Pyrokinins mediate visceral muscle contractile activity (myotropic activity). Pyrokinins (PK) 1 to 4 are expressed in the retrocerebral complex. PK 1 is expressed in central brain, anntennal lobes and abominal ganglia. PK 2 is expressed in optical lobes and in gnathal, thoracic and abdominal ganglia. PK 3 is expressed in optical lobes and in thoracic and abdominal ganglia (at protein level). Pyrokinin 1. Pyrokinin 2. Pyrokinin 3. Pyrokinin 4. The structural similarity of PK1 and PK2 to CAPA-PK might compensate for the apparent absence of CAPA-PK in ants. Belongs to the pyrokinin family.