Malpighian tubules produce an iso-osmotic filtrate which is

Malpighian tubules produce an iso-osmotic filtrate that is High in K⁺ and low in Na⁺.
Malpighian tubules are slender tubes normally found in the posterior regions of arthropod alimentary canals.
Pre-urine is formed in the tubules, when nitrogenous waste and electrolytes are transported through the tubule walls. Uric acid precipitates out, and sodium and potassium ions are actively absorbed by the rectum, along with water via osmosis. Uric acid is left to mix with feces, which are then excreted.