Neurotrophin which may function as a ligand for the Toll-related receptors Toll-6 and Toll-7 (PubMed:23892553). Binds to Toll-7 and Toll-6, and probably acts as their ligands in the promotion of motor axon targeting and neuronal survival in the central nervous system (CNS) (PubMed:19018662, PubMed:23892553). Involved in synaptic targeting of ISNb/d motorneurons and also some SNa motorneurons (PubMed:19018662). May be involved in the normal development of specific neurons at the neuromuscular junction (PubMed:24124519). Homodimer; disulfide-linked. Detected in the fan-shaped body which is a component of the locomotion center in the central nervous system (CNS) (at protein level). In larvae, expressed in a punctate pattern along the CNS axons (at protein level) (PubMed:23892553). Expressed in larval body wall muscles and in the synaptic boutons (PubMed:24124519). In embryos, expressed in the CNS midline and muscles (PubMed:19018662). 'Spaetzle' means 'noodles' in German.