Box 3.4. Cryptonephric systems
Many larval and adult Coleoptera, larval Lepidoptera, and some larval Symphyta have a modified arrangement of the excretory system that is concerned either with efficient dehydration of feces before their elimination (in beetles) or ionic regulation (in plant-feeding caterpillars). These insects have a cryptonephric system in which the distal ends of the Malpighian tubules are held in contact with the rectal wall by the perinephric membrane. Such an arrangement allows some beetles that live on a very dry diet, such as stored grain or dry carcasses, to be extraordinarily efficient in their conservation of water. Water even may be extracted from the humid air in the rectum.
In the cryptonephric system of the mealworm, Tenebrio molitor (Coleoptera: Tenebrionidae), shown here, ions (principally potassium chloride, KCl ) are transported into and concentrated in the six Malpighian tubules, creating an osmotic gradient that draws water from the surrounding perirectal space and the rectal lumen. The tubule fluid is then transported forwards to the free portion of each tubule, from which it is passed to the hemolymph or recycled in the rectum.