Secretion‐grooming in the water bug Plea minutissima: a chemical defence against microorganisms interfering with the hydrofuge properties of the respiratory region

Abstract

Abstract. 1. The water bug Plea minutissima Leach (Hemiptera: Pleidae) from time to time crawls out of the water and distributes secretion from its metathoracic glands over the ventral part of its body. By this ‘secretion‐grooming’ antiseptic gland substances, mostly hydrogen peroxide, are applied to the hydrofuge hairs which hold the respiratory air bubble.2. When the bugs are prevented from leaving the water, their mortality increases significantly, and individuals lacking the air bubble are observed. Examination of such bugs with Scanning Electron Microscopy techniques reveals bacteria growing on their hydrofuge hairs, whereas in bugs allowed to secretion‐groom the hairs are not contaminated by microorganisms.3. Artificial application of diluted metathoracic gland secretion on the body surface of P. minutissima kills bacteria, while specific ciliates, which never colonize the respiratory region, are not harmed by the secretion.4. It can be concluded that secretion‐grooming prevents the contamination of the hydrofuge respiratory region that leads to the wetting of the hydrofuge respiratory regions, resulting in the loss of the air bubble, and, finally, the drowning of the animals.5. Other possible functions of the secretion, such as chemical defence against predators or wetting of the body surface, could not be demonstrated or seem unlikely.