My firstborn, Brahmā, shed his skin in the rain this morning and entered 2nd instar. His skin is visible as a gelatinous mess at his tail and his old head capsule is on the leaf beside him:
Brahmā is a wild caterpillar, living at about 1000m in the Alps. He hatched on 3rd August and passed his first instar almost entirely in the cold and rain - but this does not seem to have delayed this transition. I think 13 days for first instar is normal.
Later in the day he coloured up: