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The Malpighian tubule system is a type of excretory and osmoregulatory system found in some insects, myriapods, arachnids and tardigrades. It has also been described in some crustacean species, [ 1 ] and is likely the same organ as the posterior caeca which has been described in crustaceans.
The digestive system is typical of insects, with Malpighian tubules discharging into the midgut. Carbohydrates are digested mainly in the crop, while proteins are digested in the ceca of the midgut. Saliva is abundant but largely free of enzymes, helping to move food and Malpighian secretions along the gut.
They act either on the Malpighian tubules to inhibit urine production, or on the hindgut to stimulate reabsorption. To date, the only insect for which both diuretic and antidiuretic hormones (acting directly on tubules) have been isolated is a beetle, the mealworm Tenebrio molitor (Tenebrionidae).
A grasshopper has an open circulatory system, where hemolymph moves through interconnected sinuses or hemocoels, spaces surrounding the organs. Above is a diagram of an open circulatory system. An open circulatory system is made up of a heart, vessels, and hemolymph.
Stylised diagram of insect digestive tract showing Malpighian tubule (Orthopteran type) Stomatodeum (foregut): This region stores, grinds and transports food to the next region. [7] Included in this are the buccal cavity, the pharynx, the oesophagus, the crop (stores food), and proventriculus or gizzard (grinds food). [4]
Stylized diagram of insect digestive tract showing malpighian tubule, from an insect of the order Orthoptera. The first section of the alimentary canal is the foregut (element 27 in numbered diagram), or stomodaeum. The foregut is lined with a cuticular lining made of chitin and proteins as protection from tough food.
The Malpighian tubules are not free in the hemocele but are bound to the wall of the rectum by the perinephric membrane. This structure allows efficient resorption of water from diuresis and absorption of atmospheric water that is present in the hindgut as humidity. An adaptation for water conservation. [1] [2]
To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood by Malpighian tubules, structures that also evolved independently in insects. [3] While the marine horseshoe crabs rely on external fertilization, air-breathing chelicerates use internal but usually indirect fertilization.