Silk...

The caterpillars do a lot of silk spinning. If you peel them off of a stem while they are napping, you will find that most of their appendages seem to be glued lightly to the stem with silk. When they are preparing to pupate, they appear to coat the area upon which they will pupate with a fairly extensive layer of silk.

While unsuccessfully trying to photograph their prolegs from beneath through a piece of plastic wrap, I noticed that the caterpillars, after investigating the plastic wrap for a few seconds to a minute or so, would start moving their heads back and forth along the surface of the plastic wrap. It turned out they were depositing silk, and it could be clearly seen if the angle of light was correct. The images have been lightened to make the silk show up more distinctly.

Below, the job has just begun.

Nice loopy shapes...sometimes figure-eights, sometimes not.

After 5 minutes or so of hard work...

Perhaps this is an instinctive response to a very smooth surface to prepare it for habitation, or at least for temporarily hanging on.

The experiment: When placed on fine emory cloth (rough surface), the test caterpillars (2 subjects) were not interested in depositing silk. They napped instead. Perhaps not a definitive test, but it is suggestive.

Serious silk...
It turns out that quite a bit of silk is deposited by a caterpillar when it is preparing to pupate---it coats the surface that it will attach itself to not just at the tail and harness anchor points, but all along the surface opposite itself. 

Below are two shots of pupas that were, along with their silk, carefully detached from a dangerous location and relocated to a paper towel. In the first shot below you can see the caterpillar-length silk and where the harness attaches to it. 

When this pupa was first detached, the silk didn't look as ropy as it does in the above shot---it was a lacy sheet as shown in the shot of a different pupa, below:

This pupa appears a bit fuzzy because you are viewing it through its lacy silk. As you can see, the caterpillar did a lot of pre-attachment spinning. The surface that this caterpillar was detached from was quite smooth, and it may be that more silk is laid down on a smooth surface than on a rough one as suggested by the emory cloth experiment described above.

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