Excretion

The typical insect nephritic organs (Malpi-ghian tubules) are long, thin, blindly end ing tubes arising from the gut near the junction of midgut and hindgut and extending freely in the body cavity. Their numbers vary among different groups from a few to hundreds. The wall of the tubule is one cell thick, encircling a lumen. These cells extract waste products of metabolism from the blood, nitrogenous by-products usually in the form of uric acid but also as urea and ammonia. Potassium, sodium, and other inorganic ions are also eliminated, along with a quantity of water.

The maintenance of constant salt levels, water, osmotic pressure in the hemolymph, and elimination of nitrogenous wastes are the main excretory tasks (Maddrell 1971) of the Malpighian tubules in insects. Those organs are present in the other groups, although they may be replaced by ne-phridial glands in arachnids and some chilopods.

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