Hormonal Control

As in other systems with a homeostatic role, the activities of the excretory system, including both production of nitrogenous waste and osmoregulation, need to be regulated to suit the specific but changing requirements of the insect. This coordination is effected by hormones.

Though there is strong circumstantial evidence for involvement of hormones in the synthesis of uric acid, neither the nature of the factor(s) involved nor the site of action is known. Changes in the rate of uric acid production, for example, occur at specific stages in an insect's life and these can be correlated with fluctuations in the levels of juvenile hormone and ecdysteroids. Similarly, removal of the corpora allata or corpora cardiaca may markedly affect uric acid production though, again, it is uncertain whether the response is direct or indirect, for example, via an influence on amino acid or protein metabolism.

In contrast, the occurrence of both diuretic and antidiuretic hormones is firmly established (Phillips, 1983; Spring, 1990; Coast, 1998a, 2001; Gade, 2004). In most species, neurosecretory cells in the brain produce these hormones that are then stored in the corpora cardiaca, though there are many reports of diuresis-modifying factors in extracts from other ganglia in the ventral nerve cord. Almost all identified osmoregulatory hormones are peptides, though in some insects (e.g., Rhodnius, locusts and crickets) serotonin also appears to be an important diuretic factor.

One or more diuretic hormones are released after feeding in many terrestrial insects. In Rhodnius, which takes intermittent large blood meals, stretching of the abdominal wall brings about hormone release (Maddrell, 1964). In Schistocerca, Dysdercus, and other insects that feed more or less continuously, it is probably the stretching of the foregut that causes release of hormone (Mordue, 1969; Berridge, 1966). Diuretic hormones appear to act primarily on the Malpighian tubules, stimulating them to secrete potassium ions at a greater rate, thereby creating an enhanced flow of water across the tubule wall (Pilcher, 1970). This primary action of diuretic hormone will affect both osmoregulation and the excretion of uric acid. In some insects, for example Calliphora and Schistocerca, diuretic

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