Tiny Amphibious Water Striders

Unveiling the behavior and fluid dynamics of Microvelia, one of the tiniest water striders

*Microvelia* traversing across water, creating vortices in its wake.

The world of water-walking presents its own challenges

Many ask what lurks beneath the water, but what lurks right on top of it? Water-striders (Gerromorpha) are a family of insects that are found lurking on the surface of lakes, streams, and even the ocean. These insects leverage surface tension and hydrophobic hairs on their legs to stand and walk on water surface. Amongst many genera of water striders, Microvelia is one of the tiniest yet most interesting water strider.

Microvelia typically lives on low-flow creeks and ponds where it effortlessly traverses on both water as well as solid surfaces owing to their specialized tripod gait. Most common water striders such as Gerridae exhibit rowing gait, which restricts their ability to walk on water surface. Grab a lens and let’s dive (or rather stride) into the world of Microvelia!

A high-speed (2000 FPS) recording of a *Microvelia* on water, slowed down 66x.

We reveal the fluid dynamics behind Microvelia’s water locomotion and insights on the role of vortical interactions through PIV, physical model, and CFD. Their gait adaptation and kinematics for different tarsal conditions on different surfaces were also evaluated.

Trajectory trace of *Microvelia* as it skates across water.

Dr. Johnathan O’Neil describing the Microvelia work during his PhD Defense in Aug 2024.

Major questions

How does Microvelia walk on water and other terrains?
What is the role of different legs in their locomotion on water?
What principles of fluid dynamics does Microvelia leverage to enhance its thrust on water?

What we’ve discovered

Tiny Alternating Tripod Gait

Alternating tripod gaits are well studied in multiple terrestrial six-legged insects, such as ants and cockroaches, but are not common amongst water walkers. Other water walking insects, which are multiple times the size of a *Microvelia* (such as Gerridae, pictured left), use a six-legged rowing gait. Even larger lizards and birds quickly sprint across the water with two legs. Yet, the tiny *Microvelia* remixes an old classic alternating tripod gait for venturing onto water.

**Amphibious Locomotion of Microvelia**

Water isn’t the only surface these critters can conquer— only a few epineuston species can leave the water, and *Microvelia* is one of them. On water, the middle legs act as propulsors whereas the hind legs are rudders. On terrestrial surfaces, they adjust their tripod gait and biomechanics for effective locomotion. These organisms can use the same gait to traverse on a variety of surfaces, including water covered with duckweed.

Adaptability and Specialized Leg Dynamics Overcomes Limb Loss

Predators lurk beneath the water, from the air, and on the surface and shore, along with competition from its fellow water striders. *Microvelia* must be able to adapt in the face of injury in order to survive. We discover how a *Microvelia* compensates its locomotion when parts of its leg are removed, revealing the underlying roles of each leg in propulsion on water. Using its middle legs as propulsors, hind legs as rudders, and front legs for stability, the *Microvelia* are still able to locomote through injury unless both of its hind legs are injured.

**Epineuston Vortex Capture in Water Skaters**

Using high-speed imaging and PIV, we discovered that tiny *Microvelia* exhibit Epineuston Vortex Capture (EVC) to enhance their thrust on water. In this process, their middle legs sheds vortices which are captured by their hind legs. Using physical model and CFD simulations, we show the role of vortical interactions on their motion.

Modeling Vortex Capture

We used physical models and CFD simulations to demonstrate that vortex capture leads to higher coefficient of thrust when compared to the case of no interaction.

A New Frontier: Vortex Recapture Utilized at the Interface

Wake capture has been observed in the aerial flights of fruit flies and butterflies, as well as in the underwater swimming of jellyfish. Here, we present one of the first reports on vortex capture at the air-water interface (epineuston), as *Microvelia* utilize vortices to enhance their thrust on the surface of water. Our study extends wake capture principles from bulk fluids to the neuston, offering insights into biological fluid mechanics and providing a foundational understanding that can inspire micro-amphibious robots in the future.

Read the papers

Limb loss and specialized leg dynamics in tiny water-walking insects, ICB (2024)

Tiny amphibious insects use tripod gait for seamless transition across land, water, and duckweed, ICB (2024)

Epineuston vortex recapture enhances thrust in tiny water skaters, Biorxiv (2024)

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