Harold Oldroyd (24 December 1913 – 3 September 1978)[1] was a British entomologist.
He specialised in the biology of flies, and wrote many books, especially popular science that helped entomology to reach a broader public.
Although his speciality was the Diptera, he acknowledged that they are not a popular topic: "Breeding in dung, carrion, sewage and even living flesh, flies are a subject of disgust...not to be discussed in polite society".
He also debunked the calculation that a single pair of house-flies, if allowed to reproduce without inhibitions could, within nine months, number 5.6×1012 individuals, enough to cover the Earth to a thickness of 14.3 m (47 ft).
Oldroyd calculated that such a layer would only cover Germany, but remarked "that is still a lot of flies".