Studies on the Water Relations of Adult Locusts — III The Water Balance of Non-Flying Locusts

Abstract

The object of this work was to draw up a balance sheet of water gains and losses in non-flying adult Locusta. Metabolic water is produced at the rate of 0,6 mg/locust/h and is insufficient to balance transpiratory losses (10,3 mg/locust/h in dry air). Adult locusts are unable to absorb water vapour from air of up to 97,5 % R.H., nor are they able to absorb liquid water through the body surface or anus. They do, however, drink and can gain as much as 120 mg water/g weight in 30 min. It is shown that desiccation or exsanguination elicit a strong drinking response. Food is shown to be the most important source of water-51,3 mg/locust/h if the food is fresh grass and losses in the faeces in this case amount to 30,7 mg/locust/h. If fed on dried grass the locust gains only 0,2 mg/locust/h, partly because of feeding inhibition when only dry food is available. At the same time faecal water loss is 1,6 mg/locust/h and taken with transpiratory water loss leads to a substantial deficit Transpiratory water loss is discussed and evidence given for the reduction in ventilation of Locusta, Schistocerca and Chortoicetes when severely desiccated by feeding on dry food. Survival under conditions of non-replenishment of food and water reserves was assessed. Locusta starved at 91% R.H. died after an average of 189h when their fat reserves were reduced to 6,6% Give weight basis) but had normal water content. Locusta starved at 0% R.H. died after an average of 134h when the water content was reduced to 55%, the fat content still being 9,1 %. It is shown that 20% of the water in normally hydra ted locusts is present in the blood, and most of this appears to be available as a water reserve. Estimates of the fat consumption of starving locusts (0,26 mg/g/h) show good agreement with the fat consumption calculated from oxygen consumption data. The water balance sheet clearly indicates that, in feeding locusts, the gain of water in the food and loss in faeces are the most important avenues of gain and loss. Regulation of water loss in the faeces is discussed.