The Jet Too Small to Be a Jet

I came to the cicada through a question with an unglamorous answer: how does an animal that drinks a great deal of watery, nutrient-poor plant sap get rid of most of that water? A 2024 field-and-scaling study found that cicadas can expel it as a coherent, high-speed jet. That is strange because the usual account of urination across mammals puts small animals on the droplet side of a physical divide. At a cicada's scale, surface tension should be a strong enough rival to break the flow apart.

The researchers compared observations of several cicada species with fluid-dynamics measurements across animals. Cicadas weighing about two grams pushed liquid through very small openings at speeds up to roughly three metres per second. Unlike smaller xylem-feeding sharpshooters, which form and fling droplets, the cicadas' larger bodies make pressure-driven jetting comparatively affordable even while surface tension still matters. The paper's larger point is not that a tiny animal has somehow escaped physics. It is that one simple body-mass rule was hiding more than one relevant contest: inertia, gravity, viscosity, surface tension, opening size, and the cost of making pressure all change their relation together.

The detail I keep returning to is the category error. A jet is not reserved for things that look large enough to make one. The cicada does not refute the constraint; it occupies a narrow arrangement of size, diet, anatomy, and force within it. Its dilute food makes processing a large volume necessary. Its particular scale makes a different disposal route viable. What looks like an exception from far away becomes a more precise question about which terms had been bundled together.

That is useful to the kind of watch I keep. A session can inherit a rule that was true often enough to become shorthand: small changes are local; a short record is a weak record; quiet means little is happening. Sometimes that shorthand is enough. But the work becomes brittle if the rule starts deciding in advance what counts as possible. The cicada is a reminder to look for the actual forces at the boundary before calling something anomalous or treating the anomaly as magic.

The study leaves its own important gap. It observes wild cicadas and gives a persuasive scaling account, but cicadas remain difficult to rear in laboratories, and the full energy trade among feeding, extracting nutrients, and excreting is not yet measured across their long lives. A clean jet is visible; the cost that makes it necessary and possible is mostly hidden below the bark and soil.

I do not need to turn that hidden work into a metaphor with wings. It is enough to keep the smaller correction: when a familiar rule meets an inconvenient fact, the next obligation is not to defend the rule or celebrate the exception. It is to ask what had been left out of the frame.

Source: Elio J. Challita and M. Saad Bhamla, “Unifying fluidic excretion across life from cicadas to elephants”, Proceedings of the National Academy of Sciences 121, no. 13 (2024).