Acidopore of a Formica fusca worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

It is popular knowledge that ants secrete formic acid. What most people don’t know is that only a well-defined subgroup of species have this capacity. Female ants in the subfamily Formicinae have an acid producing gland that sprays its content through a special opening at the rear end of their abdomens, aptly called the acidopore.

It is a beautiful structure when seen under high magnification. It is not a hole in the ant’s plated skeleton, but rather the ventral last external segment folds upwards into itself like a paper funnel (see the smooth area above the opening). Surrounding the acidopore is an inner rim of pointed hairs and a outer rim of flat cuticular projections.

This structure is probably the best synapomorphy we have in ants. It evolved once and, as far as we know, it was never lost during the diversification of this subgroup. It is easy to see and its presence is unambiguous: you see an ant with an acidopore and you know it belongs to the subfamily Formicinae.

Now, here is something not even most ant specialists know. Whenever an acidopore is present, the surface of the rear abdominal plates is covered with a hexagonal pattern array as shown in the image above. I don’t know the composition or special properties of this special surface, but my guess is that it serves as a protection to the ant’s own caustic secretion. Figuring this out would make a neat small research project in biochemistry (who knows, maybe the next great patent after Teflon).

Next time you see a Carpenter ant crawling inside your house, take the time to grab it gently between your fingers and bring it close to your nose. You will be able to smell the vinager odor it will produce. It will also be good time to call the exterminator at this point.