A healthy young man, upon placing the palm of his hand on a wooden floor, experienced an immediate severe burning sensation. A small black shiny beetle was found lying partially crushed. The initial burning pain in the palm subsided, but a dull discomfort persisted for several hours. Examination revealed two areas of dark yellow-brown discoloration, resembling the colour of iodine, on the palm (Box 1). These measured about 1 × 2 cm, with a surrounding halo of mild erythema. The affected skin had normal texture, markings, and sensation, and no blistering or vesication was seen. The patient remained systemically well throughout the episode. No evidence of external biting, stinging or defensive apparatus was found on inspection. The skin changes lasted 3–4 days, and resolved completely.

The beetle was identified as a species of bombardier beetle by Daniel Bickel of the Australian Museum, Sydney.

Bombardier beetles belong to the large Carabidae family of active predatory ground beetles, and comprise the subfamily Brachininae, with some 500 species. One species, Pherosophus verticalis, is widespread in Australia (Box 2). Chemicals are used defensively by many ground beetles, usually discharged as a fine spray when threatened. At least nine diverse groups of noxious chemicals have been found in these insects, including acids, phenols, hydrocarbons and quinones.1

When threatened, bombardier beetles emit a defensive spray of heated benzoquinones from the tip of the abdomen.2 The quinones are produced explosively at the moment of ejection. This involves a two-chamber system — an inner reservoir chamber stores a solution of hydroquinones and hydrogen peroxide, and a separate reaction chamber contains oxidative enzymes (catalase and peroxidase).3 Mixing of the reaction components leads to catalytic events, with a rapid build-up of pressure from liberated oxygen, and explosive discharge of oxidised benzoquinones (Box 2). This can be accompanied by an audible "pop". The temperature of the emitted reaction mixture has been measured at 100°C.3 This boiling spray can be directed in all directions by a revolvable turret arrangement on the abdominal tip, and effectively deters both vertebrate and invertebrate predators.4

Australia is rich in beetles, with about 20 000 native species recorded.5 Despite this diversity of forms, beetles are rarely directly harmful to humans.6

1: The affected areas of the patient's hand

2: The bombardier beetle and its chemical weapon

1: Pherosophus verticalis, the Australian bombardier beetle. 2: A schematic diagram of the internal defensive gland. The secretory apparatus (A) produces an aqueous solution of hydroquinones and hydrogen peroxide, which is stored in the reservoir (B). When threatened, the beetle releases fluid from the reservoir into the reaction chamber (C), which contains a mixture of catalases and peroxidases. These enzymes catalyse an explosive reaction. The boiling mixture discharges in a spray through the abdominal tip. Reproduced with permission from NSW Agriculture.