Thaddeus William Harris (November 12, 1795 – January 16, 1856) was an American entomologist and botanist. For the last few years of his life Harris was the librarian of Harvard University.
Harris was a native of Dorchester, Massachusetts. His father, Thaddeus Mason Harris, was a Unitarian minister who had also for a time served as librarian of Harvard. Harris himself received his undergraduate education at Harvard, and then went on to study medicine there, receiving his M.D. in 1820. He went into medical practice with a Mr. Holbrook, whose daughter Catherine he married. Thaddeus and Catherine had 12 children.
Read Thaddeus William Harris' description of the "measure worm," free from Google Books.
In 1831 Harris became the librarian of Harvard. He replaced Benjamin Peirce who had just died in this position. Harris' study of insects was a result of his having interacted with William D. Peck while a student at Harvard. Harris lectured on various topics related to his work as a naturalist while he was Harvard librarian, and originated the Harvard Natural History Society for the students. In 1837 he was appointed one of the commissioners for a zoological and botanical survey of Massachusetts, the result of which was his Systematic Catalogue of the Insects of Massachusetts in which 2,350 species are enumerated. He sought a permanent appointment to the faculty in 1842, but the position was given to Asa Gray instead.
He was one of the founders of the Massachusetts Horticultural Society. He also published A Treatise on some of the Insects of New England which are Injurious to Vegetation (1842).
In 2008, Clark Elliot, a retired Harvard archivist, had his book Thaddeus William Harris (1795–1856): Nature, Science, and Society in the Life of an American Naturalist published by Lehigh University Press.