Nitrogenous Excretion And Salt

hormone has a dual action, causing accelerated secretion through the tubules and a slowing water balance down of water resorption through the rectal wall. In Rhodnius and other insects that have two diuretic factors, it is believed that these work synergistically so that a small change in the amount of either will induce a large change in the rate of production of the primary fluid. In other words, this arrangement endows the excretory system with great sensitivity, enabling the Malpighian tubules to react quickly to changes in water load within the body.

An ancillary effect of diuretic peptides and serotonin is to stimulate contraction of the muscles on the outside of the tubules, enhancing their writhing movements. This will reduce the thickness of the unstirred layer of hemolymph adjacent to the tubule, as well as improving fluid flow within the tubule (Coast, 1998b).

The nature and mode of action of antidiuretic factors are less well understood, and until recently it was thought that they act only at the level of the rectum, enhancing fluid uptake. One possible means of achieving this would be through stimulation of ion secretion as proposed in the scheme for water resorption outlined earlier (Figure 18.5). A chloride transport-stimulating peptide (CTSP) (molecular weight 8000) isolated from the corpora cardiaca of Schistocerca stimulates resorption of these ions from the rectal lumen (Spring, 1990; Phillips and Audsley, 1995), but whether it promotes water resorption has not been determined. Curiously, a second peptide (ion transport peptide [ITP]) from the same source and having a similar molecular weight (8500) has been identified (Phillips et al., 1998). ITP also stimulates chloride ion resorption, but in the ileum, and the consensus is that they are different hormones.

For a few insects, including Rhodnius and Tenebrio, the antidiuretic hormone acts on the Malpighian tubules, inhibiting the active transport of potassium ions across the tubule wall (Eigenheer et al., 2002).

A further aspect of hormonal control is that some insects produce both diuretic and antidiuretic hormones simultaneously after feeding, an action that may appear to be counterproductive. Proux et al. (1984) suggested, however, that this action may enhance fluid cycling to clear metabolic wastes from the hemolymph.

Because, presumably, there is a direct relationship between the amount of food consumed, the amount of hormone released, and the quantity of water removed across the tubules or resorbed from the rectal lumen, it is difficult to see how such a simple arrangement will work other than in insects such as Rhodnius whose food has a constant water content. The physical stimulus of "stretching" alone would not provide a precise enough mechanism for water regulation in insects whose food differs in water content. Some other control system must therefore operate. Perhaps an insect can monitor the water content of its food or, alternatively, the resorptive power of the rectal wall may be controlled directly by the hemolymph itself. Phillips (1964b) showed that the rate at which sodium and potassium ions are resorbed is dependent on the ionic concentration of the hemolymph.

Very little work has been done on the hormonal control of salt and water balance in aquatic insects. However, as a result of his experiments on Aedes aegypti, Stobbart (1971) suggested that accumulation of sodium ions across the wall of the anal papillae is under endocrine control. Inthe water boatman, Cenocorixa blaisdelli, and larvae of the dragonfly, Libellula quadrimaculata, extracts of the head and thoracic ganglia, respectively, stimulate Malpighian tubule secretion (Spring, 1990).

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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