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.
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. Not a definitive test,
but 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 it's
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. So many experiments to do, so little time...
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