There are two larvae (caterpillars) here, of two different instars. The white things on the horizontal branch are woolly aphids being tended by ants. The larvae are predatory on the aphids and are often disguised by hiding under the mass of aphids. The ants are trying to protect the aphids which they tend (rather like we tend cattle) and protect, in order to eat the sweet honeydew the aphids secrete. This is the only butterfly in North America with carnivorous larva!! To see the attractive adults, please look at the next photo.