Thermoregulation

Insects, as well as other invertebrates, are generally heterothermic, meaning that their body temperatures are determined primarily by ambient temperature. Rates of metabolic activity (hence, energy and carbon flux) generally increase with temperature. Developmental rate and processes also are temperature dependent. However, at least some species regulate body temperature to some degree through physiological or behavioral responses to extreme temperatures.

Insect species show characteristic ranges in temperatures suitable for activity. Aquatic ecosystems have relatively consistent temperature, but insects in terrestrial ecosystems often experience considerable temperature fluctuation, even on a daily basis. As a group, insects can survive at temperatures from well below freezing to 40-50°C (Whitford 1992), depending on adapted tolerance ranges and acclimation (preconditioning). Some insects occurring at high elevations die at a maximum temperature of 20°C, whereas insects from warm environments often die at higher minimum temperatures. Chironomid larvae living in hot springs survive water temperatures of 49-51°C (R. Chapman 1982).

In general, developmental rate of heterotherms increases with temperature. Both terrestrial and aquatic insects respond to the accumulation of thermal units (the sum of degree-days above a threshold temperature) (Baskerville and Emin 1969,Ward 1992,Ward and Stanford 1982). Degree-day accumulation can be similar under different conditions (e.g., mild winter/cool summer and cold winter/hot summer) or quite different along elevational or latitudinal gradients. Anthropogenic conditions can significantly alter thermal conditions, especially in aquatic habitats. Discharge of heated water, artificial mixing of thermal strata, impoundment, diversion, regulation of water level and flow, and canopy opening in riparian zones, through harvest or grazing, severely modify the thermal environment for aquatic species and favor heat-tolerant individuals and species over heat-intolerant individuals and species (Ward and Stanford 1982).

A number of insects survive temperatures as low as -30°C, and some Arctic species survive below -50°C (N. Hadley 1994, Lundheim and Zachariassen 1993). Freeze-tolerant species can survive ice formation in extracellular fluids but not ice formation in intracellular fluids (N. Hadley 1994, Lundheim and Zachariassen 1993). Ice-nucleating lipids, lipoproteins, or both inhibit supercooling to ensure that ice forms in extracellular fluids at relatively high temperatures (i.e., above —10°C) (N. Hadley 1994). Extracellular freezing draws water osmotically from cells, thereby dehydrating cells and lowering the freezing point of intracellular fluids (N. Hadley 1994).

Other species have various mechanisms for lowering their freezing or supercooling points. Voiding the gut at the onset of cold conditions may prevent food particles from serving as nuclei for ice crystal formation. Similarly, nonfeeding stages may have lower supercooling points than do feeding stages (N. Hadley 1994, Kim and Kim 1997). Some insects prevent freezing to temperatures as low as —50°C by producing high concentrations (up to 25% of fresh weight) of alcohols and sugars, such as glycerol, glucose, and trehalose, as well as peptides and proteins in the hemolymph (N. Hadley 1994, Lundheim and Zachariassen 1993). In many cases, a multicomponent cryoprotectant system involving a number of compounds prevents accumulation of potentially toxic levels of any single component (N. Hadley 1994). Cold tolerance varies with life stage, temperature, and exposure time and can be enhanced by preconditioning to sublethal temperatures (Kim and Kim 1997). Rivers et al. (2000) reported that cold hardiness in a pupal parasitoid, Nasonia vitripennis, was enhanced by encasement within the flesh fly host, Sarcophaga crassipalpi, and by acquisition of host cryoprotectants, especially glycerol and alanine, during larval feeding.

Many insects also can reduce body temperature at high ambient temperatures, above 45°C (Casey 1988, Heinrich 1974,1979,1981,1993). An Australian montane grasshopper, Kosciuscola, can change color from black at night to pale blue during the day (Key and Day 1954), thereby regulating heat absorption. Evaporative cooling, through secretion, regurgitation, ventilation, or other means, can lower body temperature 5-8°C below high ambient temperatures when the air is dry (N. Hadley 1994). Prange and Pinshow (1994) reported that both sexes of a sexually dimorphic desert grasshopper, Poekiloceros bufonius, depress their internal temperatures through evaporative cooling. However, males lost proportionately more water through evaporation, but retained more water from food, than did the much larger females, indicating that thermoregulation by smaller insects is more constrained by water availability.

