Two maggots

     "Yes, I've got a new theory of the crime, which knocks yours into a cocked hat. I've got evidence to support it, too."

    "Which crime, by the way?"

    "Oh, the Epping Forest business. I don't believe the old Dawson person was murdered at all. That's just an idea of yours."

    "I see. And you're now going to tell me that Bertha Gotobed was got hold of by the White Slave people."

    "How did you know?" asked Parker, a little peevishly.

    "Because Scotland Yard have two maggots which crop up whenever anything happens to a young woman. Either it's White Slavery or Dope Dens - sometimes both. You are going to say it's both."

    "Well, I was, as a matter of fact. It so often is, you know."

(from Unnatural Death, by Dorothy L. Sayers)