Attacking one of them, it pierced right through
the part where we first take our nourishment;
and then it fell before him at full length…
The serpent stared at him, he at the serpent;
one through his wound, the other through his mouth
were smoking violently; their smoke met…
Let Ovid now be silent…
I do not envy him; he never did
transmute two natures, face to face, so that
both forms were ready to exchange their matter.
These were the ways they answered to each other:
the serpent split its tail into a fork;
the wounded sinner drew his steps together.
The legs and then the thighs along with them
so fastened to each other that the juncture
soon left no sign that was discernible.
Meanwhile the cleft tail took upon itself
the form the other gradually lost
its skin grew soft, the other’s skin grew hard…
And while the smoke veils each with a new color,
and now breeds hair upon the skin of one,
just as it strips the hair from off the other,
the one rose up, the other fell; and yet
they never turned aside their impious eyelamps,
beneath which each of them transformed his snout:…
his tongue, which had before been whole and fit
for speech, now cleaves; the other’s tongue, which had
been forked, now closes up; and the smoke stops.
The soul that had become an animal,
now hissing, hurried off along the valley;
the other one, behind him, speaks and spits. (Inf. XXV, 85-138)
Thieves fail to recognize the boundaries between their own property and that of others. To Dante, this indicates a basic flaw in their humanity; they lack the human reason to distinguish between what is theirs and what belongs to others. This blatant misuse of their intellects renders them more animal than human. Thus, in the Eighth Circle, they mutate into hideous pseudo-serpentine creatures. And because they did not honor the boundaries of property in life, they "exchange their matter" with each other, merging and morphing into the bodies of other thieves. In this way, they can hardly retain their own identities and have thus come to embody their sin.