees, however, are always endothermic at medium to low Ta ( Heinrich, 1979a, Schmaranzer and Stabentheiner, 1988 and Kovac and Schmaranzer, 1996). In our water foragers endothermy was at a low level or absent only at high Ta (>∼30 °C; see below and Fig. 6, Fig. 7 and Fig. 8). The same was observed in water foraging vespine wasps (Vespula; Kovac et al., 2009). However, the thermoregulatory behavior of our water foraging bees differed from that of vespine wasps ( Fig. 6A–C, Table 3) at moderate Ta (∼20–30 °C). The bees’

thorax temperature excess decreased slightly with increasing radiation whereas it increased in Vespula. At high Ta (>∼30 °C), by contrast, the thorax temperature excess increased in both. The relation between body temperature and ambient temperature shows impressively the thermoregulatory ability of the water foraging honeybees (Fig. 3 and Fig. 6). The thorax temperature was regulated independent of Ta (in sunshine and shade) in a broad range of Ta (∼3–30 °C). This resembles an investigation on honeybees collecting water in shade ( Schmaranzer, 2000). Similar to our study he reported mean thoracic temperatures of 36.0–38.8 °C (Ta = 13.6–27.2 °C). Bees foraging from other natural resources like flowers regulate their thoracic temperature at a somewhat lower level.