How do water striders sense an insect the instant it falls in the water?
― They read tiny ripples with the tips of their legs
A water strider resting on a pond will suddenly spin around and dart toward an insect that has fallen in some distance away. It isn't looking for it. When the insect struggles, it sends out fine ripples across the surface, and those ripples reach the strider's feet — carrying clues about both direction and distance.
An irrigation channel by an autumn rice paddy, or a park pond. Several water striders float on the surface, legs splayed wide.
A small winged insect drops in and thrashes its wings. The striders glide across the water toward it, one smooth stroke after another.
If we rest a finger at the pond's edge, we feel nothing. It's the same surface — yet only the striders receive the message.
Just two reasons they notice
Water's surface behaves like a taut, stretched membrane trying to shrink. When an insect struggles, this membrane shakes and sends fine ripples spreading outward. These ripples have an odd property: the finer they are, the faster they travel.
The strider feels these vibrations through the tips of its outspread legs. Which leg the ripple reaches first is thought to reveal direction, while the gap between the arrival of fine and coarse ripples is thought to reveal distance.
For a water strider, the water's surface is both floor and telephone line, carrying news of distant events. Let's look at each part in turn.
Finer surface ripples travel faster
Waves on water are pulled back toward flat by two different restoring forces. One is gravity: water that bulges up is pulled back down by its own weight. The other is surface tension: the water's surface tries to shrink its area, so any bumps get smoothed away.
In large waves, like ocean swells, gravity is the dominant force, and longer waves travel faster. But in fine ripples — the size of a fingertip — surface tension takes over. The relationship then flips: the finer the ripple, the faster it travels.
Look at Figure 1. The boundary between the two regimes sits at a wavelength of about 1.7 cm, where waves are slowest, traveling at roughly 23 cm per second. Ripples finer than this get faster the further left you go. The vibrations from a struggling insect fall squarely in that left-hand world.
Six legs as a direction-and-distance sensor
A water strider's legs are covered in fine hairs that repel water, letting it rest on the surface. Near each leg joint sits what's thought to be a sensory organ tuned to vibration. With six legs spread wide, the body forms a small "antenna array" set right on the water's surface.
As in the left half of Figure 2, when an insect falls in, ring-shaped ripples spread outward. The legs nearer the insect feel the vibration just slightly before the legs farther away. This tiny time difference is thought to be how the strider judges direction.
Distance is thought to be read from how the ripples are "arranged" as they arrive. Look at the right half of Figure 2. Because finer ripples travel faster, fine ripples reach the strider's feet first, followed later by coarser ones. The farther away the ripples originated, the more the fine ripples pull ahead during the journey, stretching out the arrangement.
In other words, a slow shift from fine to coarse ripples means far away; a fast shift means close. It's much like judging the distance of thunder by ear — except the water strider does it through its legs.
How do other creatures use surface vibrations?
Water striders aren't the only ones reading ripples. Backswimmers, which float upside-down just under the water's surface in ponds, are thought to locate prey by the ripples a fallen insect makes. Fishing spiders that live near water also rest their legs on the surface and wait for vibrations.
Water striders also use ripples as a kind of "voice." Males tap the water's surface rhythmically with their legs, sending out ripples with a distinct pattern, reported to serve as signals to other males or courtship calls to females. Instead of sound, they talk through waves on the water.
Our own fingertips, by contrast, can barely detect the fine vibrations on a pond's surface. Our bodies are simply too large, and these ripples too small. On the very same surface, creatures of different sizes are tuned into entirely different streams of information.
Fine ripples are quickly damped out by the water's own viscosity, unlike large ocean swells that can travel thousands of kilometers. Water striders are thought to be able to detect only relatively nearby prey. When it rains, the whole surface fills with ripples, and news of prey gets drowned out much like a voice lost in noise.
Summary
Water striders notice prey with their legs, not their eyes. A struggling insect sends out fine ripples driven by surface tension. Because finer ripples travel faster, the order in which they arrive carries information about distance. Water striders are thought to read these timing differences across their six legs to judge both direction and distance.
For a water strider, the water's surface is both floor and telephone line.
The order in which fine ripples arrive tells the story of "how far."
For how ripples spread across a water surface, see Why do circular ripples spread out when you drop a stone in a pond?, and for other creatures' hidden senses, see How do sharks find prey buried in the sand? and How do bats avoid obstacles in the dark?.
- Fill a basin or tray with water and place it where window light or a lamp reflects on the surface.
- Touch the surface once with the tip of a toothpick. Then tap the same spot repeatedly, quickly.
- Watch the spreading rings through the wobble of the reflected light. You should see fine rings race outward first, then weaken and fade.
Try adding a drop of dish soap and repeating this: it lowers the surface tension and changes how the ripples behave. If you spot a water strider on a pond, try gently tapping the surface nearby with a blade of grass — it may turn and react.
Want to know more? ― terms, formulas, and how this connects to textbooksWe flag which level each part belongs to, from middle-school science up to university-level courses
- MScovered in middle-school science
- HScovered in high-school "Physics" or "Biology"
- HS+high-school extension material, or textbook sidebar content
- Univ.not covered in high school — university-level specialist courses (fluid dynamics, sensory physiology)
- Researchnot settled even at university level — an area researchers are actively studying
MSTerms: this phenomenon has names
- Surface tension: a liquid's tendency to minimize the area of its surface. Water's is especially strong.
- Capillary wave (ripple): a fine water-surface wave for which surface tension is the main restoring force.
