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a Data mainly from Stobbart and Shaw (1974).

a Data mainly from Stobbart and Shaw (1974).

tubules, showed that the isosmotic condition is retained over a wide range of external concentrations. Indirectly, however, the tubules are important in regulation, as the rate at which ions and water are excreted from the body is the difference between their rate of secretion into the tubule lumen and their rate of resorption by the rectum.

As noted in Section 3.2, in some insects the tubules show regional differentiation, secretion taking place in the distal part and resorption beginning in the proximal part of the tubule. In most species, however, resorption occurs mainly in the rectum, though the ileum may also modify the fluid. In the rectum major changes occur in the osmotic pressure and composition of the urine (Table 18.2). Generally, the urine becomes greatly hypertonic to the hemolymph, but when much water is available a hypotonic fluid may be excreted.

In the rectum, water is resorbed against a concentration gradient; that is, it is an active process and energy is expended. Phillips (1964a) showed that in Schistocerca the rate of water movement across the rectal wall is independent of the rate of salt accumulation. The rate at which water is resorbed depends on the osmotic gradient across the wall, and, as the gradient increases during resorption, the point is reached at which the rate of active accumulation is balanced by the rate of passive diffusion back into the rectal lumen; that is, the concentration of the rectal fluid reaches a maximum value. However, this value varies according to the water requirements of the insect. For example, locusts that have been kept in a dry environment and given strong saline to drink have a rectal fluid whose osmotic pressure is about twice that of insects with access to tap water. The physiological basis of this increased ability to concentrate the urine is not known.

The precise mechanism of water uptake is still unclear. Though models have been proposed in which water per se is actively transported across the rectal wall, there is now direct evidence that water movements occur as a result of active movements of inorganic ions, especially sodium, potassium, and chloride (secondary transport of water) (Phillips, 1977). Fine-structural studies of the rectal wall of Calliphora (Berridge and Gupta, 1967), Periplaneta (Oschman and Wall, 1969) and other insects, and the elegant work of Wall and Oschman (1970) and Wall et al. (1970), who used micropuncture to obtain fluid samples from the subepithelial sinus and intercellular spaces in the rectal epithelium, led to the following scheme for water absorption from the rectum (Figure 18.5). Ions are actively secreted into the intercellular space between the highly convoluted plasma membranes of adjacent epithelial cells so that local pockets of high salt content are formed. Thus, an osmotic gradient is developed down which water flows from the rectal lumen to the intercellular spaces via the cytoplasm of the epithelial cells. Water may also enter the intercellular spaces directly from the rectal lumen if the apical septate junctions are leaky as has been proposed. The entry of water into the spaces produces a hydrostatic pressure that forces the ions and water toward the hemolymph. As the fluid moves through the larger (inner) intercellular spaces and subepithelial sinus, active resorption of ions occurs across the epithelial cell membrane. However, relatively little water moves into the cells because the spaces have a low surface area/volume ratio (i.e., the plasma membrane of the cells is not convoluted in these regions as it is in the distal intercellular spaces).

Ramsay's early work on Rhodnius and Dixippus provided a strong indication that the rectum is also capable of resorbing salts, and this has been confirmed by Phillips (1964b) in Schistocerca. This author showed that sodium, potassium, and chloride ions are accumulated

FIGURE 18.5. Scheme to explain water absorption from the rectum. Active secretion of solute into the intercellular channels induces passive movement of water into the channels from the epithelial cells and, perhaps, directly from the rectal lumen. The intercellular fluid thus formed flows toward the hemocoel, and, as it moves through the sinuses, solute is actively resorbed by the cells for recycling. For further details, see text. [After S. H. P. Maddrell, 1971, The mechanisms of insect excretory systems, Adv. Insect Physiol. 8:199-331. By permission of Academic Press, Ltd. and the author.]

FIGURE 18.5. Scheme to explain water absorption from the rectum. Active secretion of solute into the intercellular channels induces passive movement of water into the channels from the epithelial cells and, perhaps, directly from the rectal lumen. The intercellular fluid thus formed flows toward the hemocoel, and, as it moves through the sinuses, solute is actively resorbed by the cells for recycling. For further details, see text. [After S. H. P. Maddrell, 1971, The mechanisms of insect excretory systems, Adv. Insect Physiol. 8:199-331. By permission of Academic Press, Ltd. and the author.]

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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