Third instar caterpillar of the giant swallowtail, Papilio cresphontes. Early stages resemble a fresh bird dropping and this caterpillar is highly dependent upon its camouflage for protection, but when disturbed it extends a pair of normally hidden scent horns. They produce a foul smelling substance that is thought to deter predation. Giant swallowtail butterflies are found over a large portion of the southeatern US west through the desert southwest and south into much of South America. During summer it extends its range northward as far as Canada. Photo, Dallas, TX on a citrus, a plant that this caterpillar is often a pest of. In citrus growing regions this caterpillar is often known as the "orange dog".