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6. Abdominal pedicle and gaster
Ants have one or two segments called abdominal pedicle between the thorax and the abdomen. This pedicle is composed either of petiole alone or of petiole + postpetiole.
Petiole: The petiole corresponds to the second segment of the abdomen. Its shape is diverse. The petiole of Amblyopone silvestrii has a broad connection to the abdomen. In many ants of the subfamily Myrmicrinae, the petiole has a peduncle and a node. The petiole of Iridomyrmex itoi assumes a scale-like form. The petiole of genus Tapinoma is a small tube-like form. Its shape is thus important for classification of ants. The ventral plane of the petiole sometimes has plate- or spindle-shaped processes called subpetiolar processes. The presence or absence of this process and its shape are often used for classification.
Postpetiole: The postpetiole corresponds to the third segment of the true abdomen. This term is usually used only when this segment can be clearly distinguished by constriction from the successive segments, as in the subfamily Myrmicrinae. Although this term is sometimes used even in ants of the subfamily Amblyopone, we call the third segment of the true bone of this subfamily "the first segment of the abdomen".
Gaster: The term gaster indicates the part posterior to the abdominal pedicle. The number of segments constituting the gaster varies depending on the presence or absence of postpetiolar differentiation. In cases where the postpetiole has developed, the gaster indicates the fourth and subsequent segments of the abdomen and is composed of 4 segments. In cases where the postpetiole is absent, it indicates the third and subsequent segments of the abdomen and is composed of 5 segments. The number of segments constituting the gaster is often the same between males and workers. The end of the gaster is important, primarily for classification at the subfamily level. The end of the gaster has trigger-hair, a slit-shaped opening or a conical opening. The structure of the gaster end of males differs markedly from that of females and workers. The number of gaster segments on the notum of males is larger by one than that of females and workers. The number of gaster segments on the sternum of males is larger by 2 than that of females and workers. The ninth sternum of males is called the subgenital plate, which is often important for classification.
(Comments by Kazuo Ogata)