  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.
