The common physical agents that cause injuries are: too high or too low temperature, high or low level of humidity, radiation, oxygen and carbon dioxide level in environment.
Insects vary in their reactions to unfavourable physical agents as well as in their tolerance to these agents. The disease, resulting from a physical agents, is generally manifested in the coagulation of proteins in the cell, in the malfunction of enzymes and hormones, and in the mechanical alteration of the non-cellular parts of insects.
Insects have range of optimum temperature and beyond the upper and lower temperature limits insect can be injured or even die. The upper lethal limits are usually between 40 and 50°C, but some insects as desert insects can withstand temperature about 60°C. An insect can usually withstand high temperature better in a dry atmosphere than in a moist atmosphere. Also insects in nature are generally able to escape from overheating, but escape from high temperature in captivity is really difficult.
The effects of too high temperature results in:
At the cellular levels, there may be denaturation of proteins and the melting of lipids and phosphatides.
Before death from heat takes place, the insects become motionless and may draw their legs up tightly against their bodies but still react when pressed between the fingers or otherwise stimulated. Sometimes the reaction up to this point is reversible and sometimes it is not.