Giant willow aphid is regarded the largest of it's kind, sometimes reaching 6mm big. They are sometimes described as mysterious because scientists still know shockingly little about their biology. Unlike most aphids, giant willow aphids feed on stems and twigs, penetrating the bark with their adapted mouthpieces. Like most species of aphid, the aggregate, forming large colonies. They are grey-brown with black dots, but more interestingly, possess a distinctive dorsal fin like a shark. The purpose of this is yet to be proven by scientists; however, it's thought it may be used for camouflage when feeding in thorned trees.