Droplet superpropulsion in an energetically constrained insect

Authors: CHALLITA, ELIO - Georgia Institute Of Technology; SEHGAL, PRATEEK - Georgia Institute Of Technology; Krugner, Rodrigo; BHAMLA, M - Georgia Institute Of Technology

Submitted to: Nature Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2023
Publication Date: 2/28/2023
Citation: Challita, E.J., Sehgal, P., Krugner, R., Bhamla, M.S. 2023. Droplet superpropulsion in an energetically constrained insect. Nature Communications. 14. Article 860. https://doi.org/10.1038/s41467-023-36376-5.
DOI: https://doi.org/10.1038/s41467-023-36376-5

Interpretive Summary: Sharpshooter vectors of the plant pathogenic bacterium Xylella fastidiosa feed exclusively on plant’s xylem sap, an extremely low nutrient source (95% water) that generates large volumes of droplet excreta. While sap ingestion behaviors and host plant preferences have been studied in detail for insect vectors, how excretion dynamics shape and constrain sharpshooters remain largely unexplored. A combination of mathematical models, computational fluid dynamics, and biophysical experiments showed for the first time in a living organism that superpropulsion of droplets is exploited by these insects to eliminate the water-based droplet excreta, providing insights to ecologically-based studies on insect nutrition. From an engineering perspective, the principles and limits of superpropulsion outlined in this study can inform designs of energy-efficient self-cleaning structures and microfluidic ejectors for powering small, soft robots.

Technical Abstract: Food consumption and waste elimination are vital functions for living systems. Although how feeding impacts animal form and function has been studied for more than a century since Darwin, how its obligate partner, excretion, controls and constrains animal behaviour, size, and energetics remains largely unexplored. Here we study millimeter-scale sharpshooter insects (Cicadellidae) that feed exclusively on plant’s xylem sap, a nutrient-deficit source (95% water). To eliminate their high volume excreta, these insects exploit droplet superpropulsion, a principle discovered for the first time in a living organism. We combine coupled-oscillator models, computational fluid dynamics, and biophysical experiments to show that these insects temporally tune the frequency of their anal stylus to the Rayleigh frequency of their surface tension-dominated elastic drops as a single-shot resonance mechanism. Unlike larger animals, our model predicts that for these tiny insects, superpropulsion of droplets is energetically cheaper than forming jets, enabling them to survive on an extreme energy-constrained xylem-sap diet. The principles and limits of superpropulsion outlined here can inform designs of energy-efficient self-cleaning structures and microfluidic ejectors for powering small, soft robots.