I recently indentified the plant that the catepillars are hosting on in these pictures as: Water Hemlock "Cicuta Maculata". Warning! DEADLY POISONOUS PLANT! In the pictures below you will see porter weed and leaves mixed in. There is one picture that shows leaves only and that is from this plant.

This blooms vary, I guess as to the age of the plant or bloom. Notice how different this one looks on this young plant. This is it's first bloom.