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Ito, F.; Billen, J. P. J. 1998. Larval hemolymph feeding and oophagy: behavior of queens and workers in the primitive ponerine ant Prionopelta kraepelini (Hymenoptera, Formicidae). Belgian Journal of Zoology 128:201-209.

Billen, J. P. J. 2000. A novel exocrine gland in the antennal scape of the army ant Eciton burchelli. Belgian Journal of Zoology 130:159-160.

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Billen, J. P. J. 2002. A novel epithelial intramandibular gland in the ant Pyramica membranifera (Hymenoptera, Formicidae). Belgian Journal of Zoology 132:175-176.