The scene is Volpone's house. This scene consists entirely of a soliloquy by Mosca. He enters, and expresses fear at his growing narcissism. This increasing self-love is the result of the successful way he is helping Volpone conduct his con-game. He then discusses what it is to be a "parasite," presenting it as an "art" in which most of the world, in fact, takes part: "All the world is little else, in nature, / But parasites or sub-parasites."
Corbaccio's son Bonario, enters. Mosca begs to talk to him, but he scorns him, deriding him for being a parasite. Mosca pleads with him not to be so harsh and asks for his pity. Bonario responds to Mosca's plea. But then Mosca informs Bonario that his father has disinherited him. The son does not believe it at first, but Mosca asks him to follow his lead. He promises to let Bonario see his father Corbaccio in the act of disinheriting him. They exit the stage together.
Mosca opens the act with a soliloquy. The soliloquy is a speech given by a character while alone on stage, spoken directly to the audience, and it usually gives the audience direct access to the inner workings of the character's mind. In other words, the soliloquy is an opportunity for villains to speak their plans and heroes to voice their doubts, and it gives the playwright an opportunity for characterization, defining the character's motivations, problems, and quirks. In the case of Mosca, this soliloquy is especially important in terms of characterization. This is the first time in the play we see Mosca without Volpone, and most of the things Mosca has said up to this point have been clever lies told in order to benefit his master. Mosca has remained a very shadowy, elusive character. Jonson does use the speech for this purpose, but he does it obliquely. And initially he uses the speech to foreshadow later developments in the play. Mosca is growing increasingly independent in the play; we just saw him arrange Volpone's seduction of Celia by himself, and now we have him alone, on stage. Jonson hints that this increasing independence will be a problem for Volpone. "I fear" are the first words of the Act, and he worries that he is growing too strong, too confident, too in love with himself: "I could skip/ Out of my skin now, like a subtle snake, / I am so limber." The snake, a symbol of temptation, signals danger; and the imagery of transformation, of slipping out of one's skin, indicates that Mosca is becoming less satisfied with his identity as a lackey to Volpone.
This line later developments in Act V, where there is a reversal of fortune between Mosca and Volpone, and Mosca "slips out of his skin" to become a nobleman (if only temporarily). Mosca only refers to himself indirectly, through an abstract discussion of parasitism. The word "parasite" had a slightly different meaning for Jonson than it does today; the "parasite" was a stock character of Greek and Roman comedies, similar to the "Fool" character, except that they were buffoons instead of witty. These characters were usually pathetic, and poor: they performed tricks, told stories, and generally debased themselves in order to feed at the table of the rich. Parasitism, in this sense, seems to be a trait that defines Mosca completely; he is dependent on Volpone for his life and for his food (a true parasite), and he expresses this dependence, at least in public, with displays of servility and pathos. In Act I, scene ii, he is so gushing toward Volpone that the Fox, embarrassed, asks him to stop, and his pathetic display in front of Bonario again reinforce this impression. But when Mosca talks about how "all the wise world is little else, in nature, but parasite or sub-parasites," he begins to overturn this image. He identifies the true parasite as being strong, quick, agile, inventive and able to fake any emotion in a second if necessary; being a parasite is a mark of being wise, and not being a "clot-pole," forced to work the earth to make one's living. He is superior to those he feeds on, not inferior.
People that are called "parasites", like Mosca, play the same game as everyone else, but just play it better, with more cunning, which demonstrated in his dealings with Bonario. They show weakness in order to hide their true strength; the goal of the game is to feed off the wealth and livelihood of others without doing any real work yourself, except the work involved in keeping them credulous. This statement of moral equality—"we are all parasites"—clearly refers not only to the legacy hunters, but to Volpone as well; it is indicative that Mosca considers himself less and less a subordinate to Volpone and more of an equal. Mosca gradually develops into an antagonist for Volpone. But his honesty, his resourcefulness, and the correctness of his appraisal of the situation—everyone in the play does in fact attempt to live off of the work of others—make him a sympathetic character. This creates a tension in the play, as to who we are to side for in the battle, who is the protagonist and who is the antagonist. In the end, the answer will seem to be neither, as both characters are punished for their actions. But as we will see, there are problems in the way Jonson brings this result about.