The greatest enemies of ants are other ants- including members of the same species- so when these little insects encounter each other outside the safety of the nest they make a quick chemical assessment: nestmate or foreigner? Friend or foe?
These tropical Dolichoderus workers apparently belonged to the same colony. After the greeting photographed above, each continued in peace along her own path. Had they originated from separate colonies, they would have run away or fought, depending on the context.
photo details: Canon EF 100mm f/2.8 macro lens on a Canon EOS 7D
ISO 100, f/14, 1/250 sec, strobe bounced off overhead paper