Insects require certain amount of moisture in their environment just as they need certain degrees of warmth. Actually the requirements of insects for moisture is closely correlated to that of temperature. Moisture alone, nonetheless, is important. Most terrestrial arthropods are at least 70% water by weight. Insects vary in their moisture requirements for normal growth, development and reproduction. Excess moisture or humidity may cause:
Most adverse effects of moisture are due to its scarcity or absence, and many insects are irreversibly injured when their moisture content drops 20%. Desiccation at the time of molting and pupation may cause deformities in the insect following ecdysis.
Insects lose most of their moisture through their tracheal and excretory systems. The loss of water through the tracheal system is controlled by the types of spiracles and by their opening and closing. The amount of moisture lost through the integument is generally small because of the waxy epicuticular covering. Removal of the waxy layer results in rapid dessiccation and death of the insect. This is the basis for the control of some household insects by the use of abrasive and sorptive dusts.