Figure 6.2 (a) Changes in Tth in an anthophorid bee, Creightonella frontalis, at the end of tethered flight, showing both brief and prolonged rises in Tb. Periods of flight shown by solid bars. (b) Thoracic temperatures (Tth) of female C. frontalis measured by the 'grab and stab' method, compared with stable flight temperatures (SFT).
Note: SFT + 10 s (dashed line) indicates body temperatures 10 s after cessation of tethered flight.
Source: Stone and Willmer (1989a).
However, Stone and Willmer (1989a) have criticized the 'grab and stab' method of measuring insect body temperatures on the basis that there may be rapid and unpredictable changes in Tb when a flying insect is captured. Although it has commonly been assumed that passive cooling takes place between capture and insertion of the thermocouple, Tb can rise at the end of a period of tethered flight, either briefly or on a more prolonged basis (Fig. 6.2a) (Stone and Willmer 1989a). Honeybees flying on a roundabout show increases in surface temperature of the thorax immediately after flight (Jungmann et al. 1989). 'Post-flight warm-up' during the interval between grabbing and stabbing is the result of heat production continuing after convective cooling has stopped, and can lead to significant errors in the slope of the Tth/Ta regression used to determine the extent of thermoregulation (Fig. 6.2b). Beetles close their elytra on capture and this may also lead to potential overestimates of Tth (Chown and Scholtz 1993). When more than one body segment is sampled, the common practice of measuring Tth first, because it has the largest gradient (e.g. Coelho and Ross 1996) may also be a source of error. Watt (1997) has further criticized the 'grab and stab' method for the following reasons: (1) its apparent simplicity encourages anecdotal observation; (2) only active members of an insect population are sampled; and (3) each data point comes from a killed or injured insect. The use of chronically implanted thermocouples provides more balanced and reliable information on the thermal ecology of insects (e.g. Ward and Seely 1996a). Shallow insertion of implanted thermocouples is recommended by Stone and Willmer (1989b) to minimize tissue damage (and is justified because temperature gradients within the thorax are unlikely). The appendix to the latter paper also provides an indication of the likely extent of heat loss along the thermocouple wire, which can be significant for small insects.
Infrared thermography (Stabentheiner 1991) avoids the problems associated with invasive measurements of body temperature. Continuous recording of the IR radiation emitted by an unrestrained insect is converted to surface temperatures using careful calibration, and the thermal behaviour is recorded on videotape. The surface temperature of a bee heated to 20°C above Ta is approximately 1oC lower than that just beneath the cuticle, and relative measurements can be made with an accuracy of ±0.25°C. Provided all of the insect is in focus, thermal imaging can give simultaneous remote measurements of different body regions (Schmaranzer and Stabentheiner 1988; Farina and Wainselboim 2001). However, the IR camera remains in a fixed position and this technique can not be used for flying insects. It is ideal for studies of bees visiting an artificial feeder or leaving the hive entrance (see Section 6.6.2), and extension cables have increased the field possibilities (Kovac and Schmaranzer 1996). Infrared thermography also has advantages when the 'grab and stab' method might be risky for researchers: it has been used to show how the Japanese honeybee Apis cerana japonica kills its hornet predators by engulfing them in a cluster heated to about 47°C, which is lethal to the hornets but not the bees (Ono et al. 1995).
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