The TOR and Jak/STAT signal pathways are highly conserved from Drosophila to mammals, but it is unclear whether they interact during development. The proline-rich Akt substrate of 40 kDa (PRAS40) mediates the TOR signal pathway through regulation of TORC1 activity, but its functions in TORC1 proved in cultured cells are controversial. The Drosophila gene Lobe (L) encodes the PRAS40 ortholog required for eye cell survival. L mutants exhibit apoptosis and eye-reduction phenotypes. It is unknown whether L regulates eye development via regulation of TORC1 activity. We found that reducing the L level, by hypomorphic L mutation or heterozygosity of the null L mutation, resulted in ectopic expression of unpaired (upd), which is known to act through the Jak/STAT signal pathway to promote proliferation during eye development. Unexpectedly, when L was reduced, decreasing Jak/STAT restored the eye size, whereas increasing Jak/STAT prevented eye formation. We found that ectopic Jak/STAT signaling and apoptosis are mutually dependent in L mutants, indicating that L reduction makes Jak/STAT signaling harmful to eye development. In addition, our genetic data suggest that TORC1 signaling is downregulated upon L reduction, supporting the idea that L regulates eye development through regulation of TORC1 activity. Similar to L reduction, decreasing TORC1 signaling by dTOR overexpression results in ectopic upd expression and apoptosis. A novel finding from our data is that dysregulated TORC1 signaling regulates the expression of upd and the function of the Jak/STAT signal pathway in Drosophila eye development.