They've hatched, and they're hungry!
The caterpillars of the Cinnabar Moth, feast on Tansy Ragwort, a noxious & invasive species. They head for the flower buds first, work down the stalk eating everything, and then move on to the next plant. Almost all of the tansy plants in an acre area, had an egg sack attached to it. Keep on eating you wonderful little caterpillars!
For more pictures of the Cinnabar caterpillars & moths, click here.