Impact of ambient humidity on traction forces in ladybird beetles (Coccinella septempunctata)

Many insects possess hairy adhesive foot pads, supplemented with a tarsal secretion, which allow them to securely and reversibly attach to various kinds of natural substrates, also under complex environmental conditions. Conditions such as ambient temperature and humidity have been shown to influence the attachment ability of the dry adhesive foot pads in spiders and geckos. For insects, however, the influence of the environment on attachment ability is lacking. In the present study, we therefore studied the attachment ability of the seven-spotted ladybird beetle (Coccinella septempunctata) at different humidities by traction force experiments. Our results indicate an optimal range of relative humidity with maximal traction forces. At both low (15%) and high (99%) relative humidity a decrease of attachment ability was found. At 60% humidity, the highest attachment forces were revealed. This relationship was found both in female and male beetles, despite of a deviating structure of adhesive setae and a significant difference in forces between sexes. These findings demonstrate that humidity similarly affects the function of both dry and wet adhesive pads, although both types of adhesive systems (wet and dry) are supposed to be based on different physical interactions (capillarity versus van der Waals forces).