This photo of a lantern bug with an ant appearing to be taking a drop of sweet liquid from it was taken in the Sinharaja Forest in Sri Lanka in October 2013. Could this be indicative of some sort of symbiosis between the two insects?? I would love to know what it is.
I photo a lot of moths in my garden here in Cyprus (at lights atop low walls), and often there are ants about. Most of the time the ants ignore the moths, or if an ant stumbles into a moth, it sometimes steps back (almost apologetically!) while the moth just sits there. However, from time to time, I find that an ant is particularly interested in a moth. It may go back and forth around it, obviously 'sniffing' something of interest. In some of those cases, the ant seems to pay particular attention to the moth's 'knee' joints (or should that be arm-pits?)!
This is not leading up to an explanation for the behaviour you observed; it is just another observation along similar lines, and wanting explanation.
(Did you know that the scent used by ants for trail-marking is so powerful (and/or the ants' sense of smell is so acute) that one gramme of it would be enough to lay an effective trail right roung the earth?!)