Measurement chambers

1. Resting and active metabolism: Fig. 9 shows a brass variant of a measurement chamber. For temperature control the chamber can be immersed in a water bath. If the chamber is not completely submerged in the water and the lid is exposed to the laboratory air in order to allow visual or thermographic observation of the bees (Fig. 9; Stabentheiner et al., 2012) the air temperature in the chamber may differ from the water bath temperature. The strength and direction of this deviation depends on the difference between water bath and laboratory air temperature. The resulting temperature gradients inside the chamber require placement of a temperature sensor (see section 5.) inside the chamber. Completely submersing the chamber (and the inlet tubing) avoids this effect.

2. Outdoor measurement: A measurement chamber variant for outdoor measurements of foraging honey bees of about 8 ml inner volume was presented in Stabentheiner et al. (2012; Fig. 2 and Fig. 5 therein). Its lid could be opened and closed quickly to give the bees fast access to an artificial flower inside. It was also operated in serial mode. These measurement chambers can, of course, also be used in parallel mode. In cases where the sun shine is hits the measurement chamber, cooling via a water bath is indispensable, since the temperature inside can quickly reach critical levels and the bees will no longer enter it (Stabentheiner et al., 2012).

In parallel mode, a different configuration is useful especially for field measurements (Stabentheiner et al., 2012). A set of two identical measurement chambers can be placed at the air inlets of reference and measurement air streams, one for the measuring and the other for the reference air stream. These chambers can be constructed out of a small plastic film cylinder attached to a glass laboratory funnel. Attaching the base of the cylinder to an iron spacer ring as a counterpart of pieces of hard disk magnets fits the chamber to an underlying artificial flower. Air inlets are in the base of the artificial flower.

3. Compensation of measurement gas loss: Chamber opening may lead to an unwanted loss of measurement gas. This is especially critical when measuring foraging honey bees where the duration of stay at a food source can be shorter than a minute during unlimited feeding. In such cases as it is not possible to simply cut out a section of the measurement signal and to calculate an average over a certain time interval, special calibration is necessary. Very briefly, such a procedure compares the washout volumes from the chamber containing certain concentrations of CO2 (or O2) with and without chamber opening. Stabentheiner et al. (2012) provide a detailed description of how to compensate for such losses.

4. Measurement of larvae and pupae: For the measurement of tiny honey bee larvae, measurement chambers have to be very small (e.g. 0.5–2 ml). This can be realized by adapting a plastic syringe (Petz et al., 2004), or with plastic photometer cuvets. For very young larvae and for eggs, the closed chamber method has to be used (see section 6.1.8.).

5. Flight metabolism: The other extreme of chamber size is required in measurements of flight metabolism. For this purpose, large measurement chambers of about 0.3 to 1 l volume have to be used. Such chambers allow respiration measurements in agitated free flight (Harrison and Hall, 1993; Harrison et al., 1996; Wolf et al., 1996; Roberts and Harrison, 1999; Woods et al., 2005). Measurement of bees in free hovering flight was accomplished in a wind tunnel by Wolf et al. (1989), which equals an even larger chamber size of about 4 l volume. Simulating an appropriate virtual environment to stimulate the bees to stay airborne for a longer time period without agitation is, however, a tricky task (Wolf et al., 1989).

6. Whole colony metabolism: Even larger chamber volumes will be needed for whole colony measurements (Kronenberg and Heller, 1982; Southwick, 1985).

Fig. 9. Example of a measurement chamber for use in a temperature controlled water bath. It was milled out of a brass block. Air inlet visible in the picture bottom (front), outlet on top (covered by yellow kitchen cloth). Effective chamber volume is variable by a movable, perforated brass barrier. In this setup, the window in the lid was covered by an infrared transmissive plastic film allowing visual and thermographic behavioural observations and body surface temperature measurements (Kovac et al., 2007; Stabentheiner et al., 2012). Lid tightness was assured by a Viton® O-ring (not visible). Service holes were drilled in one side (left) to accept chromatography septa (11 mm), which allowed tight insertion of thermocouple wires. Tightness has to be proofed by applying an overpressure on the submerged chamber. The rectangle at the right-hand side is a proprietary reference radiator for infrared camera calibration (see Stabentheiner et al., 2012). Photo by Anton Stabentheiner.

Figure 9