Factors Determining the Temperature of Terrestrial Arthropods in Sunlight

Abstract

1. A study has been made of the factors determining the temperature of terrestrial arthropods in sunlight. 2. For such animals the most important forms of heat exchange, which determine the equilibrium temperature, are radiation and convection. Compared with these, evaporation and metabolism are insignificant except under unusual circumstances. 3. Convection depends on the size of the body in such a way that the temperature of similar animals in similar circumstances will vary as about the square root of the linear dimension. Other morphological features affecting temperature are shape, orientation and colour. 4. The factors on which such animal temperatures depend are difficult to measure, and it is unlikely that temperatures will ever be accurately deduced from heat-balance considerations. But the insignificance of metabolism and evaporation implies that the temperature of the living animal in a given situation is likely to be very similar to that of an inanimate body of the same approximate size, shape, colour and orientation. If the temperature of such a body can be measured it forms a type of thermometer giving information of direct biological significance. 5. Terrestrial arthropods may be as warm as, or warmer than, the ‘warm-blooded’ animals, particularly on the ground where conditions change rapidly over small distances so that animal temperatures must be profoundly affected by behaviour. 6. Laboratory data on evaporation at high temperatures do not apply directly to conditions in the field, because an animal warmed by the sun will not necessarily lose water at the same rate as one warmed to the same temperature by warm air at the same relative humidity.