Evaporation from the meal-worm ( Tenebrio : coleoptera ) and atmospheric humidity

It is a matter of general agreement among entomologists that the seasonal and geographical distribution of insects is as much determined by atmospheric moisture as it is by temperature, but it is curious that very little critical work has been carried out on the precise physiology of this problem. As it appears to me, the kernel of the matter is this—it is possible that the relation between the insect and the humidity of the atmosphere in which it lives is “simple,” in the sense that the loss of water from the insect depends on some physical law. For instance, it may be that the loss of water is directly determined by relative humidity or by vapour pressure, but it is more probable that it is determined by saturation deficiency ; in saying this I argue from the analogy of the green leaf and the warm-blooded mammal, and also from the physical laws of evaporation. But it is more probable that the relation is “complex” ; for we are not considering the loss of water from a simple or smooth surface, and some at least of the loss will take place within the tracheal system, the tubes of which ramify to all parts of the insect’s body: and we may surely feel certain that the air in the tracheal tubes will be more nearly saturated with moisture than that outside, if the insect is kept in a dry atmosphere. We must also remember the possibility that the insect can control loss of water from the tracheal system by closing the spiracles through which the tracheal tubes communicate with the outside air. Furthermore, the general metabolism of the insect may be affected by the moisture or dryness of the atmosphere in which it is living.