Developmental stability is the ability of an organism to buffer given traits against environmental and intrinsic perturbations. This may involve physiological, temporal or behavioral adjustments to the developmental program. The processes leading to developmental stability have been particularly well studied in arthropods. For instance, if uncoordinated growth is induced in the larval imaginal discs (the precursors of adult appendages) of Drosophila flies, a transient delay in the onset of metamorphosis ensues, allowing extra time for all discs to achieve their species-specific size and proportion. How exactly this exquisite coordination between growth and developmental timing is achieved is not completely understood. Recently, others and us have identified a fly-specific insulin/IGF-I/relaxin peptide named Drosophila insulin-like peptide 8 (DILP8) that responds to uncoordinated imaginal tissue growth and delays the onset of metamorphosis by inhibiting, via an unknown mechanism, the biosynthesis of the major insect molting hormone, Ecdysone. Loss of dilp8 increases intra-individual asymmetry and yields individuals with a greater than normal range of size variation and time of maturation. Thus, DILP8 is a central player in the communication system that mediates the adjustments to promote developmental stability in Drosophila. Here, we plan to identify the mechanism of action of DILP8 and to understand how this new peptide became incorporated into a conserved tissue-stress sensing pathway. To reach these objectives, we will identify the target tissue/s that mediate DILP8 function(s), the signalling pathway that it acts through, and determine when it originated and became responsive to abnormal growth during dipteran evolution. Our results should provide insight into the poorly understood physiological mechanisms used for interorgan growth coordination, and maybe shed new light into the evolution of new signalling pathways.