The physiology of excretion in the cotton stainer, Dysdercus fasciatus, Signoret. IV. Hormonal control of excretion.

Considering the close analogy between the insect median neurosecretory cell (m.n.c.)/ corpus cardiacum (c.c.) complex, and the vertebrate hypothalamo-hypophyseal system (Scharrer & Scharrer, 1944), few studies have been made on the possible hormonal control of water metabolism in insects. In fact, our present knowledge of this aspect is confusing and, in some cases, contradictory (Wall & Ralph, 1962 a). Some of the confusion may arise because the control of water balance in insects is achieved by a number of different mechanisms. More critical studies must be made if this situation is to be resolved. When studying the hormonal control of physiological and biochemical mechanisms it is imperative that the mechanisms themselves be understood, and also that indirect effects, introduced by experimental procedures, can be recognized. Wrong conclusions may be reached if these elementary precautions are not observed. Such dangers are inherent in some techniques used to study neurosecretion in insects. For example, removal of m.n.c. is a common operative technique used to study the way in which these cells influence a particular physiological process. It is now known that these cells control a large number of physiological processes (Wigglesworth, 1964). Considerable caution must therefore be exercised when interpreting such experiments, especially in recognizing indirect effects caused by disturbance of related physiological events. For this reason a somewhat indirect method has been used to study the possible hormonal control of excretion in Dysdercus. In this insect there are two distinct phases of excretion (Berridge, 1965 a). The excretory phase may be considered as a period of diuresis; the Malpighian tubules generate a rapid flow of liquid and there is a large output of urine, because the rectum reabsorbs little of this liquid (Berridge, 1965 c). Conversely, since no urine is excreted during the post-excretory phase, this may be considered as an antidiuretic period. The primary cause of this antidiuresis is complete inactivity of the Malpighian tubules (Berridge, 19656). Clearly, the two phases of excretion are intimately connected with changes in Malpighian tubule activity. Investigations have been made on isolated Malpighian tubules in order to determine how these changes in activity may be controlled. The advantages of these isolated preparations are twofold: first, indirect effects are largely avoided, and secondly, because the rate of urine flow is easily measured, the isolated tubules can be used as a sensitive method of biological assay.