What are they doing with their curved gaster?

by M. L. de Andrade and C. Baroni Urbani
Institut für Natur-, Landschafts- und Umweltschutz (NLU)
Biogeographie
Neuhausstrasse 31
CH-4057 Basel Switzerland
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E-mail: Cesare.Baroni-Urbani@unibas.ch

     Members of the tribe Proceratiini are remarkable and easy to recognize for their gaster curved downwards. This character is characteristic but not unique to the tribe since it appears in an unpredictable way in other ants and in other Hymenoptera as well.

                                                  
                                 FIG. 1 Proceratium itoi Forel from Japan




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                                  FIG. 2 Discothyrea sexarticulata Borgmeier from Brazil.



                                                   
                                 FIG. 3 Bradoponera meieri Mayr from Baltic amber

     Very little is known about the function of this structure. Poldi (1964) reports that P. melinum uses the fourth abdominal segment (i. e. the curved first gastral) to close the nest entrance when extraneous ants are introduced into the arena. Brown (1980) observed Proceratium silaceum using the tip of the gaster to bring prey spider eggs to the mouth.
Poldi's and Brown's explanations are not mutually exclusive but both are based on occasional, fragmentary observations.
     During a monographic revision of the genus Proceratium indirect morphological evidence supporting Poldi's explanation was gathered. In fact a number of Proceratium species exhibit a lighter spot interpreted as a glandular reservoir seen in transparency through the integument on the fourth abdominal segment. In some cases an orifice, probably the glandular opening, was also observed. The fourth abdominal segment, however, is the sole or the main body part remaining exposed in case of phragmotic closure of the nest entrance.

                                                   
                                 FIG. 4 Proceratium croceum (Roger) from the USA.



                                                   
                                 FIG. 5 Proceratium undescribed species from Sarawak: first gastral tergite to show the folding.
      It seems, hence, that at least some Proceratium species dispose of unique chemical weaponry that can be oriented towards intruders during occlusion of the nest entrance by means of the gaster.
     These and other amazing novelties can be found in the forthcoming Proceratium revision by Baroni Urbani & de Andrade (2003) and in a revision of Bradoponera with a generic reassessment of the Proceratiini by de Andrade & Baroni Urbani, nearly completed.

References
Andrade, M. L. (de) & Baroni Urbani, C. 2003. The Baltic amber ant genus Bradoponera               (Hymenoptera: Formicidae) with description of two new species and a reassessment of the               Proceratiini genera. Submitted.

Baroni Urbani, C. & Andrade, M. L. (de) 2003. The ant genus Proceratium in the Extant and fossil record               (Hymenoptera: Formicidae). Monografie no. 36. Museo Regionale di Scienze Naturali - Torino. 480               pp.

Brown, W. L. 1980. A remarkable new species of Proceratium with dietary and other notes on the genus              (Hymenoptera: Formicidae). Psyche 86 (1979): 337-346.

Poldi, B. 1964. Alcune osservazioni sul Proceratium melinum Rog. e sulla funzione della particolare struttura              del gastro. Atti Accad. naz. Ital. Entom., Rend. 11: 221-229.