FMRFamide-like neuropeptide (PubMed:12821653, PubMed:14555955). Involved in modulating locomotion quiescence during the sleep-like state called lethargus which occurs during molting between larval and adult stages, acting via the G-protein coupled receptor npr-1 (PubMed:23764289). Plays a role in modulating social and feeding behavior (PubMed:14555955). Ligand to G-protein coupled receptor npr-1. Expressed in the ADL, ASE and ASH sensory neurons, the URA motor neurons and the MC, M2 and M4 pharyngeal neurons. May be processed by convertase egl-3. Exacerbates bordering behavior, where animals accumulate at the edges of culture media (PubMed:14555955). Enhances clumping/aggregation behavior in a G-protein coupled receptor npr-1 mutant background (PubMed:14555955). Decreases locomotion quiescence, in an npr-1 mutant background, during the sleep-like state called lethargus which occurs during molting between larval and adult stages (PubMed:23764289). Belongs to the FARP (FMRFamide related peptide) family. It has been reported that many phenotypes associated with the Bristol N2 reference allele of the receptor npr-1 may reflect a neomorphic gain-of-function sensitivity of the receptor to flp-18 in addition to sensitivity to flp-21.