There were actually two Cecropia caterpillars present on this little Box Elder tree.
Both had been parasitized by Tachinid flies.
The one one the left actually completed a cocoon but did not pupate, dying within a few days.
The photo of the opened coccoon is the same one.
The cycle from egg laying to pupation of the fly larvae was about 7 days.
The larva on the right started to spin silk but did not complete a coccoon. It died and fell off the branch. Below, where it landed, prepupal tachinids were found in the soil.