Theological necessity was among the main reasons which led St. Isidore of Seville, in the seventh century, to incorporate this theory [of the later development of insects out of carrion, following the initial creation], supported by St. Basil and St. Augustine, into his great encyclopedic work [] which gave materials for thought on God and Nature to so many generations. He familiarized the theological world still further with the doctrine of secondary creation, giving such examples of it as that "bees are generated from decomposed veal, beetles from horseflesh, grasshoppers from mules, scorpions from crabs," and, in order to give still stronger force to the idea of such transformations, he dwells on the biblical account of Nebuchadnezzar, which appears to have taken strong hold upon medieval thought in science, and he declares that other human beings had been changed into animals, especially into swine, wolves, and owls.
Isidore of Seville