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excretory organs in the majority of arachnids, myriopods, and insects; they are blind tubular appendages of the intestine, on the boundary between the mesenteron (midgut) and proctodeum (hindgut). In arachnids and myriopods there is one pair of Malpighian tubules, in orthopterous insects up to 120 pairs, and in hymenopterous insects up to 150 pairs. In the wall of the Malpighian tubules there is a muscle layer which enables them to contract. In arachnids the Malpighian tubules excrete primarily guanine; in insects and myriopods, uric acid. In aquatic insects the Malpighian tubules perform an osmoregulatory function.