The ligand for Torso receptor signaling in terminal determination is thought to be trk. tsl is required for torso signaling, but its role is unclear. It has been proposed to act as an anchor for trk or control localized cleavage of trk (FBrf0224259). However, in vitro, tsl has been shown to act as a torso agonist alone and with trk (FBrf0234821).

The eggshell proteins encoded by fs(1)N, fs(1)M3 and clos are required for terminal signaling, possibly by anchoring or stabilizing of tsl at the vitelline membrane (FBrf0235104).

tor also acts as a receptor for the Prothoracicotropic hormone, Ptth, regulating ecdysone production during metamorphosis (FBrf0209466).