The fascinating physics of insect pee | Saad Bhamla (re-release)

Host Elise Hu opens with October imagery—pumpkins, fake cobwebs, and Halloween costumes—and explains that TED Talks Daily’s seasonal playlist gathers five “haunting” talks about subjects including grieving crows and zombie roaches. She frames the collection as spooky but also as a way to encounter strange discoveries about the natural world. The featured talk is Saad Bhamla’s 2021 TED Monterey presentation, later published in February 2023. Bhamla begins by saying he loves bugs because he sees them as “nature’s tiny engineers.” He has previously studied spiders that use webs as slingshots in the Amazon, worms that knot themselves into shape-shifting blobs in sewers and caves, and aquatic beetles that carry their own scuba gear when diving. This time, however, he focuses on a behavior he and his student Elio Cholita noticed in their Atlanta backyards: a tiny insect appeared to urinate continuously for hours, with the action happening so quickly they could barely see it. Bhamla says the talk will answer three questions: how insects pee, why this insect pees so much and in such an unusual way, and why anyone should care. The central subject is the glassy-winged sharpshooter, Humulodisca vitripennis, an insect that feeds on plant xylem fluid.

Bhamla first establishes that insects do urinate in one form or another because their bodies are closed systems: what goes in must come out. The glassy-winged sharpshooter is a xylem-fluid feeder, essentially a plant’s “mosquito.” Like a mosquito that leaves behind pathogens after taking blood, the sharpshooter can spread bacteria into plants. Those bacteria cause serious diseases and can kill plants, creating a major agricultural problem, including millions of dollars in damage to California vineyards. The researchers observed that the sharpshooter forms a droplet of urine and launches it at an acceleration of roughly 40 G—“40 times faster than the sprint of a cheetah,” as Bhamla puts it. Looking under a microscope at the insect’s rear end, they found a structure that Bhamla humorously calls a “butt flicker.” It contains springs and latches arranged like a catapult, allowing the insect to repeatedly hurl droplets with very high acceleration. The first puzzle came from comparing the speed of the flicker with the speed of the droplet. The droplet moved at about 150% to 200% of the flicker’s speed. Bhamla compares this with a baseball: if a player’s hand moved at 50 miles per hour but the ball reached 100 miles per hour in midair, the extra speed would need an explanation. The sharpshooter’s urine was doing something similarly counterintuitive.

To explain the speed difference, Bhamla and his colleagues revisited their videos and noticed that a urine droplet is not rigid like a baseball. At this tiny scale, surface tension makes the droplet squishy and capable of storing energy just before launch. They tested the idea by turning a kitchen table into a laboratory and placing droplets on a speaker, which could compress them rapidly. Their experiment showed that water behaving like an ordinary liquid at the scale of a faucet can act differently at insect scale. With the right timing, surface tension allows a droplet to receive a “kick,” store energy, and launch at very high speed. Bhamla compares the effect to a child being propelled by a trampoline. To test the role of timing more directly, the researchers built a small catapult—“a catapy,” in his joke. If the mechanism moved too slowly, the droplet did not launch, but at the right speed it was flung away. The sharpshooter’s butt flicker therefore combines a spring-and-latch mechanism with the physics of surface tension. The insect is not simply pushing urine away; it is timing the flick so that energy stored in the droplet helps produce a launch faster than the flicking structure itself. Bhamla describes the result as a way of achieving “racket-breaking speeds” while solving the insect’s basic waste-removal problem.

Bhamla says the sharpshooter’s unusual urination is driven first by what it eats. Xylem fluid rises from plant roots and contains mostly water and some minerals, making it extremely poor in nutrients. By contrast, phloem fluid carries sugar produced through photosynthesis. He compares the sharpshooter’s diet to a human trying to live entirely on diet lemonade: the insect must drink constantly because the food provides so little energy, and it must also urinate constantly. That still leaves a second question: why does the sharpshooter produce droplets instead of a continuous jet? Bhamla points to cicadas, related xylem-fluid feeders that do urinate in jets, so diet alone cannot explain the difference. The key is the sharpshooter’s small size—it is smaller than Bhamla’s pinky. At that scale, surface tension dominates gravity and causes liquid to stick to the body. The droplet-launching mechanism is therefore necessary to detach the urine from the insect. The same surface tension that makes the droplet an effective energy-storage device also creates the problem the insect must overcome. The sharpshooter has evolved a way to drink and eliminate large amounts of low-nutrient fluid without allowing droplets to cling to it. Bhamla jokes that in this respect the insect resembles “other engineers” he knows on a Friday night at a bar. His broader lesson is that when exploring unfamiliar organisms, what may look like an oddity is often a functional adaptation: “It’s not a bug, it’s a feature.”

Bhamla offers a possible engineering application for the research: the principles behind the sharpshooter’s droplet ejection might someday help designers build more efficient water-ejector systems for smartphones, watches, or hearing aids. But he immediately says that practical technology is not the real reason he has spent four years obsessing over insect urine. His main motivation is what happens when he shares the work with children. Talking about insect pee makes children’s eyes light up; they laugh, run into their backyards, look at bugs, and start asking questions. For Bhamla, the subject demonstrates that marvelous discoveries can be found in ordinary places, including one’s own backyard, if people look closely enough. The tiny insect’s behavior is both an example of sophisticated biological engineering and an invitation to investigate things most people would normally overlook. Elise Hu closes by identifying Bhamla as the speaker at TED Monterey in 2021 and noting that the talk was originally published in February 2023. The episode then includes production credits and directs listeners to TED’s curation guidelines and podcasts.ted.com. After the sign-off, additional recorded advertisements promote Capital One business cash back and checking accounts and a Metro by T-Mobile phone offer.