Thermoregulation
Objectives
- Gain an insight into the structural, behavioural and physiological processes by which insects thermoregulate.
Termite mound in Northern Territory
Overview
Some definitions:
Homeothermy |
Body temperature is relatively constant and independent of ambient temperature. The term is usually applied to birds and mammals. |
Poikilothermy |
Body temperature is variable and dependent on ambient temperature. |
Endothermy |
Heat that determines body temperature is produced by the animal's own energy metabolism. The only continuously endothermic terrestrial animals are birds and mammals. |
Ectothermy |
Heat that determines body temperature is acquired from the environment by radiation, convection, or conduction. |
Heterothermy |
Endothermic part of the time and ectothermic part of the time. |
Thermoregulation is the active control of heat gain or loss.
Insects are very flexible in their thermal homeostasis patterns. Within a single order there are species that fly with a thoracic temperature of 0°C and others that operate during flight with a thoracic temperature of 45°C. Thus phylogenetic arguments do not explain the patterns of temperature regulation seen in insects. Insect size is a much better indicator.
Heat balance can be described by the equation:
dH/dt :change in heat in the body per unit time
M : amount of heat produced, eg. Metabolic heat
C : the body’s conductance or tendency to lose heat
Tb-Ta : the difference between the body’s temperature and the ambient temperature.
Smaller insects have much higher rates of heat loss because their conductance is greater due to the larger surface area: mass ratio.
Heat can be lost by conduction, convection, radiation, and evaporation.
Activities
- Minilecture on thermoregulation for this module.
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Mini-lecture:
Insect Flight
Flight requires a high level of energy expenditure. Power output cannot drop below a certain (relatively high) threshold, otherwise the insect would drop to the ground.
Types of flight muscle: DLM and DVM
Synchronous and asynchronous
Winter flying moth can fly with body temperature around 1°C. The muscles have an increased sarcoplasmic reticulum: Ca++ transport
Warm-up by Shivering
Larger insects are forced to operate at elevated temperatures because they cannot lose heat rapidly enough to operate at lower temperatures. They have therefore evolved to operate most efficiently at elevated temperatures. They are relatively inefficient at low temperatures and require a warm-up mechanism.
The primary mechanism is for the flight muscles—DLM and DVM—to work against each other. They fire synchronously and the wings do not flap but work is being done. Sometimes visible in the form of high frequency “whirring” of the wings. Common in moths and dragonflies.
Honey- and bumblebees have “myogenic” (asynchronous) flight. Stretching is required to fire the muscles at normal flight frequency: nerve impulses are sent at lower frequency than the actual wing beat frequency. To utilise shivering warm-up they show a special pattern of nerve impulses. Long bursts of action potentials cause the muscles to undergo prolonged tetanus-like contractions. The bursts are sent synchronously to the opposing muscle groups so that there is no net movement.
Shivering produces progressive temperature increase, which increases the efficiency of the muscle and produces even more heat until heat loss by conductance stabilises body temperature. There are certain minimum temperatures below which shivering is not capable of warming the insect to flight temperatures.
Warm-up has to rapid otherwise heat is lost by conductance.
Some social hymenoptera forage when ambient temperatures are 1 or 2°C—temperatures too low to allow shivering warm-up. They can fly because the nest is kept warm enough to allow them to begin flight.
Warm-up by Basking
Passive warming in sunlight. Less predictable than shivering but uses less energy. Because of their small mass insects warm relatively rapidly in sunlight, eg. a 100 mg insect would warm at about 10°C per minute.
Typically take up positions that maximise solar input and minimise heat loss by convection.
Orient the body to the sun, hold the wings to minimise air flow over the body.
Pumping wing movements are common in basking butterflies. It has been suggested that this is a mechanism to prevent localised overheating and tissue damage in the wings.
Heat Loss
Temperatures at or above 45°C are generally dangerous.
Small insects rarely reach these temperatures because the conductive heat loss is so high that the body temperature is near ambient even during flight.
Flight costs are relatively independent of flight speed, so that heat build-up during hovering is a possibility because convective heat loss is greatly reduced at low velocities.
Behaviours
Tiger beetles (Cicindela spp) found on hot sand. At sand temperatures above 40°C they “stilt”, raising their bodies off the substrate away from the hottest temperatures nearer the ground. At higher temperatures they undergo short flights. The convective heat loss far outweighs the extra heat generated by flight metabolic activity.
Dragonflies take on a vertical basking position whereby exposure to solar radiation is minimised and convective heat loss is maximised (called the “obelisk” position).
