  The detailed evolution of the magnitude of the local Jahn-Teller (JT)
distortion in La_{1-x} Ca_x Mn O_3 is obtained across the phase diagram for 0 <
x < 0.5 from high quality neutron diffraction data using the atomic pair
distribution function (PDF) method. A local JT distortion is observed in the
insulating phase for all Ca concentrations studied. However, in contrast with
earlier local structure studies its magnitude is not constant, but decreases
continuously with increasing Ca content. This observation is at odds with a
simple small-polaron picture for the insulating state.
