550 against a concentration gradient and independently of the movement of water. Furthermore, the rate of accumulation of these ions depends on their concentrations in the rectal fluid and the hemolymph. In this way the requirements of the insect can be satisfied. In water-fed locusts ions are resorbed from the rectum as quickly as they arrive in the fluid from the tubules, and low rectal concentrations are found (Table 18.2). At the other extreme, in saline-fed animals, the rates of resorption are low and a greatly hyperosmotic fluid is produced.

The cryptonephridial arrangement of Malpighian tubules increases the power of the rectal wall to resorb water against high concentration gradients. The system is particularly well developed in insects that inhabit dry environments, enabling such species not only to extract the maximum amount of water from their feces but also, under fasting conditions, to take up water from moist air (relative humidity at least 88%) by holding open their anus (Machin, 1980). According to Ramsay (1964), the perinephric membrane is impermeable to water, and, under dry conditions, the osmotic pressure of the perinephric cavity is raised mainly because of the presence of potassium chloride. Thus, the concentration gradient across the rectal wall is reduced, facilitating water uptake. Ramsay suggested that potassium and chloride ions are actively transported into the lumen of the perirectal tubules (which contain many mitochondria), with an accompanying movement of water. Resorption of ions and water occurs across the wall of the parts of the tubule bathed in hemolymph. An apparent lack of mitochondria in the perinephric membrane and leptophragma cells led Ramsay to conclude that ions move passively across the membrane in order to balance those removed by the tubule. However, the fine-structural and experimental study of Grimstone et al. (1968) has shown this conclusion to be wrong. These authors found that the leptophragma cells have a normal complement of mitochondria. Active transport of potassium ions occurs across the cells, which are, however, impermeable to water. Chloride ions follow passively. The scheme is summarized in Figure 18.2D.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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