What is the fly doing here?

This fine portrait by the Flemish artist Petrus Christus has a huge, ugly fly at the very front of the painting.

Portrait of a Carthusian, 1446

 What is the reason for it to be there?

• A memento mori?
• Or the depiction of the levels of beauty in Nature
• Or is simply a peculiar signature of the Artist, who was fascinated by the concept of the linear perspective

via Daniel Arres and shakko_kitsune