- Mechanoreceptor: a sensory organ that detects vibration or pressure. In water striders, thought to sit near the leg joints.
MSHSCheck it with a formula: how fast does a struggling insect's ripple reach the strider?
Suppose an insect struggles 10 cm away and produces ripples with a wavelength of 5 mm. Let's find the speed of a surface-tension-dominated wave and calculate the travel time.
| In symbols | c ＝ √( 2π × σ ÷ ( ρ × λ ) ) |
| In words | ripple speed ＝ the square root of "2π × surface tension" divided by "water density × wavelength" |
| Where it comes from | It comes from the balance between surface tension, which flattens a curved surface back out, and the inertia of the shaking water (its tendency to keep moving). Since wavelength λ sits in the denominator, finer waves are faster. |
| c | ripple speed (in meters per second) |
| σ | water's surface tension (in newtons per meter) |
| ρ | water's density (in kilograms per cubic meter) |
| λ | wavelength — the distance between wave crests (in meters) |
| Water's surface tension (around 20°C) | about 0.073 newtons per meter |
| Water's density | about 1000 kilograms per cubic meter |
| Ripple wavelength (assumed) | 5 mm = 0.005 meters |
| Distance to the insect (assumed) | 10 cm = 0.10 meters |
| Width of the strider's spread legs (assumed) | about 1.5 cm = 0.015 meters |
| 2π × surface tension | 6.28 × 0.073 ≒ 0.458 |
| density × wavelength | 1000 × 0.005 ＝ 5 |
| division | 0.458 ÷ 5 ≒ 0.092 |
| take the square root | the square root of 0.092 is about 0.30 (about 30 cm/s) |
| time from insect to strider (seconds) | 0.10 ÷ 0.30 ≒ 0.33 |
| time gap between near and far legs (seconds) | 0.015 ÷ 0.30 ＝ 0.05 |
A struggle 10 cm away reaches the strider in about a third of a second. The gap between its near and far legs is about a twentieth of a second — much shorter than a single blink. Striders are thought to judge direction from this tiny difference. Note that even the exact formula, which includes gravity's contribution, gives almost the same speed at this wavelength.
HSHS+Wave speed and "dispersion"
HSIn high-school physics you learn that wave speed = frequency × wavelength. At 30 cm/s with a 5 mm wavelength, that's a wave oscillating roughly 60 times a second. On water, speed depends on wavelength — a property called dispersion.
HS+When speed depends on wavelength, a mixture of waves generated at one spot re-sorts itself by speed as it travels. That's exactly what's behind "fine waves first, coarse waves later" in Figure 2. It's the same kind of phenomenon as a prism splitting light into separate colors.
Univ.The dispersion relation for gravity-capillary waves
In fluid dynamics, the relationship between a surface wave's angular frequency and its wavenumber — the dispersion relation — is derived as the sum of a gravity term and a surface-tension term. This is the dispersion relation for gravity-capillary waves. Phase velocity is minimized at a wavelength of about 1.7 cm; shorter than this, and the wave is a capillary wave (ripple). For capillary waves, group velocity is 1.5 times phase velocity, and this difference determines the order in which energy arrives. Separately, it's been shown that the mechanism by which water striders themselves move across the surface is by transferring momentum to vortices created by their legs (Hu, Chan and Bush's study).
📖 For the full derivation and further reading: Wikipedia, "Capillary wave" ／ The fluid dynamics of water-strider locomotion (Hu, Chan and Bush's paper)
ResearchWhat's still not fully understood
- How precisely can distance be read? The idea that distance is read from how the ripple shifts from "fine to coarse" was proposed from experiments on backswimmers and similar insects. Just how much water striders rely on this cue is still debated.
- How do they tell prey from a signaling companion? A male's courtship ripples and a struggling insect's ripples are said to differ in rhythm. Exactly how the strider's nervous system tells them apart isn't well understood.
- Sensing in rain or wind. A natural water surface is constantly disturbed by raindrops and wind-driven ripples. How the strider picks out the important vibration from that noise is still being studied.
In other words, even this article describes things only "as far as we currently understand." The sensory world at the tip of a tiny insect's leg still holds plenty of unknowns.
Connections to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science — properties of water, animal bodies and senses | surface tension, sensing vibration through legs |
| HS | Physics — wave properties / Biology — reception of stimuli | wave speed = frequency × wavelength, mechanoreceptors |
| HS+ | Physics — dispersion (extension) | why fine waves arrive first |
| Univ. | Fluid dynamics / sensory physiology | gravity-capillary wave dispersion relation, group velocity |
| Research | Behavioral ecology / biophysics | reading distance, telling signals from prey |
| ― | Everyday connections | watching ponds and paddies, the basin experiment |
- Hu, D. L., Chan, B., Bush, J. W. M. (2003) The hydrodynamics of water strider locomotion. Nature 424.
- Wilcox, R. S. (1972) Communication by surface waves: mating behavior of a water strider. Journal of Comparative Physiology.
- Wiese, K. (1974) The mechanoreceptive system of prey localization in Notonecta. Journal of Comparative Physiology.
- Wikipedia, "Water strider" (アメンボ, Japanese-language edition)
- Wikipedia, "Surface tension" (表面張力, Japanese-language edition)
- Landau, L. D. & Lifshitz, E. M., Fluid Mechanics (Tokyo Tosho edition), section on capillary waves
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanism. Some aspects of how living creatures sense their environment remain subjects of ongoing research and differing views.