A study published in the Journal of Chemical Ecology reports on nine species of Bruneian ants, whose workers "are known to have hypertrophied mandibular glands and release their glandular contents suicidally from the head by rupturing the inter- segmental membrane of the gaster." In other words, their heads explode. Exploding ants will instinctively commit suicide when their colony is attacked, killing not only themselves but often their attackers as well.
Jones, T.H., D. A. Clark, A. A. Edwards, D. W. Davidson, T. F. Spande and R. R. Snelling. The Chemistry of Exploding Ants, Camponotus SPP. (Cylindricus COMPLEX). Journal of Chemical Ecology 30, 8 (2004).