Figure 2From: Stability of the volume of air trapped on the abdomen of the water spider Argyroneta aquatica Structure of the layer of hair covering the abdomen of the spider Argyroneta aquatica (schematic drawing). a: Vertical cut across three horizontally oriented neighbouring hairs on the abdomen (ab) of the spider. The hairs are part of a layer of the feathered hair with a hydrophobic surface trapping under water (w) a volume of air (a) on the surface of the abdomen. The feathered structure of the hair is presented in Figure 2 b by short “bristles” (short straight lines originating from the surface of the long hair covering the abdomen). To be specific, it is assumed that the solid structures have a hydrophobic surface. The tips of the bristles are in contact with water. The locations at which three phases (air - water - bristle) are in contact are marked by black dots (i.e. points of three-phase-contact). The air/water surface connecting two points of three-phase-contact is curved. The origin of the radius of curvature is located within the aqueous phase. b: Side view on the air (a)/water (w) surface of two short sections of the same or of two different long hairs (h) with a hydrophobic surface. Each section of the surface carries a bristle (br). Two points of three-.phase-contact are marked by small black dots which are a distance (2 r) apart. R e is the radius of curvature of the air/water surface connecting two points of three-phase-contact. The straight line marked by the symbol t is the tangent to the curved air/water surface at one of the points of three-phase-contact. The contact angle θe is measured across the liquid phase (by definition). The drawings refer to an arbitrary value of θe > 90°. The contact angle θe ' is given by: θe ' = 180° − θe.Back to article page