So I have a purple Passion vine that draws in Fritillary butterflies this year I have actually gotten to see the caterpillars and was wondering if there was a way to tell when there were ready to cocoon the biggest one is about 2 and 1/2 inches almost 3 so I would think he is getting close but I was wondering if there was a way to tell. I have had tons of caterpillars in the past but never this species. So I was hoping to learn a bit more..