"I couldn’t find a lot of literature on these guys, but my best guess is the almost 1 foot long silk string it hangs from and the detailed lattice structure would do well to protect against ants while minimizing investment in an all-encompassing cocoon as many moths have." (http://www.neatorama.com/2013/01/14/A-Moth-Inside-Its-Cocoon/).
One of the strangest and most beautiful cocoons is that of the Amazonian moth Urodus ( Urodidae ) which has a coarse open mesh design with an exit at the bottom, and hangs like a pendulum from a 20cm long silk cord. It seems likely that the cord may function to isolate the pupa from marauding ants, but little is known about the biology of this species. (http://www.learnaboutbutterflies.com/Lifecycle%207%20-%20chrysalis.htm).