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A Malpighian tubule is an anatomical structure found in the digestive system of insects that live on land. Usually, these tubules excrete solid waste products, creating an efficient way for the insect to expel toxins and products left over from the breakdown of the insect’s food. Sometimes, a Malpighian tubule is used for functions that are specific to a certain type of insect. For example, larvae of certain species of beetles use Malpighian tubules to produce silk.
The structure takes its name from Marcello Malpighi, an Italian scientist who lived during the 17th century. Malpighi was one of the first scientists to use the microscope in his studies. He made important discoveries using its lenses. Malpighi was the first to see capillaries, and he saw the first Malpighian tubule. He is commonly considered to be the father of comparative anatomy and microscopy.
The tubules are located at the junction between the insect’s midgut and hindgut. The tubules empty into the rectum, where the waste is stored for excretion. The number of tubules varies by species. Some have only a few, while others might have hundreds of tubules. The length of the tubules varies according to how many there are: insects with few tubules have long, twisted ones while insects with many tubules have short structures.
The tubules absorb uric acid, which is a byproduct of insects’ digestive process, from the midgut. The tubule cells convert the acid, which is in solution, into crystals of uric acid. This creates a solid waste product that is similar to a bird’s.
Some species have additional Malpighian tubules that have special functions. In the larval stage, many beetle larvae and caterpillars have tubules that excrete silk. They use the material to form the cocoon in which they transform from one stage to the next, and the insects lose the ability to create the silk when they mature to the adult stage. Leaf beetles, however, keep a specialized set of Malpighian tubules into adulthood. They create a sticky material with which the beetle coats its eggs.
The Malpighian tubule is advantageous for insects because they allow the insects to create solid waste as an alternative to the liquid waste produced by mammals, fish and water insects. The production of solid waste conserves water, allowing the insects to go longer without drinking in case access to water is cut off. It also enables the insects to be lighter because the crystals of uric acid are lighter than the acid would be in solution.
I believe some spiders also have Malpighian tubules, but there's not much information I can find on them, such as whether they are used in the same way or if they are part of the evolutionary process.