In Act 3, Scene 1, the audience gets a chance to peek inside Mosca's deceitful mind for the first time. In a soliloquy, Mosca egotistically muses on his own parasitic virtue and espouses the versatility of his grift:
O! your parasite
Is a most precious thing, dropt from above,
Not bred 'mongst clods and clodpoles, here on earth. I muse, the mystery was not made a science,
It is so liberally profest! Almost
All the wise world is little else, in nature,
But parasites or sub-parasites.
This soliloquy is something of a manifesto for the social parasite. Plenty of people, Mosca argues, are somehow parasitic—by which he means capable of shape-shifting into any societal role and acting the part. This is a virtue, in Mosca's opinion; he even goes so far as to suggest that he has been sent from heaven.
This passage explicitly furthers Jonson's exploration of appearance and deception in Volpone, and Mosca will go on to show throughout the rest of the play just how much one stands to gain if they share his ability to adopt any persona and argue from any angle.
At the same time, this soliloquy fosters a certain amount of dramatic irony in the audience—Volpone himself sees Mosca's parasitic nature as a sign of his inherent lowliness, a talent to exploit rather than exalt, and his underestimation of Mosca's facility for acting and deception is ultimately the undoing of them both. Mosca's parasitic behavior is not evidence of a lack of ambition but rather the very source of it.
In Act 5, Scene 5, Volpone leaves Mosca alone in his house as he leaves to check in on the court proceedings. Soliloquizing on his machinations to the audience, Mosca builds upon the metaphorical animal identities of the play's characters and reveals his plot against Volpone, or "the fox."
[...] My fox
Is out on his hole, and ere he shall re-enter,
I’ll make him languish in his borrowed case,
Except he come to composition with me.[...]
So, now I have the keys and am possessed.
Since he will needs be dead afore his time,
I'll bury him, or gain by him. I'm his heir,
And so will keep me, till he share at least.
To cozen him of all were but a cheat
Well placed; no man would cònstrue it a sin.
Let his sport pay for 't. This is called the fox-trap.
Volpone has left his "hole," or burrow—that is, he has left his house, which is where Mosca now plots against him. Mosca's plan to fleece Volpone for his fortune, meanwhile, builds upon this metaphor when Mosca opts to give it the name of "the fox-trap." Mosca is now using the language of the hunt and establishing himself as the hunter—an aristocratic occupation befitting his newfound disguise as a member of the nobility, which he displays for Volpone and the audience at the beginning of this scene.
Mosca's great strength in Volpone is his ability to manipulate his identity and his status through the versatility of his language, and moments like this soliloquy show his skill in full form, thus creating some dramatic irony for the audience—only they are privy to the nature of Mosca's ambition, as he sets out to write his own story by moving against Volpone.
In Act 5, Scene 11, as the rest of the characters head to court for their final judgement before the avocatori, Volpone addresses the audience in a soliloquy laden with metaphor:
To make a snare for mine own neck!
And run
My head into it wilfully, with laughter!
When I had newly ‘scaped, was free and clear!
Out of mere wantonness! O, the dull devil
Was in this brain of mine when I devised it,
And Mosca gave it second; he must now
Help to sear up this vein, or we bleed dead.
At last, it would seem that Volpone’s lies have caught up with him. He has spent the whole play pretending to be sick and dying, and now he uses the metaphor of actual illness and physical injury to highlight his predicament: he is bleeding out and needs Mosca to help “sear up this vein” to stop the flow. Language has power in Volpone, and this soliloquy presents the effect of Volpone’s own words as capable of causing actual, physical damage to the man. Whereas previously in the play Volpone has treated gold and gifts like medicine that could “cure” him, now his game has run out—no amount of money will spare him from the judgement of the Venetian court.
