Malpighian Tubules

Malpighian tubules are extensions of the insect digestive system. These blind tubules are attached just before the hindgut and they vary in number from 2 to more than 100 tubules. Urine is formed differently in insects because there is no pressure differential between the body fluids and the excretory organ. Consequently, there is no filtration. Uric acid, potassium, and sodium are secreted into the Malpighian tubules. Water follows because the osmotic concentration of the tubules is much greater than the osmotic concentration of the body fluids.

As the tubular fluids move into the hindgut, several changes occur. First, the fluids become more acidic. Uric acid is less soluble in acid and it precipitates out of solution. Second, sodium and potassium are reabsorbed. As a result, water follows the sodium and potassium leaving crystals of uric acid. The uric acid crystals mix with undigested foodstuffs and are eliminated as near solids. The Malpighian tubules allow the insects to survive on very little water.

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