The diverse excretory systems of insects exhibit several features that appear unusual when comparisons are made with the mammalian kidney. Secretion by the Malpighian tubules of a fluid that is unlike the blood in composition, substitutes for glomerular filtration. Various reabsorptive functions, such as volume reduction, regulation of individual electrolytes, adjustment of osmotic concentration and pH regulation, which are associated with distinct renal segments in the mammalian kidney, all occur simultaneously in the rectum of terrestrial insects. Involvement of an extracellular molecular sieve in selective reabsorption is novel. As far as water transport is concerned, the rectal pads of the cockroach and locust appear to accomplish, across a single layer of cells, the same function as the countercurrent multiplier system of the mammalian kidney with its several epithelial layers. Direct absorption of water vapor in the rectum of some insects from atmospheres of low relative humidity, clearly involves quite different and unknown mechanisms. Finally, saline-water insect larvae produce hyperosmotic excreta by direct secretion of ions into the rectal lumen. They can adjust individual transport processes to form various secretions, which are appropriate to the natural waters of diverse chemical types in which these larvae thrive.