The ferromagnetic and insulating state observed in La$_{1-x}$Ca$_{x}$MnO$_3$, 0.125$<$x$<$0.2, is characterized by structural and magnetic anomalies below T$_C$, similar to those observed in the x$_{Sr}$$approx$1/8. A neutron scattering study of the superlattice {bf Q$_0$}= (0,0,1+/4)$_{cub}$ peak, and of the magnetic excitations are reported in the x$_{Ca}$=0.2 sample. The occurrence of this superstructure is associated with the observation of a gap in the spin dynamics, at a {bf q$_0$} wave-vector ({bf q$_0$}={bf Q$_0$}-$tau$) with the same modulus $|${bf q$_0$}$|$ in all directions, which divides the dispersion into two regimes. For $|${bf q}$|$$<$$|${bf q$_0$}$|$ the dispersion is splitted into two or three curves. For $|${bf q}$|$$>$$|${bf q$_0$}$|$, magnetic excitations lock on acoustic and optic phonon energies, revealing a new kind of magneto-vibrational coupling. We suggest an analysis in terms of two distinct magnetic couplings, associated with two ferromagnetic media involved into a collective state.
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