Spray mechanism of the most primitive bombardier beetle (Metrius contractus).

The bombardier beetle Metrius contractus discharges its defensive secretion as a froth that clings to its body. When attacked from the rear, it allows the froth to build up over the gland openings near the abdominal tip; when attacked from the front, it conveys the secretion forwards along special elytral tracks. M. contractus has two-chambered defensive glands typical of bombardier beetles, and its secretion, like that of other bombardiers, is quinonoid and hot. Its frothing mechanism, however, is unique for bombardiers and possibly illustrative of the ancestral glandular discharge mechanism of these beetles. M. contractus, thus, could be the least derived of extant bombardiers.