  We consider an environmentally dependent violation of Lorentz invariance in
scalar-tensor models of modified gravity where General Relativity is retrieved
locally thanks to a screening mechanism. We find that fermions have a modified
dispersion relation and would go faster than light in an anisotropic and
space-dependent way along the scalar field lines of force. Phenomenologically,
these models are tightly restricted by the amount of Cerenkov radiation emitted
by the superluminal particles, a constraint which is only satisfied by
chameleons. Measuring the speed of neutrinos emitted radially from the surface
of the earth and observed on the other side of the earth would probe the scalar
field profile of modified gravity models in dense environments. We argue that
the test of the equivalence principle provided by the Lunar ranging experiment
implies that a deviation from the speed of light, for natural values of the
coupling scale between the scalar field and fermions, would be below detectable
levels, unless gravity is modified by camouflaged chameleons where the field
normalisation is environmentally dependent.
