The sulfakinin (SK) family of neuropeptides have only been identified in crustaceans and insects. For most species there is the potential for producing two sulfakinin peptides one have a short sulfakinin sequence The function of the sulfakinins is difficult to assess. For the American cockroach, various forms of the endogenous sulfakinins have been shown to be active on the hindgut, and also on the heart. In C. vomitoria the peptides act as neurotransmitters or neuromodulators, linking the brain with all thoracic and abdominal ganglia. In adults of P. monodon they appear to be restricted to a few neurones in the brain with a neural pathway extending along to the ventral thoracic and abdominal ganglia.