A pair of acrobat ants (Crematogaster sp.) feed on extrafloral nectaries in Iowa. Some plants produce these little packets of nutrients as part of their defensive strategy - ants who feed on them will defend the plant for the renewable resource they provide.
Pergid sawfly (Symphyta: Pergidae) from Iowa.
Velvet ant, Mutillidae, from Monteverde, Costa Rica. These "ants" (actually parasitic wasps) are well known for the painful sting they can inflict.