Method and measurement

Miniaturization of electronic data loggers is facilitating the collection of detailed information on microclimate parameters. Rather than measuring air and substrate temperatures, these devices are often used to record operative (environmental) temperature (Te), which gives an integrative thermal index of microclimate and provides an estimate of the Tb achieved without metabolic heating or evaporation (Bakken 1992). For studies of insect thermal biology, field measurements of Te are usually obtained from thermocouples implanted in dead insects (known as Te thermometers). These have the same size, shape, and radiative properties as live animals, so react similarly to air temperature, air movement, and solar radiation. Bakken (1992) discussed measurement of Te and its applications in detail. Physical models such as copper pipes are widely used to measure Te in field studies of reptile thermoregulation, and the attributes of such models (size, reflectance, orientation, and substrate contact) have been shown to have surprisingly little effect on estimates of Te (Shine and Kearney 2001). Corbet et al. (1993) measured the minimum black globe temperature for flight activity in various social bees, arguing that for interspecific comparisons this method was more appropriate than the use of taxidermic models of different sizes.

The extent of thermoregulation is commonly assessed by linear regression of Tb on ambient temperature (Ta), with a zero slope indicating perfect thermoregulation (Tb independent of Ta) and a slope of one indicating thermoconformity. The 'thermoregulatory performance index' used by Bishop and Armbruster (1999) in their study of Alaskan bees was defined as the slope of the relationship between thoracic temperature (Tth) and Te. They argued that Te is more realistic as a reference temperature and permits comparison of insects captured under different microclimatic conditions.

A simple regression of Tb on Ta has sometimes been criticized as simplistic: by examining Tb for various categories of Ta, it is possible to assess an insect's thermoregulatory ability in more detail—its ability to raise Tb above the minimum threshold (at low Ta), to maintain Tb near the performance optimum (at intermediate levels), or to avoid overheating (at high Ta), as demonstrated for grasshoppers by Willott (1997). Dreisig (1995) modelled these three successive levels of behavioural regulation (basking, graded, and heat-avoidance phases) and showed that Hipparchia semele (Lepidoptera, Satyridae) passed through all three phases as Te increased. For ectotherms showing a graded series of temperature-modulating behaviours, a polynomial regression may be the best fit describing the relationship of Tb to Ta or Te (see Stone 1993; Schultz 1998).

The methods commonly used to evaluate thermoregulation in ectotherms, particularly lizards, have been criticized by Hertz et al. (1993). Low variance in Tb measurements may reflect a thermally homogeneous environment, not careful thermoregulation, and Tb versus Ta regressions suffer from the limitations of measuring only Ta. These authors advocate the collection of three kinds of data—Tb measurements for a representative sample of animals, the distribution of operative temperatures Te at the study site (which may require many models), and laboratory measurements of preferred Tb to give an independent indication of the target Tb that the animal attempts to achieve. Effective thermoregulation is indicated by field-active Tb being closer to preferred Tb than Te is, and field Tb near average Te merely represents thermoconformity. Unfortunately this approach is not applicable to flying or endothermic insects, in which there are dramatic changes from rest to flight and Tb differs among body segments. However, close correspondence between field-active Tth and laboratory-measured optimal temperature has been recorded in dragonflies (Marden 1995b; Marden et al. 1996).

Thermocouples and thermography Thermocouples are easy to construct and use, and have generated a wealth of information on insect thermoregulatory abilities (Heinrich 1993).

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