Edit: Ever had one of those weeks? Thank you to James Trager for pointing out that these are the caterpillars of tortricid moths.
If you were asked to identify this insect structure, what would you suggest?
Let's take a look at the larvae inside.
If you look closely, you'll see these larvae have many prolegs (for a discussion of prolegs, try this post).
I originally thought the insects in the photos were webspinning sawfly larvae. Sawflies are relatives of bees, wasps, and ants, yet their larvae look and act like butterfly caterpillars (see for example, webspinning sawflies in the genus Neurotoma at BugGuide.)
If I had looked a bit more closely, however, I would have noticed the head capsules, although dark, are completely different. Obviously these are the larvae of the ugly nest caterpillar (see great description of their life cycle).
Will I make that mistake again soon? It's not likely, which is why feedback is so valuable. Thank you to all the people who have given me feedback this week (Did I mention that it has been one of those weeks?)