A few moments' Googling produced the probable origin of the anecdote, and a different interpretation: that the African viewers were in fact joking, but were misinterpreted by colonialist observers predisposed to think them primitive and visually illiterate. James Burns, a historian of African culture traces it to a colonial film maker William Sellers.
“After the war, when the magazine Colonial Development reported on the work of the Colonial Film Unit in Africa, it managed to squeeze two of Sellers’ stories into the article’s first paragraph.Burns goes on to suggest an alternative interpretation:
‘On one occasion, during the showing of a serious instructional film in Nigeria, the audience was unaccountably rocked with laughter. It was afterwards discovered that the behaviour of a white hen that had strolled into the picture had distracted attention from the main purpose of the film. A film on malaria being shown to a bush audience made little impact because at one point a greatly enlarged picture of a mosquito filled the screen so that its structure could be explained. The audience declared that there was no reason to fear the tiny mosquitoes they knew, which were quite different from the huge and terrifying creature they saw on the screen.’
George Pearson, who became the Colonial Office’s chief film maker in 1939, related the story in an article he published in 1949. ‘The reaction among the natives was ruinous to the film purpose, for they said there would be no need for them to worry about the little mosquitoes they knew; those in the film were enormous and terrible things quite different from anything in their country!’ Pearson then explained the obvious lesson: ‘What had been overlooked was the complete ignorance of the primitive mind and magnification’. Pearson could not resist retelling it a decade later in his autobiography. The story lives on in Southern African today. The historian Tim Burke found it circulating among White professionals in the advertising industry in Zimbabwe in 1991.”
“One’s first impression is that these stories are apocryphal, particularly since the specifics of the incidents are rarely given. And if true they are certainly open to alternative interpretations. Megan Vaughan, in discussing the reaction of the audience to the mosquito on the screen, pointed out that Sellers and his successors never considered that such comments might have been meant ironically. My own experience in Zimbabwe suggests the likelihood of this possibility. Two separate former mobile cinema operators of the Rhodesian Information Service recounted to me their experiences showing rural people films explaining the lifecycle of a new strain of maize. The use of time-lapse photography inspired members of two separate audiences to ask ‘Why does our government not give us this maize which grows so fast?’ Both informants related this story as evidence of the credulity of their audiences. However, when I told a third retired cinema operator this story he merely laughed: ‘Did they not realise the people were only joking?’.”James Burns (2000) Watching Africans Watch Films: theories of spectatorship in British Colonial Africa, Historical Journal of Film, Radio and Television, 20: 2, 197 – 211
William Sellers (1941) The production of films for primitive people, Overseas Education: A Journal of Educational Experiment and Research in Tropical and Sub-Tropical Areas, (October 1941), p. 221.