Long-term exposure to high temperatures requires high body water content or access to water because death results from dessication at low humidity (R. Chapman 1982, N. Hadley 1994). N. Hadley (1994) described experiments demonstrating that males of a Sonoran Desert cicada, Diceroprocta apache, maintain evaporative cooling by ingesting xylem water from twigs on which they perch while singing. Although this species has high cuticular permeability, even at nonstressful temperatures, water loss ceases at death, indicating active cuticular pumping of body water. A 0.6-g cicada maintaining a temperature differential of 5°C must siphon at least 69 mg xylem fluid hour-1. Laboratory experiments indicated that maintaining this temperature differential resulted in a 5% increase in metabolic rate over resting levels. These cicadas probably have additional energetic costs associated with rapid extraction and transport of ingested water to the cuticle.

Thermoregulation also can be accomplished behaviorally. Heinrich (1974, 1979,1981,1993) and Casey (1988) reviewed studies demonstrating that a variety of insects are capable of thermoregulation through activities that generate metabolic heat, such as fanning the wings and flexing the abdomen (Fig. 2.10). Flight can elevate body temperature 10-30°C above ambient (R. Chapman 1982, Heinrich 1993). A single bumble bee queen, Bombus vosnesenskii, can raise the temperature of the nest as much as 25°C above air temperatures as low as 2°C, even in the absence of insulating materials (Heinrich 1979).

Insects can sense and often move within temperature gradients to thermally optimal habitats. Light is an important cue that attracts insects to sources of heat or repels them to darker, cooler areas. Aquatic insects move both vertically and horizontally within temperature gradients to select sites of optimal temperatures (Ward 1992). Terrestrial insects frequently bask on exposed surfaces to absorb heat during early morning or cool periods and retreat to less exposed sites during warmer periods (Fig. 2.11). Some insects use or construct shelters to trap or avoid heat. Others burrow to depths at which diurnal temperature fluctuation is minimal (Polis et al. 1986). Seastedt and Crossley (1981a) reported significant redistribution of soil or litter arthropods from the upper 5 cm of the soil profile to deeper levels following canopy removal and consequent soil surface exposure and warming in a forested ecosystem. Tent caterpillars, Malacosoma spp., build silken tents that slow dissipation of metabolic heat and increase colony temperature above ambient (Fig. 2.12) (Fitzgerald 1995, Heinrich 1993). L. Moore et al. (1988) reported that overwintering egg masses and tents of the western tent caterpillar, Malacosoma californicum, occurred significantly more often on sides of trees, or isolated trees, exposed to the sun. Tents of overwintering larvae of the arctiid moth, Lepesoma argentata, occur almost exclusively in the exposed upper

Dragonflies Thermoregulation
FIG. 2.10

10 15

Ambient temperature (°C)

| Thermoregulation by insects. Thoracic and abdominal temperatures of high Arctic and temperate queen bumblebees foraging in the field as a function of air temperature; Tb = body temperature, Ta = ambient temperature. Arctic queens forage at significantly higher abdominal temperature than do temperate queens. From Heinrich (1993) with permission from Harvard University Press and Bernd Heinrich.

Bernd Heinrich
| Many insects, such as dragonflies, raise their body temperatures by basking. Heat absorption is enhanced by dark coloration and orientation. Photo courtesy of S. D. Senter.

canopy and significantly more often on the south-facing sides of host conifers in western Washington in the United States (D. Shaw 1998).

Some insects regulate body temperature by optimal positioning (Heinrich 1974,1993). Web-building spiders adjust their posture to control their exposure to solar radiation (Robinson and Robinson 1974). Desert beetles, grasshoppers, and scorpions prevent overheating by stilting (i.e., extending their legs and elevating the body above the heated soil surface) and by orienting the body to minimize the surface area exposed to the sun (Heinrich 1993).

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