A Demonstration of Active Transport Using Insect Malpighian Tubules

The insect excretory system consists of a group of tubes, the Malpighian tubules, which extend into the body cavity from the anterior portion of the hindgut (fig. 1). The Malpighian tubules remove nitrogenous wastes from the blood and, in addition, regulate water and salt content (Borror, DeLong, and Triplehorn 1981). Insects such as crickets, grasshoppers, cockroaches, mealworms, and milkweed bugs have many large tubules which are easily exposed by ventral dissection of the abdomen. Individual tubules can be removed and placed in insect Ringer's solution where they will remain functional for several hours. An active transport system in the wall of the tubule pumps potassium ions from outside the tubule (blood) into the inside of the tubule (lumen). In the intact animal, water and metabolic waste materials move along with the postassium ion flow and eventually all molecules enter the rectum. Water and potassium are reabsorbed through the walls of the rectum and the remaining metabolic wastes become part of the fecal material (Eckert and Randall 1978). The following simple experiments will demonstrate the basic properties of the active transport system in the Malpighian tubule.