Insecticidal toxin (PubMed:11689233). It inhibits insect voltage-gated sodium channels (Nav) by partially blocking the channel pore (1 uM inhibits 55.7% sodium current in DUM neurons from the American cockroach), not by acting as a gating modifier (PubMed:25559770). The inhibition is only partially reversible (about 25% recovery) after prolonged washout (PubMed:25559770). In vivo, the toxin causes flaccid paralysis followed by death when injected into Heliothis virescens larvae (PubMed:11689233). It also causes uncoordinated movements followed by full paralysis to sheep blowflies (Lucilia cuprina) (PubMed:25559770). Expressed by the venom gland. The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. LD(50) is 10 mg/kg on H.virescens larvae. PD(50) are 2.2 +-0.2 and 1.5 +-0.3 nmol/g (native and recombinant toxins, respectively) when injected into sheep blowfiles (L.cuprina). Could be considered as a biological insecticide candidate, when fused to Galanthus nivalis agglutinin (GNA), a protein with the potential to cross the insect gut (PubMed:15037094, PubMed:16206236). This chimeric Sf1a/GNA variant shows a gain in toxicity against insects when administered orally, while maintaining similar effects as the wild-type toxin when injected (PubMed:15037094). This variant has been found to be orally toxic against larvae of the tomato moth (Laconobia oleracea), the rice brown planthopper (Nilaparvata lugens), and the peach-potato aphid (Myzus persicae) (PubMed:15037094, PubMed:16206236). It does not show significant effects on voltage-gated calcium channel (Cav) and voltage-gated potassium channel (Kv) currents in cockroach DUM neurons (PubMed:25559770). In vivo, it does not induce any toxic effects when injected intravenously into adult mice at a dose of 1.5 mg/kg body weight (PubMed:25559770). Exists in two forms, due to cis-trans isomerization at 31-Arg-Pro-32. Belongs to the neurotoxin 16 (SFI) family.