Stink bugs are notorious pests known for their foul odor, but the query of whether they “pee” requires biological clarification. Like all insects, stink bugs must excrete metabolic waste, but they do not produce liquid urine in the manner of mammals. The liquid often observed is a separate substance, based on the different biological processes insects use to manage water and nitrogenous waste.
How Insects Handle Nitrogenous Waste
Insects, including stink bugs, have evolved a highly efficient system for waste management that conserves water. Unlike mammals that excrete nitrogenous waste as water-soluble urea, insects use specialized organs called Malpighian tubules. These tubules act as the insect equivalent of kidneys, filtering waste products, ions, and water from the hemolymph. The primary nitrogenous waste product is converted into uric acid, a poorly water-soluble compound. This uric acid is transported into the gut where most of the associated water is reabsorbed. This process results in the excretion of nitrogenous waste as a dry or semi-solid paste, which is mixed with the feces and passed out through the rectum.
The Liquid Mistaken for Urine
The liquid commonly observed when a stink bug is threatened or crushed is not the product of metabolic excretion, but a defensive secretion. This fluid is actively released from specialized glands located on the underside of the thorax, between the first and second pair of legs. This liquid is a voluntary defense mechanism, deployed when the insect is disturbed or injured. The defensive liquid is sprayed or oozed onto a rough patch of the exoskeleton, known as the evapatorium, which helps the chemical components quickly vaporize into the air. The purpose of this foul-smelling fluid is to deter predators. The irritating nature of this secretion can be potent enough to cause temporary discomfort to eyes or skin if contact is made.
The Chemical Makeup of the Stink
The pungent odor of the defensive fluid comes from a cocktail of volatile organic compounds. These compounds are chemically distinct from the uric acid paste that constitutes the bug’s actual waste. The major components that create the smell are aldehydes, such as trans-2-octenal and trans-2-decenal. These aldehydes are highly volatile, allowing the smell to quickly spread and function as an effective repellent. The chemical structure of these compounds makes them irritating and even toxic to small predators. The odor lingers because these chemicals cling to surfaces, serving as a warning signal long after the bug has departed.