Superfast excretion of viscous particle-laden droplets in phloem feeding insects*

Fluid ejection is a tricky problem in biological systems, especially depending on the size scale and rheological properties of the fluid. An example is sharpshooter insects that feed on xylem sap and expel water droplets using a stylus at an ejection velocity of 0.4 m/s. In contrast to xylem feeders, spotted lanternflies (Lycorma delicatula) are insect species that feed on phloem sap rich in sugars and excrete sticky particle-laden honeydew droplets. Here, we explore how spotted lanternflies (SLFs) can flick sticky honeydew microdroplets. We analyze the rheological properties of fresh honeydew samples excreted by SLFs to quantify the viscosity and surface tension coefficients of honeydew. Using high-speed imaging technique, we found that SLFs fling droplets at an ejection velocity of 1.5 m/s using a stylus to expel their viscous liquid waste, seemingly similar to the catapult system of sharpshooters. The scaling analysis, however, suggests that droplets expelled from SLFs are governed by inertia rather than surface tension (Weber number ~ O(10¹)) and viscous force (Capillary number ~ O(10^-1)), implying the ejection concept differs from sharpshooters that exploit the surface tension-driven droplet superpropulsion. Our findings on the superfast excretion behaviors of SLFs will inform us about how insects at small scales can overcome surface tension and viscous forces. This research sheds light on biofluid dynamics and development of self-cleaning bioinspired fluid ejectors at the millimeter-scale.