The lie could be made in order to save Volpone from the accusations against him, which Bonaire and Celia accuse him of; or to convince Corvino to let his wife sleep with Volpone. Regardless of how you look at it, Mosca seems to have no scruples about deceiving others.
His most important deception, however, is the one he effects on Volpone and the audience, hiding his true nature and intentions from both Volpone and the reader. In the opening acts, Mosca appears to be exactly what he is described as: a clinging, servile parasite, existing only for the sake of Volpone.
His entire existence is to serve Volpone. This impression is reinforced by several cringing speeches that he gives, all in praise of Volpone.
But in Act Three, we have the beginning of what seems an assertion of self-identity by Mosca, when he begins to grow confident in his abilities. But then this confidence again is left unvoiced, and Mosca seems to go back to being Volpone's faithful servant, helping him get out of the troublesome situation with Bonario and Celia. But it turns out that Mosca’s aid in this situation may have been motivated as much by personal interest as it was by a desire to aid Volpone, for when he is presented with an opportunity to seize Volpone's wealth, he takes it. Mosca himself is possessed by greed, and he attempts to move out of his role as parasite—a harmless fly, circling around a great beast—to the role of great beast himself. But his attempt fails, as...