Francesco Redi's main contribution to biology was proving that maggots did not erupt spontaneously from rotting meat, but were deposited there in the eggs of flies. He was an early pioneer in the study of parasitology, observing that many types of parasites developed from eggs and did not spontaneously generate.
Spontaneous generation, a theory that maggots, fleas, worms and other living organisms developed from inorganic or dead organic matter, was the prevalent viewpoint of scientists for around 2,000 years, since Aristotle first posited a description of the phenomenon.
In 1668, Francesco Redi conducted the first experiment to challenge this theory. He placed various types of meat in six jars. He covered three jars with gauze, and he left the other three open. Maggots appeared on the open meat but only on the gauze covering the other jars. Next, he used three jars, corking one, covering one with gauze and leaving the other open. Maggots appeared on the meat in the open jar and on the gauze but not in the closed jar. In the 19th century, Louis Pasteur expanded on Redi's experiments to disprove spontaneous generation conclusively.
Redi also studied parasites in great detail, writing descriptions and creating illustrations in books and treatises. His observations backed up his conviction that parasites laid eggs from which offspring developed and did not grow spontaneously.