What was that tentacular crown of wonders?
Tucker got there first, for all ten points: it was the acidopore of a formicine ant. Júlio Chaul picks up two bonus points for getting the genus: Oecophylla.
Rather than explain the acidopore myself, I’ll point you to Roberto Keller:
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.