Those white things are wasp larvae eating this manduca caterpillar from the inside out.

Parasitic wasps lay their eggs in it. The eggs are coated with a virus that temporarily turns off the manduca's immune system. When it's immune system reboots, it is tricked into thinking that the eggs are part of the caterpillar. The eggs hatch, and they eat their way out and turn into pupae and the cycle repeats.