6.1.3. CO2 production
For the measurement of honey bee CO2 production in open flow respirometry, a differential infrared gas analyser (DIRGA) is recommended. These measure infrared light absorption in the λ = 2.5–8 μm wavelength range by the CO2 present in air (photometric principle according to the Lambert/Beer law). These devices are very sensitive down to the sub-ppm range and usually offer a high baseline stability if their internal temperature is regulated accurately. Internal construction is mostly a set of two tubes (for reference and measurement signal) with an infrared lamp on one side and a detector on the other.
Portable instruments, originally designed for field measurements of plant photosynthesis, can also be used for field measurements of honey bee respiration (e.g. Li-COR LI-6400XT Portable Photosynthesis System). This device also offers the possibility of simultaneous measurement of air H2O content.
Fig. 10 shows a
respiratory CO2 trace of a honey bee. The bee was kept in the dark
during the night, which made her soon entering the resting state. At rest she
showed typical patterns of discontinuous respiration. At hive temperatures of
30 °C or 34 °C a bee breathes on average only once every 37 or 28 seconds,
respectively (Kovac et al., 2007).
This trace, however, also shows the bees' high proneness to disturbance when
caught in a respiratory chamber against their own will. When the experimenter
entered the laboratory ('!' in Fig. 10) and afterwards switched on the light,
the trace characteristic changed and the bee eventually heated up. Such
endothermy increased energy turnover by more than the 100-fold.
Fig. 10. Respiratory trace of a honey bee (Apis mellifera carnica). The bee was kept overnight in a 18 ml respiratory chamber (Fig. 10; 0 min = 07:21 in the morning). Left trace part: discontinuous gas exchange cycles during rest (light off), the bee was ectothermic; exclamation mark: experimenter entering the laboratory (with subsequent light on); right trace part: endothermy (bee prepared for immediate flight) and was actively seeking for an exit out of the chamber. Ambient temperature near the bee: ca. 22 °C. For more resting traces see Kovac et al. (2007). Graph by Anton Stabentheiner.