Japan’s Dragonfly King: Unveiling the Charm of the Oniyanma, Insect World’s Top Hunter (Oniyanma Ecology)
I’m Ken Kuwako, your Science Trainer. Every day is an experiment!
The other day, at Mine Park in Gunma Prefecture, I finally managed to catch an Oniyanma, often called “the king of Japanese dragonflies”! With a body nearly 10 cm long, this giant dragonfly is an incredible sight as it zips around right in front of you. Its sheer size and power left me absolutely speechless.
After the stick insect, I’d like to turn the spotlight on another fascinating creature: the “king” of dragonflies, the Oniyanma. In this article, we’ll take a close look at its lifestyle and some of its truly remarkable features. Why is the Oniyanma known as “the king”? As we uncover the secrets behind its reputation, I’m sure you’ll find yourself fascinated by this amazing dragonfly too.
And there’s much more to the Oniyanma than simply being big. From its role as a fearsome hunter at the top of the insect food chain, to its importance as an indicator of environmental health, and even a clever way of catching it that takes advantage of one of its surprising weaknesses, let’s explore the little-known world of the Oniyanma together!
The Name Comes from “Oni,” the Japanese Demon
The Oniyanma is one of the largest dragonflies in Japan, with a body that can grow to nearly 10 cm long. Its most striking feature is its vivid black-and-yellow striped pattern. One popular explanation for its name is that these stripes resemble the loincloth traditionally worn by an oni, the fearsome demons of Japanese folklore. Another theory is simply that its enormous size makes it look like a dragonfly fit for a demon!
A Ferocious Hunter of the Insect World
Adult Oniyanma are fearsome predators near the top of the insect food chain. They hunt all kinds of insects, including flies, horseflies, wasps, and even cicadas and smaller dragonflies. With their powerful jaws, they snatch prey right out of the air and devour it on the spot. That’s a true predator of the insect world! The fact that a dragonfly can take down a wasp is pretty incredible.
A Long Life as a Larva—and a Connection to Water Quality
Adult Oniyanma live for only a few months, but their larval stage, known as a naiad, is remarkably long. They spend three to five years living underwater. During this time, the larvae grow by feeding on water fleas, mosquito larvae, and even small fish such as medaka and tadpoles. Oniyanma prefer clean streams and clear-running mountain brooks, so finding them in an area can also be a sign that the local natural environment is in good health.
Exceptional Flying Skills and Emerald-Green Compound Eyes
Oniyanma are superb fliers, skillfully controlling all four of their wings to hover in place, make sharp turns, and even fly backward. They are said to reach speeds of more than 60 km/h, allowing them to chase down prey at speeds faster than even hornets. Their large, emerald-green compound eyes are made up of around 20,000 tiny individual lenses. This gives them an incredibly wide field of view and excellent ability to detect moving objects, allowing them to spot even the slightest movement and catch their prey with astonishing accuracy.
Can an Oniyanma Keep Insects Away?
Because Oniyanma are natural predators of troublesome insects such as hornets, figurines modeled after them have become popular as insect-repellent gadgets. I even have one myself! The idea is that when you wear one, flies, horseflies, wasps, and other insects mistake it for a lurking predator and stay away. My kids wear one at home too. It certainly gives you an idea of just how terrifying an Oniyanma must look to other insects!
There’s also a fascinating traditional way of catching Oniyanma that takes advantage of their strong territorial instincts. It’s known as “yanma fishing.” The basic idea is to attach a thread to a small stone or similar object and swing it around in front of the dragonfly. The Oniyanma may mistake it for an intruder entering its territory and swoop down to attack. Apparently, this makes it possible to catch one without even using a bug net. I’ve never tried it myself, but I definitely want to give it a shot!
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