How insects use hydrophobic hairs to control droplet ejection

Adult sharpshooters (Cicadellidae) are insects that use a specialized catapult mechanism to rapidly super-propel droplets during waste excretion. Sharpshooters feed exclusively on nutrient-poor food sources (xylem sap – 95% water), so it is hypothesized that the catapult is beneficial for conserving critical energy. However, in sharpshooters, none of the nymphal instars possess the anal stylet needed for the adult’s catapult mechanism. Here we examine how juvenile sharpshooters (0.15 - 0.8x smaller than adults) are capable of fluid ejection without the use of the anal stylet. We collected high-speed videography of fluid ejection in juvenile sharpshooters from three species, Homalodisca vitripennis, Graphocephala atropunctata, and Graphocephala coccinea (167 trials; N = 4-6 individuals per species, 1-13 trials per individual). Juvenile sharpshooters possess hydrophobic hairs surrounding the anal pore that deform waste droplets as they form through elastocapillary interactions. Using physical models, we examine the role that these hydrophobic hairs play in controlling the direction and debonding of water droplets during fluid excretion. These experiments inform how sharpshooters control fluid ejection at small scales and provides insights into the emergence of biomechanical systems and the fluid mechanics of droplets at small scales.