At temperatures below the metal-insulator transition of La _1-xSr_xMnO₃ with 0.1 < x < 0.15, a peculiar ferromagnetic and insulating phase is observed which has been intensively discussed over the last few years. We present a detailed investigation of this phase by means of resonant and high energy x-ray scattering along with measurements of the electrical resistivity, thermal expansion, magnetization, and specific heat. Interestingly, the data show that the metal-insulator transition of lightly doped manganites is accompanied by an orbital rearrangement. The microscopic information provided by the x-ray scattering studies together with the analysis of the macroscopic properties implies that the orbital reordering maximizes the gain of double exchange energy and, at the same time, induces an insulating behaviour. The relevance of the double-exchange mechanism for the stabilization of the ferromagnetic insulating phase is further substantiated by studies of (La_1-yPr _y)_7/8Sr_1/8MnO₃: with increasing praseodymium content, the metal-insulator transition is dramatically suppressed which can naturally be explained by a reduction of the band width upon praseodymium doping.