This sounds like the perfect job for: Surface Flow!

Check your docs for it, and it will do the job perfectly for you, with a little tweaking.

If you have Maya 4.5, here is the direct link:
C:\Program Files\AliasWavefront\Maya4.5\docs\en_US\html\UserG uide\Dynamics\helpCreateSurfaceFlow.html

Scene setup tip:

One idea, would be to import your image into maya as a background.

Then, model low-res, degree 2 planes of your scene pieces that your particle spiders would be running on.

Select your pieces, and apply SurfaceFlow.
(you can also select ALL of your pieces together, and check on "createParticle per flow" in the Surface Flow option box to create a flow of spider particles on all your surfaces at once)

JUST REMEMBER, surface flow can be problematic with degree 1 surfaces. It's best to use degree 2 surfaces with enough spans so that you can choose a row of verts as, say, a "drawer corner", and just move the rows of verts on either side of your "corner" row closer to your chosen row to make it look like a sharper corner.

Once you've got your flow working, open the attribute editor to play with the Random speed settings to make your spiders look like they are moving at random speeds etc.

Apply a "useBackground" shader to your nurbs pieces.

Then, tweak your SurfaceFlow settings to taste

Good luck Nem!