  In this letter, we present the results of our study of galaxy-galaxy lensing
in massive cluster-lenses spanning $z = 0.17$ to 0.58, utilizing high-quality
archival {\it Hubble Space Telescope} ({\it HST}) data. Local anisotropies in
the shear maps are assumed to arise from dark matter substructure within these
clusters. Associating the substructure with bright early-type cluster galaxies,
we quantify the properties of typical $L^*$ cluster members in a statistical
fashion. The fraction of total mass associated with individual galaxies within
the inner regions of these clusters ranges from 10--20% implying that the bulk
of the dark matter in massive lensing clusters is smoothly distributed. Looking
at the properties of the cluster galaxies, we find strong evidence
($>3$-$\sigma$ significance) that a fiducial early-type $L^\ast$ galaxy in
these clusters has a mass distribution that is tidally truncated compared to
equivalent luminosity galaxies in the field. In fact, we exclude field galaxy
scale dark halos for these cluster early-types at $>10$-$\sigma$ significance.
We compare the tidal radii obtained from this lensing analysis with the central
density of the cluster potentials and find a correlation which is in excellent
agreement with theoretical expectations of tidal truncation: $\log [r_t*]
\propto (-0.6\pm 0.2) \log [\rho_0]$.

Astrophysics