Physiology of Nitrogenous Excretion

Uric acid is produced in the fat body and/or Malpighian tubules (occasionally the midgut) and released into the hemolymph. How the highly insoluble uric acid is transported in the hemolymph remains unclear though the most likely means seems to be as the sodium or potassium salt, or in combination with specific carrier proteins (Cochran, 1985). The uric acid is secreted into the lumen of the tubules as the sodium or potassium salt, along with other ions, water, and various low-molecular-weight organic molecules. In a typical insect, for example Dixippus, secretion occurs along the entire length of the tubule. No resorption of materials takes place across the tubule wall, and urate leaves the tubule in solution. In the rectum resorption of water and sodium and potassium ions occurs, and the pH of the fluid

FIGURE 18.4. Movements of water, ions, and organic molecules in the excretory systems of (A) Dixippus and (B) Rhodnius. [After R. H. Stobbart and J. Shaw, 1974, Salt and water balance: Excretion, in: The Physiology of Insecta, 2nd ed., Vol. V (M. Rockstein, ed.). By permission of Academic Press, Inc. and the authors.]

FIGURE 18.4. Movements of water, ions, and organic molecules in the excretory systems of (A) Dixippus and (B) Rhodnius. [After R. H. Stobbart and J. Shaw, 1974, Salt and water balance: Excretion, in: The Physiology of Insecta, 2nd ed., Vol. V (M. Rockstein, ed.). By permission of Academic Press, Inc. and the authors.]

decreases from 6.8-7.5 to 3.5-4.5. The combined effect of water resorption and pH change is to cause massive precipitation of uric acid. Useful organic molecules such as amino acids and sugars are also resorbed through the rectal wall. The Malpighian tubule-rectal wall excretory system thus shows certain functional analogies with the vertebrate nephron. The excretion of uric acid in Dixippus is summarized in Figure 18.4A.

In Rhodnius, whose tubules show structural differentiation along their length, the process of excretion is basically the same as in Dixippus. However, in Rhodnius only the distal portion of the tubule is secretory and resorption of water and cations begins in the proximal part. Slight change in pH occurs (from 7.2 to 6.6) as the fluid passes along the tubule and this is sufficient to initiate uric acid precipitation. Further water and salt resorption occurs in the rectum (pH 6.0), causing precipitation of the remaining waste (Figure 18.4B).

Although allantoin is the major nitrogenous waste in many insects, its mode of excretion appears to have been studiedinonly one species, Dysdercusfasciatus (Hemiptera) (Berridge, 1965). This insect is required, because of its diet, to excrete large quantities of unwanted ions (magnesium, potassium, and phosphate). This, combined with the insect's inability to actively resorb water from the rectum, results in the production of a large volume of urine. Because no resorption or acidification occurs which could cause precipitation of uric acid, this molecule is no longer used as an excretory product. Thus, allantoin, which is 10 times more soluble than uric acid (yet of equally low toxicity), is preferred. However, the insect does not possess a mechanism for actively transporting this molecule from the hemolymph to tubule lumen; that is, allantoin only moves passively across the wall of the tubule. It is therefore maintained in high concentration in the hemolymph to achieve a sufficient rate of diffusion into the tubule. Whether a similar mechanism occurs in other allantoin-excreting insects remains to be seen. It may be significant that many other allantoin producers are herbivorous and have the problem of removing large quantities of unwanted ions.

The physiological mechanisms for excretion of other nitrogenous wastes are poorly understood. Aquatic insects are presumed to excrete ammonia in very dilute urine, whereas larvae of meat-eating flies such as Lucilia cuprina and S. bullata produce highly concentrated, ammonia-rich excreta, apparently by actively transporting ammonium ions across the anterior hindgut wall. Urea probably moves passively into the Malpighian tubules and becomes concentrated in the hindgut because of its inability to permeate the cuticular lining as water resorption occurs.

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.

Get My Free Ebook


Post a comment