bombardier : Idioms & Phrases

• noun beetle that ejects audibly a pungent vapor when disturbed
  

WordNet

• (Zoöl.), a kind of beetle (Brachinus crepitans), so called because, when disturbed, it makes an explosive discharge of a pungent and acrid vapor from its anal glands. The name is applied to other related species, as the B. displosor, which can produce ten or twelve explosions successively. The common American species is B. fumans.

Webster 1913