Heat Radiators
Increase the surface area available for convective heat loss.
Members of Odonata, Lepidoptera, Diptera, and Hymenoptera use a fluid-transfer cooling mechanism that shunts heated haemolymph into the abdomen from where it is dissipated. Experimental ligature of the heart results in the insect rapidly overheating.
Evaporative Cooling
Efficient but would require very large water reserves in insects because of their surface area:mass ratios.
Honeybees regurgitate fluid from the crop which evaporates from the mouthparts during flight. The close contact between the head and thorax results in convective heat loss. Temperature sensors are in the head because localised heating of the head (and not the thorax) immediately results in regurgitation.
Water wastage is not such a problem because of the honeybee’s mobility and ability to replenish water.
Normally fly with a thoracic temperature 15°C above ambient. Can even fly at ambient temperatures of 45°C and keep the thorax at about the same temperature.
Sawfly larvae can emit fluid from the anus that is spread over the body and allows evaporative cooling.
Some desert cicadas release fluid from pores on the dorsal surface.
Structures
- Insulation
- Airsacs
- Scales
- Stilt-like legs in desert insects
Bees have a coiled dorsal vessel in the petiole. Suggested to function as a counter-current mechanism for maintaining elevated temperatures of the thorax, preventing warming of the abdomen by haemolymph.
Cool abdominal haemolymph is pumped anteriorly through the petiole within the dorsal vessel.
Warm thoracic haemolymph is forced back through the petiole.
The presence of coils in the dorsal vessel maximises heat exchange.
Thermoregulation: advantages
Complete a generation in one season by basking, seen in Arctic insects.
In arctic bumblebees, the females incubate their ovaries. The abdominal temperature is elevated by comparison with the workers and other bumblebee species. Allows rapid egg development.
Cold & Heat Tolerance
Supercooling
Body tissues drop to below the freezing point without the formation of ice crystals. European wasp queens overwinter by hanging from their mandibles in an underground cavity. Minimises the probability of contacting ice crystals which would cause the body tissues to crystallise. Can supercool to -15°C.
Many supercooling insects have high concentrations of glycerol in the haemolymph, which acts as an “antifreeze”. It inhibits ice crystal formation.
Thermoregolation in a bumblebee (Bombus) (a) Sagittal section of a bee showing the features involved in regulating thoracic temperature (after Heinrich, 1976). (b) Detail of the heat exchange system in the waist region. As hot hemolymph flows back under the ventral diaphragm it loses heat to the cool, forwardly flowing hemolymph in the aorta (after Heinrich, 1976) (c) Temperature of the thorax and gaster of bumblebees in flight. As air temperature rises, the gaster gets markedly hotter The insect cannot fly without overheating when thoracic temperature exceeds 45 •C. (after Heinrich, 1975)
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Thoracic and abdominal temperature during two heating experiments of a tethered bumblebee. Heat was applied only to the thorax. The normal bee (left) prevented Tthx from exceeding 42 • C by shunting excess heat to the abdomen. When the heart was made inoperative (right), the same input of heat to the thorax killed the bee because it no longer dumped excess heat into the abdomen. (From Heinrich, 1 976a.) |
Freeze tolerance
Insect can tolerate the formation of ice crystals. They are usually formed in the haemolymph.
Heat shock proteins
Proteins that are transcribed after a heat shock or other stress. They appear to prevent protein deformation that tends to occur at high temperatures.
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Nests of Social Insects
Honeybees,Apis mellifera, must maintain their nests at or close to 32°C. There can be a major difference between inside and outside temperature in temperate regions during winter. Site selection is very important. Bees cool the hive by “fanning”. They orient along air pathways and increase the airflow.
Bumblebees make small nests which are insulated. The queens incubate the brood by sitting over it and generating heat by shivering.
Ants don’t usually thermoregulate: they tend to carry the brood to the most appropriate site within the nest.
Termites have mounds with a “thermosiphon” effect (see diagram)
Australian “compass termites” build nests that are oriented to catch morning and afternoon sun and have low exposure to the midday sun.
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The Macrotermes bellicosus mound's thermosiphon system controls the nest's climate 1, queen ceil; 2, the "cellar": 3, the "attic" above the main living chamber with the fungus gardens: 4, the air channels in the ribs or flutes where gas and heat are exchanged with the environment. |
References
Heinrich, B. (1996) The Thermal Warriors. QL495. H39
Chapter 31 in Chapman
Kerkut and Gilbert. Vol 4. Chapter 12. QL495.C64
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TOPIC REVIEWDo you know…?
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