Eastern Tent Caterpillar - bits of caterpillar?
I'm "raising" a pair of Eastern Tent Caterpillars with my daughter. One of them seems very content to eat the leaves from the tree we caught them in. The other one just wants to sit on a stick all the time. Earlier today it's "made" what looks like a couple pieces of itself on the stick, albeit a bit skinnier. We're curious, what is it doing? This particular caterpillar was handled quite a bit before we found out that it isn't good to handle them (the other was not handled except to put him in the terrarium). Is it creating some sort of defence, or is it possibly sick and "shedding" parts of itself in an effort to survive? We're located just west of Ottawa Ontario, Canada.
The caterpillar in your photo has just molted or shed its skin. Even though caterpillars seem soft-bodied, their exoskeleton is like a little armored coat that doesn't grow larger with the insect. Think of is as a set of clothes that won't get bigger as you grow, which you then need to replace with larger clothes as you get bigger. For the caterpillar (or any insect) to grow larger, it grows a new exoskeleton inside, and then splits open and sheds its old, too-tight exoskeleton. The new exoskeleton is a little larger, but sort of wrinkled and soft. While it is still soft after molting, the caterpillar uses hydraulic pressure within its body to sort of stretch out the new exoskeleton until it hardens within a few hours. The new, bigger exoskeleton gives it room to grow. So what you are seeing on the branch is the old, shed exoskeleton, now not much more than a thin hollow shell, with the "new" larger caterpillar sitting next to it. Caterpillars typically stop eating and become quiet for a day or two before they shed. Your caterpillar should starting eating soon if it hasn't already. If your caterpillar is about two inches long, it is in probably in its final larval stage, and when it reaches full size in another week or two, it will probably spin its cocoon and again shed its exoskeleton, but the quiescent pupal stage will have emerged inside the cocoon. The adult moth should emerge a few weeks later.