Charles Darney is acquitted, and the crowd is compared to a group of flies.
Dickens takes great care to have fun with the trial, pondering the accused and the spectators. He describes the Old Bailey as a “kind of deadly inn yard, from which pale travellers set out continually, in carts and coaches, on a violent passage into the other world” (enotes etext p. 39). The accused are compared to travelers.
During the trial, the crowd is described as buzzing like flies. Dickens focuses on their excitement, and their hive-like mind as they view the proceedings as entertainment.
When the Attorney-General ceased, a buzz arose in the court as if a cloud of great blue-flies were swarming about the prisoner, in anticipation of what he was soon to become. When it toned down again, the unimpeachable patriot appeared in the witness-box. (p. 42)
Sydney Carton manages to come out of his drunken stupor long enough to come up with the brilliant strategy of comparing himself to his client. Since the two men look so much alike, there is no way Darnay could be proved to have done it. Darnay is acquitted! Now, the blue-flies are baffled.
[The] crowd came pouring out with a vehemence that nearly took him off his legs, and a loud buzz swept into the street as if the baffled blue-flies were dispersing in search of other carrion. (p. 50)