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This beetle’s butt is basically a machine gun

The bombardier bum's explosive mysteries revealed. (Charles Hedgcock)
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Ever heard of the bombardier beetle? It's aptly named: When threatened, the beetle doesn't just excrete deadly chemicals. It actually mixes them up in an internal chamber, then fires the reaction off as a near-boiling, high-speed spray from its rear end. Now researchers have figured out how the beetles manage to aim and fire these noxious rounds at enemies.

In a study published Thursday in Science, scientists from MIT and the University of Arizona get to the heart of the reaction chamber in the bombardier's bum, which can fire off between 368 and 735 pulses per second.

"Twenty-five years ago, a team of scientists from Cornell University and MIT discovered that each blast from the bombardier beetle is actually a series of extraordinarily fast micro-pulses," study author Wendy Moore at the University of Arizona said in a statement. "What wasn't known is what causes each discharge to be pulsed, like a machine gun. Previous researchers suggested that the pulses were caused by muscle contractions or by a fluttering of the exit duct during the explosions."

[This snail fights enemies with shots of weaponized insulin]

But to figure out how bombardier blasts really work, the scientists used high-speed x-ray imaging to watch them do it. To aggravate the beetles in the least harmful way possible, they had to perfectly time a robotic forceps that tugged on their legs, then capture the milliseconds-long gas expulsion while firing off x-rays.

"For each experiment we had to cool the beetle down, carefully set it up in the observation chamber such that the X-ray beam was aimed precisely at the defensive glands, seal the doors, walk over to mission control, flip the switch to allow the X-ray beam to enter the room, and use robotic manipulators to remotely touch the beetle's leg so it would blast. In some cases, just turning the radiation on caused them to blast," Moore said.

[Why are there so many freaking beetles? Science may finally have the answer.]

It turns out it's a pretty passive process, and it's all due to the beetle's internal plumbing, specifically the valve between the two portions of the chemical mixing chamber. From National Geographic's Ed Yong:

When the valve opens, a droplet of reservoir chemicals enters the reaction chamber. Boom! The pressure created by the explosion forces a pulse out through the exit channel. It also pushes against a membrane that closes the valve, cutting off the supply of fuel. As the pressure in the reaction chamber drops, the membrane relaxes and the valve re-opens, letting another droplet through. Boom! This continues until the reservoir muscles finally relax and the beetle stops spraying.

The process could theoretically be copied in man-made combustion engines, the authors say. In any case, it helps explain why the bombardier is so unique.

"By having a pulsed delivery, these small beetles produce a relatively large amount of defensive spray, which they can aim precisely and with great force and speed," Moore said. "This is truly one of the most remarkable and elegant defensive mechanisms documented to date."

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