Figure 36.12: Urine Production in Insects The Malpighian tubules project into the hemocoel, the principal body cavity, which is filled with blood because of the structure of the open circulatory system. There the tubules are bathed with blood. The tubules form primary urine from the blood plasma by a secretory mechanism. Insects use at least three markedly different mechanisms to produce urine that is more concentrated than blood plasma (osmotic U/P > 1.0), usually in the rectum. They can also produce urine that is more dilute than blood plasma (U/P< 1.0).