Neurotrophin which may function as a ligand for the Toll-related receptors Toll-7 and Tollo (PubMed:22022271, PubMed:23892553). Binds to Toll-7 and probably acts as its ligand in promoting motor axon targeting and neuronal survival in the central nervous system (CNS) (PubMed:23892553). Involved in synaptic targeting of ISNb/d motorneurons and also some SNa motorneurons (PubMed:19018662). In larvae, involved in the negative regulation of the tracheal immune response to bacterial infection perhaps by acting as a ligand for the Toll-related receptor Tollo (PubMed:22022271). 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) (PubMed:23892553). Expressed in the optic lobes and brain (PubMed:19018662). Expressed throughout development (PubMed:19018662). Expressed in the embryonic CNS midline (at protein level) (PubMed:23892553). Also expressed in various longitudinal muscles (at protein level) (PubMed:23892553). In embryos, expressed in the CNS midline and in the muscles (PubMed:19018662). In larvae, expressed in the lamina of the optic lobe (PubMed:19018662). Expressed in larval body wall muscles (PubMed:24124519). 'Spaetzle' means 'noodles' in German.