On the Suggestion That the Secretion from the Metathoracic Scent Glands of a Surface-dwelling Aquatic Insect, Gerris Najas (de Geer), (heteroptera; Gerridae) Has a Waterproofing Function

The ventral surface of the pond skater, Gerris najas, is covered with a hair pile extremely resistant to water penetration. It is possible that the hydrofuge properties of the hair pile are maintained by an oil released from glands opening out on to the general body surface. Holdgate (1955) has reported findings indicating that many surface-dwelling and surface-breathing aquatic insects actively maintain high and constant contact angles during life. Many bugs are provided with scent glands located ventrally in the hind part of the adult thorax. The metathoracic scent glands of G. najas open into a median reservoir which in turn opens to the exterior by a single median pore. The secretion, an oily fluid, has a slightly unpleasant odour reminiscent of that from tfo-butyl alcohol. More than one previous observer has suggested that the hydrofuge properties of the hair pile of Gerris are maintained by the periodic release of secretion from the metathoracic scent glands. Alone amongst these, Brinkhurst (i960) has described experiments which appear to support this suggestion. The experiments are in no way conclusive, however, and reveal little other than the possible effects of various treatments on the state of filling of the reservoir. This paper describes experiments designed to measure the effect of sealing the external opening of the metathoracic scent gland system of G. najas on the resistance of the hair pile to wetting by surface forces.