  The motion of planetary nebulae (PNe) through the interstellar medium (ISM)
is thought to lead to a variety of observational consequences, including the
formation of bright rims; deformation and fragmentation of the shells; and a
shift of the central stars away from the geometric centres of the envelopes.
These and other characteristics have been noted through imaging in the visual
wavelength regime. We report further observations of such shells taken in the
mid-infrared (MIR), acquired through programs of IRAC imaging undertaken using
the Spitzer Space Telescope (SST).
  NGC 2440 and NGC 6629 are shown to possess likely interacting halos, together
with ram-pressure stripped material to one side of their shells. Similarly, the
outer halos of NGC 3242 and NGC 6772 appear to have been fragmented through
Rayleigh-Taylor (RT) instabilities, leading to a possible flow of ISM material
towards the inner portions of their envelopes. If this interpretation is
correct, then it would suggest that NGC 3242 is moving towards the NE; a
suggestion which is also supported through the presence of a 60 microns tail
extending in the opposite direction, and curved bands of H-alpha emission in
the direction of motion - components which may arise through RT instabilities
in the magnetized ISM.
