Vanni Fucci's obscene gesture at God is answered by snakes coiling over his mouth. Then Cacus appears — the centaur-son of Vulcan who stole Hercules's cattle by dragging them backward into his cave, so their tracks would mislead — a dragon on his back, fire-breathing, serpents covering his shoulders. He is different from the centaurs of the Phlegethon: he belongs here, among the fraudulent thieves, for his theft was deceptive as well as violent.
Five Florentine thieves are named and their metamorphoses described with a technical virtuosity Dante explicitly claims surpasses Ovid and Lucan. A six-legged serpent leaps onto one shade and fuses with it — its legs encircling the man's legs, its forefeet seizing his arms, its mouth covering his mouth, while its tail splits his feet. The two substances melt into each other and a single new creature emerges, neither man nor serpent. Then the reverse: a human form and a serpent exchange natures entirely, the serpent becoming human and the human becoming a serpent, their eyes changing color as they exchange essences. Theft's deepest nature is appropriation of the other's being; the punishment is the literal, physical enactment of that appropriation.