Insects and other terrestrial arthropods have open circulatory systems, with tissues bathed directly in he-molymph contained in sinuses. Excretory organs called Malpighian tubules remove NITROGENOUS WASTES . from the blood and function in osmoregulation (Figure 40.9). These organs open into the digestive tract at the juncture of the midgut and hindgut. The tubules, which dead-end at the tips away from the gut, dangle in the fluid of the body cavity.
Figure 40.9
Malpighian tubules of insects. Divertic-la(outpocketings) of the gut the tubules accumulate NITROGENOUS WASTES . and salts torn the coelomic fluid, and water follows these solutes by osmosis. Most of the alts and water are reabsorbed across the epithelium of the rectum, and the dry nitro-genous wastes are eliminated with the feces.
The transport epithelium that lines a tubule pumps certain solutes, including salts and NITROGENOUS WASTES ., from the blood into the lumen of the tubule. The fluid within the tubule then passes through the hindgut into the rectum. The epithelium of the rectum pumps most of the salt back into the blood, and water follows the salts by osmosis. The NITROGENOUS WASTES . are eliminated as nearly dry matter along with the feces. The insect excretory system is one adaptation that has contributed to the tremendous success of these animals on land, where conserving water is essential.