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cryptonephridial system An arrangement of the excretory system in certain beetles and lepidopteran larvae in which the distal blind endings of the Malpighian tubules are held against the wall of the rectum by a perinephric membrane. A high concentration of ions is established in the tubules, which draws water from the gut by osmosis. This creates a highly efficient mechanism for water conservation in species, such as mealworms (e.g. Tenebrio spp.), that feed on very dry materials.