Henri,
you should keep the cocoons in the box until they are fully intransparent and white (~ 2-3 days, looking rather dry). What to do with the feeding caterpillars depends on the size of the box and the density. If its too crowded they'll disturb the spinning ones, so you better remove them (the feeding) into another box.
After 2 days I'd remove the cocoons and pin them on a styrofoam board to fully harden and let the caterpillars pupate (1-(2) weeks). You may then store them in a closed plastic box or glas jar in the fridge (10-15°C). The lid is important to keep the air in the box moistened. I have no idea how long they last that way, mine are from before Christmas and are perfectly well.
If you wanted to induce real Diapause, Samia had to be kept at 15°C and short day conditions after L3. Storage would be at 6°C in the fridge then.
Thomas
