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41.9: Excretion Systems - Malpighian Tubules of Insects

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    14073

    • Boundless
    • Boundless
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    Learning Objectives
    • Explain how insects use malpighian tubules to excrete wastes and maintain osmotic balance

    Malpighian tubules line the gut of some species of arthropods, such as bees. They are usually found in pairs in the posterior regions of arthropod alimentary canals; the number of tubules varies with the species of insect. The system of malpighian tubules consists of branching tubules, which increase their surface area, near the hemolymph (a mixture of blood and interstitial fluid that is found in insects, other arthropods, and most mollusks) and fat tissues. They are lined with microvilli for reabsorption and maintenance of osmotic balance. They contain actin for support.

    Malpighian tubules work cooperatively with specialized glands in the wall of the rectum. Body fluids are not filtered, as in the case of nephridia. Instead, urine is produced by tubular secretion mechanisms by the cells lining the malpighian tubules that are bathed in hemolymph. Metabolic wastes, such as urea and amino acids, freely diffuse into the tubules, while ions are transported through active pump mechanisms. There are exchange pumps lining the tubules which actively transport H+ ions into the cell and K+ or Na+ ions out; water passively follows to form urine. The secretion of ions alters the osmotic pressure, which draws water, electrolytes, and nitrogenous waste (uric acid) into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low-water environments and uric acid is precipitated and excreted as a thick paste or powder. By not dissolving wastes in water, these organisms are able to conserve water; this is especially important for life in dry environments.

    image
    Figure \(\PageIndex{1}\): Malpighian tubules in bees: Malpighian tubules of insects and other terrestrial arthropods remove nitrogenous wastes and other solutes from the hemolymph.

    Key Points

    • Malpighian tubules are found in the posterior regions of insects, where they work with glands in the rectum to excrete waste and maintain osmotic balance.
    • Ions are transported through active pumps found in the malpighian tubules; as the ions are secreted, water and waste are drawn to the tubules due to the change in osmotic pressure.
    • Nitrogenous wastes, such as uric acid, are precipitated as thick pastes or powder to be excreted.

    Key Terms

    • malpighian tubule: a tubule that extends from the alimentary canal to the exterior of the organism, excreting water and wastes in the form of solid nitrogenous compounds
    • uric acid: a bicyclic heterocyclic phenolic compound, formed in the body by the metabolism of protein and excreted in the urine
    • hemolymph: a circulating fluid in the bodies of some invertebrates that is the equivalent of blood

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