Clearly, the way in which Mosca's name is derived from the Italian for fly presents us with the theme of parasitism in the play at its very beginning. Most importantly, however, is Mosca's soliloquy in Act III scene 1, where he praises the position of being a parasite, and also argues that everyone is actually a parasite, to a lesser or a greater extent. Consider what he says about the role of being a parasite:
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.
Mosca's view of the world, with everyone feeding off others in some way of course is borne out by the action in the play. All the characters are shown to attempt living off the riches of other characters, avoiding hard work themselves. Clearly, the three death-bed suitors are perfect examples of people "sucking" wealth from others, but let us also remeber that Volpone is only rich because of his ability to extract wealth from others.
What is key to realise however is the way that parasitism is presented in an extremely positive light. As Mosca shows and discusses, it takes considerable intelligence and skill to be a successful parasite. It is not about laziness. Consider how Mosca presents the "art" of being a "true" parasite:
But your fine elegant rascal, that can rise,
And stoop, almost together, like an arrow;
Shoot through the air as nimbly as a star;
Turn short as doth a swallow; and be here,
And there, and here, and yonder, all at once;
Present to any humour, all occasion;
And change a visor, swifter than a thought!
This is the creature had the art born with him;
Parasitism is therefore depicted as a form of superiority, as the successful parasite "feeds" off other characters through their abilities to manipulate and deceive.