What is the origin of the idiom 'a fly on the wall'?

This is an American phrase that originated there in the 1920s. The first citation of it that I can find is from The Oakland Tribune, February 1921: “I’d just love to be a fly on the wall when the Right Man comes along.” It is now most often used in relation to ‘fly on the wall documentaries’, which are films of real life situations supposedly made without affecting the behaviour of the participants. It is de rigeur for participants in such films to comment along the lines of ‘We just got used to the camera crew and after a while we just ignored them’. Well, maybe. I must say that if I had half a dozen strangers trooping around my house I think I would know that they were there.

For hundreds of years, in English and no doubt in other languages, flies have been cited metaphorically in connection with the idea of enjoying unseen intimacy or intelligence. Thus for example, in Romeo and Juliet (by 1595) we have this (in an 1821 edition) ... Romeo's speech is invoked (and rewritten) by Litchfield Moseley, "A Successful Elopement," in Once a Week (September 3. 1870) as part of a lover's letter to his beloved ... Also (and more specifically relevant to the modern sense of "a fly on the wall"), in Susanna Centlivre, Marplot in Lisbon (1711), reprinted in The Works of the Celebrated Mrs. Centlivre, volume 2 (1761):
"Marplot. A Letter! Wou'd I were a Fly now, that I might swop down upon the Paper and read it before his Face : Lord, Lord, what wou'd I give for an universal Knowledge!"

The earliest match in a Google Books search for the exact words "a fly on the wall" in the intended idiomatic sense is from Julia Cecilia Stretton, Woman's Devotion (1855), a three-volume novel published in London: "Had Lady Jane been a fly on the wall, and seen Frank's delighted face" ...The next match in Google Books is from 1880, followed by three matches from about the turn of the 20th century. From Adeline Dutton Train Whitney (an American author), Odd or Even? (1880), which uses the phrase four times in the course of about ten pages ... From Lucy Hill, Marion's Year in a German School (1899) ... From Bithia Mary Croker ("a prolific Anglo-Indian author," according to Wikipedia), The Happy Valley (1904) .. From Bettina von Hutton (another American novelist), The Halo (1907)

So we have five instances of "a fly on the wall" used in the modern idiomatic sense in the space of 52 years, all in novels by female writers-the first English, the next American, the third probably American but with a special interest in Germany, the fourth Anglo-Indian (but living in England when she wrote her book), and the fifth American ... Whether Julia Stretton is directly responsible for popularizing the notion of being "a fly on the wall" in order to gather information or witness a private scene is difficult to determine; but it seems fair to say that the six English and American women cited above whose relevant novels appeared between 1855 and 1907 almost certainly deserve credit for popularizing (in English) that wording for a very old idea.