In Chapter 3 of Book the Second of Dickens's A Tale of Two Cities, the symbolism attached to the simile of blue flies in the courtroom during the trial of Charles Darnay cannot be overlooked. Traditionally, the fly represents evil, death, destruction, and corruption--even Beelzebub himself. Thus, in this passage in which the flies swarm around the French aristocrat, Charles Darnay, foreshadows the other "blue flies" of the swarming revolutionaries in France who congregate to bring death and destruction to the aristocrats and Charles Evremonde after he returns to his country.
Even the crowd in the London courtroom parallels those of the other city in their degeneracy as they sadistically listen and await the condemnation of a man that they do not know, but hate because he is aristocratic.
In Book 2 - The Golden Thread - Chapter 3, "The Disappointment" Charles Darnay is being prosecuted by the Attorney General on charges of treason for passing the state secrets of England into the hands of the enemy, the French. After the Attorney General had finished his speech charging Charles Darnay with treason; the onlookers, who till then had remained silent, in the court room began to whisper and discuss amongst themselves the fate of Charles Darnay.
Dickens very poetically compares their whispering to the buzzing sound of blue-flies:
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.
In Chapter 2.3 of Dicken's book "A Tale of Two Cities" the trial is underway. The Attorney General tells the crowd that even though the man in front of them is young his actions of trespass are old. The crowd in the court room listens and murmurs. The man is presented as sublime and having no concern for his actions. The crowd in the courtroom listens and hums like a swarm of flies gathered together.
"A buzz arose in the court as if a cloud of great blue-flies were swarming."
The patriot then appears in the witness box. Mr. Jarvis Lorry is called up to testify and Dickens again refers to the crowd as flies.
"The blue-flies buzzed again, and Mr. Attorney-General called Mr. Jarvis Lorry. "