Physical conditions for trapping air by a microtrichia-covered insect cuticle during temporary submersion

The intertidal midge Clunio, which reproduces on exposed rocky seashores, becomes enclosed in an irregularly shaped air bubble during short submersion by incoming waves. This water-repellent property of Clunio's cuticle is caused by a complete cover of hydrophobic microtrichia offering an effective surf tolerance. These microtrichia not only trap a thin air layer above the cuticle but also maintain a larger air bubble between the insect's ventral side and legs. The effectiveness of the water repellence was quantitatively characterised on the basis of a known model (Crisp and Thorpe, Discuss Faraday Soc 3:210-220, 1948). The parameters of the model are the contact angle theta (>90 degrees) at the contact line of air/water/microtrichia and the distance between individual microtrichia and their radius. When the microtrichia are 1.1 microm apart and have a radius of 0.1 microm and an estimated contact angle theta of 140 degrees, the air layer is stable against hydrostatic pressures of up to 3 m water column. As shown by a modified version of the model, considerably larger air bubbles can be trapped by the microtrichia cover of the legs up to distances of 0.5 mm from the body. The widely spaced (about 8 microm apart) and longer setae of Clunio are not involved in the formation of air layers and air bubble.