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An older summary of ricinuleid internal anatomy was published by Jacques Millot. The midgut has been described, while the excretory system consists of Malpighian tubules and a pair of coxal glands. Female ricinuleids have spermathecae, presumably to store sperm. The male genitalia, sperm cells and sperm production have also been intensively studied. Gas exchange takes place through trachea, and opens through a single pair of spiracles on the prosoma. At least one Brazilian species appears to have a plastron, which may help it prevent getting wet and allow it to continue to breathe, even if inundated with water.