The caterpillar I named Yudhiṣṭhira was tiny when I found him earlier this year and I suggested he was 2nd instar. Mark ('iriscientist') agreed. He shed his skin for the first time in 2015 between 9th and 11th May. He is now laid up again for a second moult, though still only about 12-13 mm long - a typical size for the 3rd to 4th instar moult. It is not possible that this is a 4th to 5th instar moult, at which stage the larva is typically 24-25 mm long.
Unfortunately, I have suffered a high loss of larvae over the last couple of weeks. The other two I believed might have hibernated in 2nd instar have both disappeared. A third, that I am unsure about, is still alive, but I haven't logged all her moults as she's a bit off the track and when I have little time I miss out a daily check. I will try and follow Yudhiṣṭhira through his next moults but there is absolutely no doubt now that he did hibernate in 2nd instar. The evidence I have gathered over the last few years suggests this is not at all uncommon in Switzerland.
Yudhiṣṭhira on 9th May, still laid up:
Yudhiṣṭhira on 11th May, newly 3rd instar:
Yudhiṣṭhira today, 21st May, laid up for his next moult:
About 200m away in the same woods is Agni, who moulted into 5th instar on 16th May:
He was still there when I checked today ...