Gaster (insect anatomy)

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The gaster (from Ancient Greek γαστήρ 'belly, paunch') is the bulbous posterior portion of the metasoma found in hymenopterans of the suborder Apocrita (bees, wasps and ants). This begins with abdominal segment III on most ants, but some make a constricted postpetiole out of segment III, in which case the gaster begins with abdominal segment IV. The gaster in ants contains what is sometimes called the "social stomach," which is named for the fact that food can be carried within it and then shared with other members of the colony. It also contains the ant's heart as well as the rest of their digestive system. In the ant subfamily Formicinae, the gaster houses an acidiphore which they can use to spray formic acid. Certain ants in the genus Cataglyphis, including Cataglyphis bicolor and Cataglyphis fortis, have a cubiform petiole that allows them to decrease their moment of inertia by raising their gaster into an upright position. This lets them move more quickly on their sinusoidal foraging paths and evade predators by moving more erratically. The ant species Cephalotes atratus is capable of gliding by changing the angle of their gaster as they fall.