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 e
lytral tracks. M, contractus has two-chambered defensive glands typical of
bombardier beetles, and its secretion, like that of other bombardiers, is q
uinonoid and hot. Its frothing mechanism, however, is unique for bombardier
s and possibly illustrative of the ancestral glandular discharge mechanism
of these beetles. M. contractus, thus, could be the least derived of extant
bombardiers.