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.
Sources:
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).
Encyclopedia Bugtanica
The illustration is Ants by M.C. Escher