The hickory horned devil (also regal moth) is the biggest. They are the color bright teal with red horns on it's head with small black spikes along it's body.

The hickory horned devil is the largest caterpiller and is 12 cm long. 

The Regal Moth's caterpillar can get up to 140 mm, and when they mature, have a wingspan of 155 mm. The Queen Alexandra's Birdwing is a butterfly of comparable size.