ground beetle, (family Carabidae), any member of more than 40,000 insect species in one of the largest families in the insect order Coleoptera. They can be found in almost any terrestrial habitat on Earth. Ground beetles are recognized by their long legs and shiny black or brown elytra (wing covers), which are decorated with ridges and may be fused together along the midline. In many species the hind wings are reduced or absent. Ground beetles prefer moist cool areas and usually run rather than fly when disturbed. They emerge from under rocks, crevices, or litter at night in search of insects, worms, or snails. The long, slender larvae are mostly carnivorous, although those of a few species feed on seeds. They have sharp projecting mouthparts and a pair of bristly tail appendages. Many ground beetles secrete a foul-smelling liquid that discourages potential predators such as birds.
The searcher, or caterpillar hunter (Calosoma scrutator), is a common, brightly coloured North American ground beetle about 35 mm (1.5 inches) long. Its green or violet wings are edged in red, and its body has violet-blue, gold, and green markings. This and related species of ground beetles are known to climb trees in search of caterpillars. They secrete an acidic fluid that can blister human skin. The iridescent green-coloured C. sycophanta was introduced into North America from Europe to help control the spongy moth and brown-tail moth caterpillars.
The snail hunters (e.g., Scaphinotus) are a specialized group of ground beetles. Elongated, hook-shaped mouthparts allow them to extract the snail from its shell. The bombardier beetle (Brachinus in North America and Pherosophus in Africa, Asia, and the East Indies) has little sacs at the tip of its abdomen that spray a noxious fluid used to deter enemies. The beetle fires this fluid at boiling-hot temperatures, and the hot fluid vaporizes upon contact with air. The fluid itself consists of toxins called quinones that react explosively in an internal chamber with hydrogen peroxide, which is also produced by the beetle and stored in a separate body compartment. The expulsion of fluid from the abdomen is accompanied by a distinctive popping sound that may further startle the beetle’s predators. Brachinus species have dark blue, black, or blue-green wings and reddish yellow bodies and legs.
The beneficial Lebia grandis, which resembles the bombardier beetle, preys upon the Colorado potato beetle. The Malayan leaf beetle, or fiddle beetle (Mormolyce), measuring approximately 100 mm (4 inches) long, resembles a violin with its slender head and thorax and wide elytra. This flat beetle uses its long head to probe into small openings in search of prey. It hides in crevices, under tree bark, and in porous fungi. The majority of ground beetles feed on insects harmful to humans and are therefore considered to be beneficial.