4.2.2. Thermoregulation
Insects are poikilothermic, that is they lack the means to maintain homeothermy — a constant temperature independent of fluctuations in ambient (surrounding) conditions. Although the temperature of an inactive insect tends to track the ambient temperature, many insects can alter their temperature, both upwards and downwards, even if only for a short time. The temperature of an insect can be varied from ambient either behaviorally using external heat (ectothermy) or by physiological mechanisms (endothermy). Endothermy relies on internally generated heat, predominantly from metabolism associated with flight. As some 94% of flight energy is generated as heat (only 6% directed to mechanical force on the wings), flight is not only very energetically demanding but also produces much heat.
Understanding thermoregulation requires some appreciation of the relationship between heat and mass (or volume). The small size of insects in general means any heat generated is rapidly dissipated. In an environment at 10°C a 100 g bumble bee with a body temperature of 40°C experiences a temperature drop of 1°C per second, in the absence of any further heat generation. The larger the body the slower is this heat loss — which is one factor enabling larger organisms to be homeothermic, with the greater mass buffering against heat loss. However, a consequence of the mass—heat relationship is that a small insect can warm up quickly from an external heat source, even one as restricted as a light fleck. Clearly, with insects showing a 500,000- fold variation in mass and 1000-fold variation in metabolic rate, there is scope for a range of variants on thermoregulatory physiologies and behaviors. We review the conventional range of thermoregulatory strategies below, but refer elsewhere to tolerance of extreme temperature (section 6.6.2).
Behavioral thermoregulation (ectothermy)
The extent to which radiant energy (either solar or substrate) influences body temperature is related to the aspect that a diurnal insect adopts. Basking, by which many insects maximize heat uptake, involves both posture and orientation relative to the source of heat. The setae of some “furry” caterpillars, such as gypsy moth larvae (Lymantriidae), serve to insulate the body against convective heat loss while not impairing radiant heat uptake. Wing position and orientation may enhance heat absorption or, alternatively, provide shading from excessive solar radiation. Cooling may include shade-seeking behavior, such as seeking cooler environmental microhabitats or altered orientation on plants. Many desert insects avoid temperature extremes by burrowing. Some insects living in exposed places may avoid excessive heating by “stilting”; that is raising themselves on extended legs to elevate most of the body out of the narrow boundary layer close to the ground. Conduction of heat from the substrate is reduced, and convection is enhanced in the cooler moving air above the boundary layer.
There is a complex (and disputed) relationship between temperature regulation and insect color and surface sculpturing. Amongst some desert beetles (Tenebrionidae), black species become active earlier in the day at lower ambient temperatures than do pale ones, which in turn can remain active longer during hotter times. The application of white paint to black tenebrionid beetles results in substantial body temperature changes: black beetles warm up more rapidly at a given ambient temperature and overheat more quickly compared with white ones, which have greater reflectivity to heat. These physiological differences correlate with certain observed differences in thermal ecology between dark and pale species. Further evidence of the role of color comes from a beclouded cicada (Hemiptera: Cacama valvata) in which basking involves directing the dark dorsal surface towards the sun, in contrast to cooling, when the pale ventral surface only is exposed.
For aquatic insects, in which body temperature must follow water temperature, there is little or no ability to regulate body temperature beyond seeking microclimatic differences within a water body.
Physiological thermoregulation (endothermy)
Some insects can be endothermic because the thoracic flight muscles have a very high metabolic rate and produce much heat. The thorax can be maintained at a relatively constant high temperature during flight. Temperature regulation may involve clothing the thorax with insulating scales or hairs, but insulation must be balanced with the need to dissipate any excess heat generated during flight. Some butterflies and locusts alternate heat-producing flight with gliding, which allows cooling, but many insects must fly continuously and cannot glide. Bees and many moths prevent thoracic overheating in flight by increasing the heart rate and circulating hemolymph from the thorax to the poorly insulated abdomen where radiation and convection dissipate heat. At least in some bumble bees (Bombus) and carpenter bees (Xylocopa) a counter-current system that normally prevents heat loss is bypassed during flight to augment abdominal heat loss.
The insects that produce elevated temperatures during flight often require a warm thorax before they can take off. When ambient temperatures are low, these insects use the flight muscles to generate heat prior to switching them for use in flight. Mechanisms differ according to whether the flight muscles are synchronous or asynchronous (section 3.1.4). Insects with synchronous flight muscles warm up by contracting antagonistic muscle pairs synchronously and/or synergistic muscles alternately. This activity generally produces some wing vibration, as seen for example in odonates. Asynchronous flight muscles are warmed by operating the flight muscles whilst the wings are uncoupled, or the thoracic box is held rigid by accessory muscles to prevent wing movement. Usually no wing movement is seen, though ventilatory pumping movements of the abdomen may be visible. When the thorax is warm but the insect is sedentary (e.g. whilst feeding), many insects maintain temperature by shivering, which may be prolonged. In contrast, foraging honey bees may cool off during rest, and must then warm up before take-off.
