The thin green line

In addition to nutritionally supporting many caterpillars, the oleander tree was also physically supporting several vines.  One of these vines was about to bloom when I first arrived in Florida, and whenever I checked on the caterpillars I would also check on the buds.  I did not have to wait long before the first bud opened, revealing a very distinctive inflorescence.

An aroid vine (Araceae) with an open inflorescence and several buds.

I was less successful in discovering what was inside a strange object underneath one of the vine leaves.  It appeared to be a cocoon and although most of its silk was yellow, there was a dark green line going down the middle.

The mysterious yellow cocoon (?) with a green line across the middle.

The green line puzzled me -- it made the cocoon more obvious, yet it did not look like the warning signal of something dangerous or distasteful.  I hoped to find answers online, but I could not even find a picture or description resembling the cocoon.  I began to wonder if this cocoon just happened to have been woven in a peculiar way, until I found a second and then a third cocoon with the same green line.  Therefore, it now seems highly unlikely that the green line is just a random pattern; however, why there is a green line and what is behind it remain mysteries.