Note that the figure does not show the movement of chloride. We will talk about uric acid formation and excretion later, but for now please note that uric acid also moves into the tubule. Resorption occurs in the proximal portion of the tubule, with potassium moved by active processes, and water following. After movement of urine and uric acid crystals to the rectum, more water can be resorbed, along with selected ions. Some insects have homogeneous Malpighian tubules, such as the stick insect also represented in the figure.
Secretion takes place all along the homogenous Malpighian tubules.
Some movements are active processes, such as sodium and potassium pumps, while others are passive. There is no resorption until urine is moved into the rectum, where several ions, including amino acids are moved back into the hemolymph by active processes. Again, here are data to make the point. The hemolymph is typically high in sodium, while potassium is the major urine ion in the tubules. These data indicate that sodium and chloride are selectively resorbed in the rectum.