Aglais Io

Hello all!

I live in an area with a decent amount of Aglais Io. I alway's take some group of caterpillars home, so that I can release the butterflies later on, but leave most of the nests there. From time to time I go back to check on there progress but I alway's see the same phenomena happening.

Picture

The caterpillars seem to hang down and don't move at all. Does anyone know what happend? Is this a disease or parasites? Or pesticides? The last one seems farfecthed because it's near a forrest. The caterpillars I took home are healthy and almost ready to make a pupae. It's such a sad sight

Thank you in advance!

Edit: Posted the link because I can't seem to put the picture here ><