Morphology and ultrastructure of a specialized bacterial pouch in the digestive tract of Tetraponera ants (Formicidae, Pseudomyrmecinae).

The digestive tract in workers of some species of the pseudomyrmecine ant genus Tetraponera is characterized by a conspicuous pear-shaped diverticulum at the transition between the midgut and the intestine, that so far has not been found in any other ant species. As this organ is filled with a mass of bacteria, we propose to designate it as a bacterial pouch. Its distal wall is formed by a thin ectodermal epithelium, through which tens of tracheal branches penetrate into the bacterial mass that fills the pouch lumen. The proximal wall, in contrast, is formed by a cylindrical epithelium with a conspicuous microvillar differentiation of the apical cell membrane, but without a cuticular lining. The contact region between both epithelia occurs as a complex fold surrounding the pouch like a belt. The Malpighian tubules open into the pouch through the cylindrical epithelium adjacent to the belt fold. The functional significance of the bacterial pouch remains unknown, although the abundant presence of bacteria may indicate a symbiotic function. The conspicuous tracheolar supply illustrates the metabolic activity in the pouch, while the microvillar differentiation of the cylindrical epithelium may be interpreted in the uptake of metabolites from the pouch lumen.