The caterpillars are growing – they’re in their second instar now. Caterpillars shed their skin when they grow too big for it – the stages between these molts are called instars.
Each time they change their skin, their color changes a little. In their first instar they were all black. Now they’re orange with black spines.

One caterpillar was much more orange than the others. The black lump is its old skin that it has just shed.