At the end of each blind tubule of the nephridium is a ciliated flame cell . As fluid passes down the tubule, solutes are reabsorbed and returned to the body fluids. Image from W.H. Freeman and Sinauer Associates, used by permission. Image from W.H. Freeman and Sinauer Associates, used by permission. Body fluids are drawn into the Malphigian tubules by osmosis due to large concentrations of potassium inside the tubule.
Body fluids pass back into the body, nitrogenous wastes empty into the insect's gut.
Water is reabsorbed and waste is expelled from the insect. Images from W.H. Freeman and Sinauer Associates, used by permission. ALL vertebrates have paired kidneys. Excretion is not the primary function of kidneys. Kidneys regulate body fluid levels as a primary duty, and remove wastes as a secondary one.