Flies that have any direct effect on Man are only a small proportion of the eighty to one hundred thousand species which constitute the Order Diptera. "To discuss adequately a group so rich and varied it is necessary to particularize, mentioning the names of species, genera, families or larger groups," writes Professor Oldroyd. Yet, he has tried to present more of a picture of flies in evolution than a discussion of classification and arrangement. Part One is a general description of flies and their life histories: eggs, larvae, pupae, and adult flies. Part Two surveys the flies in sixteen groups, in what appears to be an ascending order of evolution, as seen from the viewpoint of natural history. "When I point out similarities of habit I do not necessarily wish to maintain that the flies concerned are closely related," he writes. "On the contrary, it cannot be overemphasized that flies have made the same evolutionary experiments again and again, and much of the interest in studying them is to see how different groups tackle similar problems." Part Three deals with the effects of flies on Man, the communal behaviour of flies, and their future in a rapidly changing world.