Abdomen of a Leptogenys sp worker from Nepal, profile view (Scanning Electron Micrograph, Roberto Keller/AMNH)

In most ants the hypopygium is a simple trapezoidal plate, yet commonly adorned by a set of long hairs pointing backwards. Hairs like these serve as tactile receptors: as arthropods, ants use them to sense the world around them in their otherwise numb medieval armored bodies. Predators, like the Leptogenys pictured here, relied on their sting (Stng) to capture prey and need to know where the rear end of their abdomen is moving.

Something interesting to note is that the hypopygium folds into itself around the protruding sting in exactly the same way as it does when forming an acidopore.

Rear end of a Leptogenys sp worker from Nepal (Scanning Electron Micrograph, Roberto Keller/AMNH)

There is a caveat with this anatomical concept, however. The hypopygium is not homologous among all insects, not even among all adult ants.  Primitively, the abdomen in insects consist of eleven segments, but the number of external, visible segments varies. Female ants have seven, male ants have nine. Since hypopygium denotes whatever the last visible abdominal segment is, it does not corresponds to the same body part between the sexes.

If you are asking yourself what happen to the other four abdominal segments in female ants relative to the eleven found in the insect groundplan, you are asking the right evolutionary question. I am not going to tell you just now, but here is a hint: it’s pointy and it hurts.