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 t...
A few years ago a paper appeared in this Bulletin (Buxton, 1931) describing methods of controlling and measuring humidity. Since that paper was written, we have evolved a number of improvements which tend to make the methods easier in practice, though none of them are novel in principle. It is now known that humidity has diverse and unpredictable effects on many insects. In experimental work it is advisable that humidity should always be measured and controlled even if it is not a factor in the experiment. Moreover, workers should state what method of measurement or control they have adopted.
It is a matter of general agreement that atmospheric humidity is often of great importance in limiting the times or places at which insects are abundant. But though one may collect facts which suggest that a particular degree of dryness or dampness favours some stage of an insect, it is not easy to devise experiments which will give unassailable facts. Some of us feel that the precise study of water relations and water balance may lead to greatly increased knowledge of the living insect, and therefore to results of economic value ; and it is certain that a fuller understanding of these matters is delayed because few know the methods appropriate for measuring and controlling humidity.The purpose of the present paper is to make known a number of practical methods, which might be of service to a worker even in a remote country. As this is my objective, I shall include material which is already well known to physicists, and as our concern is with the conditions in the places where insects are actually living, I have given particular attention to unorthodox devices which can be used in small spaces and which are portable; among the great variety of methods, I have endeavoured to show what is good and bad in each.
gave me invaluable assistance in the preparation of Chapter I (" The Desert Climate ") and Chapter III (" The Floral Environment "). To my wife, my sister, and my mother my best thanks are due for help in preparing the book for the press. P. A. BUXTON. Jerusalem. June, 1923. 7. Curve obtained from Recording Instrument showing daily Fluctuation in Relative Humidity at Gizeh, Egypt, 20th-22nd March, 1922. The figures on the side of the graph represent percentages of humidity (C. B. Williams) p. 8. Graph showing the Relative Humidity at Mosul, N. Mesopotamia, at 8 a.m. and 4 p.m. during an average Week in June and an average Week in December p.
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