<p>Water and electrolyte regulation is essential for organismal survival. In mammals, the nephron operates a futile cycle of ultrafiltration and reabsorption that accounts for most of the entropy generation of the kidney. In contrast, insect Malpighian tubules form urine primarily through electrolyte secretion. Here, we develop a thermodynamic model of Malpighian tubule function in which entropy generation is estimated from electrical dissipation, with cationic urinary flow treated as current and transepithelial voltage as the potential difference. The model is applied to fluid secretion in the Malpighian tubules of the yellow fever mosquito, the stick insect, and the fruit fly, and to fluid reabsorption in the rectum of the desert locust and the fruit fly. The criterion of efficacy is the ratio between entropy generation and water flow, representing the energy dissipated per unit urinary volume. The results are compared with estimates for humans, mice, and dairy cattle, spanning eight orders of magnitude in body mass. The excretory systems of insects are shown to be more economical than those of mammals. Nevertheless, the differences are remarkably small across all species examined, despite the enormous variation in body size. These results confirm the hypothesis, proposed in the 1980s, that Malpighian tubules operate with greater efficacy than mammalian nephrons and further suggest that fundamental thermodynamic constraints shape the efficiency of epithelial transport systems across widely divergent organisms.</p>