The Little Bombardier

Best stay out of the way.

There are more than 500 species of Bombardier beetle (a form of ground beetle – Carbidae) in the tribes Brachinini, Paussini, Ozaenini, or Metriini all displaying the highly effective defence mechanism of releasing a superheated pulsing jet of noxious chemicals sprayed directly at would-be predators.  I have always been fascinated by the ammunition of Bombardier beetles – their highly accurate and violent chemical attack brought on whenever you touch them – but it was only on reading Eisner’s essay that I started to fully grasp the incredible complexity of these beetles.

These diagrams are reproduced from Thomas Eisner’s fascinating book For Love of Insects which explores the variety of ways in which insects use chemicals for defence, signalling and prey capture. I cannot recommend this book highly enough for anyone interested in the pursuit of entomology or study of chemical ecology.

The chemical process involves hydrogen peroxide rapidly decomposing into oxygen and boiling water, while the hydroquinones oxidize into benzoquinone in the beetle’s reaction chambers. This mix explodes out of the beetle with an audible popping sound, in a volley of rapid-fire blasts – in a manner likened to the pulsing propulsion system of Germany’s V-1 “buzz bomb” in WWII. The consequent foul chemical burn (at 100°C) incapacitates smaller attackers like ants, and deters larger predators such as the unfortunate frogs in one of Eisner’s experiments.

By examining the propulsion mechanism using high-speed synchrotron X-ray imaging Eric Arndt from MIT confirmed Eisman and his colleagues’ qualitative passive ‘pulse jet’ model. This research shows that a flexible membrane and a valve passively control the spray pulsation – as pressure increases in the reaction chamber because of the chemical explosion, the membrane stretches and the valve closes. The membrane then relaxes and the valve reopens once the pressure has been reduced following the ejection of the liquid, and so the process repeats.

The only UK resident species of bombardiers are Brachinus sclopeta, the streaked bombardier beetle, and Brachinus crepitans, the common bombardier beetle, and both are rarely seen. B. sclopeta is so rare that it has only recently been accepted as a native species  (past records were  thought to be of rare migrants) and prior to 2005 had been presumed extinct since 1928. Because the beetles prefer habitats with thin soil, rubble and bare ground they tend to favour brownfield sites and have been found in east London; but with the continuous and unrelenting development of this area, these beetles’ futures are very precarious despite being listed as UK BAP priority species, and considered critically endangered by the IUCN. However, insect charity Buglife worked with developers to secure a site near London City Airport that is now the only known intact colony of the species in the UK. You can read Richard Jones highly informative blog about the relocation and conservation of this species.

Brachinus crepitans, though more common than B. sclopeta, is restricted to Southern England and Wales and especially the coastal areas of the South-East where it is considered nationally scarce. It is more commonly found in continental Europe, central Asia, the Middle East and North Africa, with central Sweden being the northernmost extreme of its range.

Usually seen in May and June,  the beetle favours calcareous grasslands, arable field margins and chalk quarries. It is usually found in dry, sunny areas – typically under stones. Little is known about its life-cycle, but it is thought that the larvae are external parasites on the pupae of other species of beetle, particularly those of the ground beetle Amara convexiuscula and a staphylinid beetle, Ocypus ater. 

I think these gorgeous and enthralling beetles definitely warrant being surveyed for this coming Summer.

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Author: Gino Brignoli

Currently employed as FSC BioLinks Project Officer for London, I am coordinating invertebrate-focused place-based volunteer training and delivering a range of recording project activities across London. I am also developing and delivering online training courses and content focusing on invertebrate taxonomy, field ID and ecology through a virtual learning platform. You can find out more about the project here. Previously ant genomics and evolution Research Assistant at Queen Mary University of London. With a keen interest in ecology and entomology, I have volunteered with the Lepidoptera department and Soil Biodiversity Group at the Natural History Museum, and on the Thorn to Orchid and Water for Wildlife projects with London Wildlife Trust. View all posts by Gino Brignoli