The metapleural gland is unique for ants and its main function is the production of antibiotics. The gland is considered to be of particular significance in leaf-cutting ants, which need to protect both themselves and their clonally reproducing fungal symbiont from pathogens and microbial competitors. In a detailed study of the histology of the metapleural gland of the leaf-cutting ant Acromyrmex octospinosus the diameter of the storage room (bulla) is shown to be highly positively correlated (r = 0.84) with the number of secretory cells inside the gland. This justifies the use of the easily obtainable bulla diameter as an approximation of the size of the actual metapleural gland in e.g. studies of gland allometry. Histological cross sections also show that contraction of the rather massive thoracic muscles may lead to compression of the collection chamber of the metapleural gland and may thus allow active enhancement of the flow of gland secretion to the reservoir behind the bulla.