The case of the rappeling shield bug

[del]What do you get when you cross a spider and a shield bug?[/del]

I was watching a shield bug that had turned up on the wall. It would stay still, move a bit, freeze… and then it dropped almost all the way down to the floor. The bug was swinging about an inch off the ground, belly up. Then it shook itself free, buzzed, and went off somewhere.

I’m still trying to figure out what happened here. If I hadn’t known better, I’d think the bug had just found a handy cobweb to rappel down. I didn’t see a cobweb, but it must have just been very thin. And I don’t understand how a cobweb could make a bug fall thee feet down.