  We map substructure in three strong lensing systems having particularly good
image data: the galaxy lens MG J0414+053 and the clusters SDSS J1004+411 and
ACO 1689. Our method is to first reconstruct the lens as a pixelated mass map
and then substract off the symmetric part (in the galaxy case) or a projected
NFW (for the cluster lenses). In all three systems we find extended irregular
structures, or meso-structures, having of order 10% of the total mass. In
J0414+053, the meso-structure suggests a tidal tail connecting the main lens
with a nearby galaxy; however this interpretation is tentative. In the clusters
the identification of meso-structure is more secure, especially in ACO 1689
where two independent sets of lensed images imply very similar meso-structure.
In all three cases the meso-structures are correlated with galaxies but much
more extended and massive than the stellar components of single galaxies. Such
extended structures cannot plausibly persist in such high-density regions
without being mixed; the crossing times are too short. The meso-structures
therefore appear to be merging or otherwise dynamically evolving systems.
