Insect thermoregulation

Insect thermoregulation is the process whereby insects maintain body temperatures within certain boundaries. Insects have traditionally been considered as poikilotherms (animals in which body temperature is variable and dependent on ambient temperature) as opposed to being homeothermic (animals which maintain a stable internal body temperature regardless of external influences). However, the term temperature regulation, or thermoregulation, is currently used to describe the ability of insects and other animals to maintain a stable temperature (either above or below ambient temperature), at least in a portion of their bodies by physiological or behavioral means. While many insects are ectotherms (animals in which their heat source is primarily from the environment), others are endotherms (animals which can produce heat internally by biochemical processes). These endothermic insects are better described as regional heterotherms because they are not uniformly endothermic. When heat is being produced, different temperatures are maintained in different parts of their bodies, for example, moths generate heat in their thorax prior to flight but the abdomen remains relatively cool.

Animal flight is a very energetically expensive form of locomotion which requires a high metabolic rate. In order for an animal to fly, its flight muscles need to be capable of high mechanical power output which in turn, due to biochemical inefficiencies, end up producing large amounts of heat. Thus, a flying insect produces heat which, as long as it does not exceed an upper lethal limit, will be tolerated. However, if the flying insect is also exposed to external sources of heat (for example, radiation from the sun) or ambient temperatures are too high, it should be able to thermoregulate and stay in its temperature comfort zone. The first mechanism that comes to mind for insects to lose heat during flight is convection because higher speeds necessarily increase convective cooling. Nonetheless, higher flying velocities have been shown to result in an increase, instead of a reduction, of thoracic temperature. This is probably caused by the flight muscles working at higher levels and consequently, increasing thoracic heat generation. The first evidence for insect thermoregulation in flight came from experiments in moths demonstrating that dissipation of heat occurs via hemolymph movement from the thorax to the abdomen. The heart of these moths makes a loop through the center of the thorax facilitating heat exchange and converting the abdomen into both a heat sink and a heat radiator that helps the flying insect in maintaining a stable thoracic temperature under different ambient temperature conditions. Thus, it was believed that heat regulation was only achieved by varying heat loss until evidence for varying heat production was observed in honeybees. Then, it was then suggested that thermal stability in honeybees, and probably many other heterothermic insects, was primarily attained by varying heat production. Whether flying insects are able or not to regulate their thoracic temperature by regulating heat production or only by varying heat loss, is still a matter of debate.

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