Nature is full of surprises, and sometimes they're explosive! Scientists have been examining how a remarkable little beetle can fire a boiling chemical spray at its attackers. The beetle in question is the bombardier beetle (Brachinus elongatulus), found in the riparian habitats of southern Arizona.
Wendy Moore, an assistant professor in the Department of Entomology at the University of Arizona, has been examining how the beetles achieve their explosive capabilities. According to Moore and her co-authors, "Understanding how these beetles produce – and survive – repetitive explosions could provide new design principles for technologies such as blast mitigation and propulsion." The study has been published in Science. You can read the press release here.
In order to withstand the explosive force (the beetle is only small, after all), the blast consists of a series of micro-pulses, similar to a machine gun. There are many ideas regarding why the beetle ejects its defensive spray in pulses as opposed to one, dramatic blast. First of all, the attack can be sustained for a longer period of time and has a wider range. Also, the beetle's body can cope with multiple smaller forces better than one much greater force.
Video of Bombardier Beetle Blast via The University of Arizona
Moore had a closer look at where the chemicals are mixed in the insect for this explosion to occur. In the beetle's abdomen, there is a reaction chamber made of cuticle: a robust mixture of chitin, proteins and waxes. This formulation also makes up the exoskeletons of beetles—so it's very strong! It protects the bombardier beetle's soft interior from the toxic chemical mix, the high temperatures and the pressure.
However, the exact internal mechanisms of the explosion wasn't fully understood since previous studies only examined it using external observations. In order to have a closer look at what was going on inside the beetle's reaction chamber, the team took hundreds of beetles to Wah-Keat Lee, a synchrotron scientist who at the time was with the Advanced Photon Source at Argonne National Laboratory.
For the first time, these beetles were filmed firing their chemical weapons using X-ray imaging, which can penetrate the hard exterior of the beetle and reveal how the reaction chamber rapid-fires its destructive spray.
Internal dynamics of bombardier beetle revealed by X-ray imaging by Eric M. Arndt et al published in Science
"Previous researchers suggested that the pulses were caused by muscle contractions or by a fluttering of the exit duct during the explosions," said Moore. But as it turns out, the explosion is an entirely passive process: chemicals enter the reaction chamber through a valve. As the chemicals pass through, they mix with enzymes in the reaction chamber. This results in a burst of oxygen gas, water vapor and heat that ejects the hot, noxious spray out of an exit pore.
"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."