Tracing the thin green line

This weekend, I decided to spend some time adding new insects and spiders to my online photo collection.  I was searching for information on one of my new entries, the spinybacked orbweaver spider (Gasteracantha cancriformis), when I stumbled upon pictures of a very familiar object: a yellow oval intersected by a green line.

The egg sac of a spinybacked orbweaver spider (Gasteracantha cancriformis).

I had seen three of these objects in our garden in Florida but had been unable to identify them.  Now, at last, I have my answer -- they were the egg sacs of spinybacked orbweaver spiders. 

Female spinybacked orbweavers produce egg sacs in the fall and winter.  Though the eggs hatch after only a couple of weeks, the spiderlings may remain inside the egg sac for several more weeks before emerging.  Thus, the egg sacs that I saw might still have been full of spiderlings.  In several months, if all goes well for the spiderlings, the females will look like this:

A female spinybacked orbweaver spider (Gasteracantha cancriformis) in the center of its web.

Although the mystery of what was hidden beneath the green line has been solved, I have not yet found any explanation of why the egg sacs are marked with a distinct green line.  There seems to have been more interest in why the webs of this species contain tufts of silk (see the picture above).  One idea is that they are a signal to birds, to prevent them from accidentally flying into the webs.