TERRESTRIAL INSECTS AND THE HUMIDITY OF THE ENVIRONMENT

Summary. The gain and loss of water by insects is discussed, also the total amount of water in the insect's body. The paper does not deal with the water content or the osmotic balance of particular organs or tissues: neither does it discuss the movement of water within the insect's body. The subject is on the borderline, where physiology appears to extend and interpret ecological observations. The majority of insects do not drink, but rely largely on the water which is contained in their food. Insects which breed in dry material or live in deserts must be able to resist loss of water, and water formed in metabolism is of great importance to them. In the fasting meal‐worm, metabolism is so adjusted as to produce as much water as is lost by evaporation: this in turn is proportional to the saturation deficiency, at any rate at 23° C. Several insects can gain water from an atmosphere which is nearly saturated. It is difficult to explain this on physical grounds: the vapour pressure of the tissue fluids, including the liquid in the tracheoles, is so close to the saturation vapour pressure of water that condensation into the insect could only occur if the external atmosphere was within 1 per cent. of saturation. Perhaps there is a secretion of water into the body of the insect: this explanation is difficult to accept at first sight, but such secretion would be no more remarkable than the activities of many types of gland. Loss of water is partly by diffusion from the respiratory system. It also takes place from the surface of the body in some insects, but apparently not in all. It is known that the duration of life, or the loss of weight during starvation, of several insects is proportional to the saturation deficiency. This is only true within certain limits: these are reached when the saturation deficiency is either very great or very small. Many insects can reduce their temperature below that of the surrounding air, at least when they are put in air which is fairly dry and above 20° C.: this is presumably due to evaporation. The thermal death‐point is also affected by evaporation. It may be lower in dry air, presumably, owing to excessive loss of water: or it may be higher in dry air, showing that the insect can cool its body by evaporating water—at any rate for a short period. Some insects do not lose water at all, and there is reason to believe that efficient cooling by evaporation is only possible for a relatively large insect: a small insect, in which the ratio of surface to volume is great, gains so much heat by convection, that if it were to compensate by evaporation it would die of desiccation in a very short period. Certain insects can maintain a particular proportion of water in the body even if external conditions change widely, but other insects lose a large proportion of their water without being killed. The normal water content alters with growth, metamorphosis, and other factors. In insects which normally hibernate, a large proportion of water is lost before dormancy. This in itself presumably lowers the temperature at which the tissues would freeze: danger of death from freezing is also reduced in many insects by binding a large proportion of water to the colloids of the body. The maintenance of a due proportion of water in the insect is partly carried out by chemical methods, but it is also due in part to behaviour. Certain insects transfer themselves to regions of less evaporation when the air is dry, or when a material proportion of water has been evaporated from their bodies. The existence of an insect in a very damp atmosphere, which is the normal environment of many of them, depends on the excretion of water through the Malpighian tubes, and on the passage of damp faeces. But a large number of insects, even among those which require a moist environment, are killed by exposure to saturated air. It is supposed that insects can only exist under very dry conditions if they possess several qualities in combination. Loss of water from the alimentary canal must be almost nil: this is assisted by the excretion of solid uric acid, and by efficient extraction of water from the contents of the hind‐gut. Certain insects also appear to reduce the loss of water through the skin to a very low figure. It is assumed that they have no control of the diffusion of water from within their tracheal system. The relation of a particular insect to atmospheric moisture is often very precise; moisture must often be a determining factor. The conditions which are most favourable may perhaps be defined in this way. If low humidity is unfavourable, then the higher the humidity the better, up to the point where elimination becomes impossible: in fact, the optimum is just below the point of danger. Similarly, if growth and reproduction are to be as rapid as possible, the temperature must be just under that which is harmful. The insect's egg may be regarded as a separate problem. Certain eggs can tolerate a considerable loss of water from their contents. In some eggs, at any rate, loss of water is directly proportional to saturation deficiency. The eggs of many insects occur normally in places where the humidity is very high: most of them do not suffer from exposure to saturated air, though a few are known to do so. The death of the egg in air which is too dry is sometimes caused by the death of the embryo, at others by the shell becoming so hard that hatching is impossible. Certain eggs tolerate dryness, which causes them to become dormant. They fall into two classes. In one class, toleration to drying may occur early or late in development, and the embryo itself loses water. In the other class, dryness is only tolerated when the egg is ready to hatch: the larva within it does not lose water, but the shell of the egg becomes water‐tight. In many insects, exposure to a low but not fatal degree of humidity increases the duration of the egg stage.