The larvae are continuing to grow very quickly. Their 'frass' (poop) is also getting bigger and bigger.
Last night this guy crawled to the mesh on the top of its jar and sat there. I thought it might be getting ready to pupate although it's not quite big enough. It may have shed another skin in the night. This morning, it was back down, eating voraciously. I have to give it a fresh milkweed leaf twice a day. Its not stretched out here but definitely chubbier and the longest of all the larvae.
I first saw the Caterpillar Castle at the Birding Center on South Padre Island. I copied down the info from this affixed tag and ordered one to bring home.
One side is clear vinyl (facing camera), three sides are solid mesh, one side is mesh with a zipper for access, and the last side has a mesh 'sleeve' which can be used to reach in. I have the sleeve twisted up so the caterpillars don't climb into it.