Ejection of honeydew droplet by phloem sap-feeding insects: Nymph-to-adult mechanism transition from capillary ratchet to catapult

Organisms excrete liquid waste into various types of fluids, such as water, urine, and honeydew, after feeding. They expel this waste using different kinematic strategies in the form of droplets, jets, or water sheets by catapulting, squeezing, or spitting the fluid. Among them, tiny millimeter-sized insects that feed on plant sap excrete liquid waste in the form of droplets, bubble foams, or jets. In contrast to xylem sap feeders, insects that feed on sugar-rich phloem sap excrete copious amounts of excess water and carbohydrates in the form of honeydew. Since this honeydew is a viscous, sugary, and particle-laden liquid waste, the phloem sap feeders require effective liquid removal strategies. Herein, we study the honeydew droplet ejection of phloem sap-feeding spotted lanternflies (SLF, Lycorma delicatula) under the hypothesis that their mechanism differs from that of xylem sap-feeding insects. Using high-speed and micro-CT imaging, we find that droplet ejection by SLF occurs in the sub-propulsion regime, as evidenced by the kinematic data of the styli and droplets. However, we find that the anal stylus changes its morphology from nymph to adult SLF, accompanying a mechanism transition from capillary ratchet to catapult. Upon this hinge-to-catapult transition, we describe the change in the droplet ejection regime using the We – Bo framework, based on rheological data of honeydew and the preceding kinematic data. Further, we mathematically show that the two-spring model fits well with the adult SLF but not precisely with the nymph SLF. We explain this deviation by proposing a Laplace pressure-driven capillary ratchet model for the nymph SLF.