As Ossein correctly commented, the naturalist who first realized that leafcutter ants were feeding on fungi rather than leaves (trivia question in the Fungus Gardener post) was Thomas Belt.
I first ran across Thomas Belt in the selection of essays about insects, Insect Lives edited by Erich Hoyt and Ted Schultz. In this collection the editors included two essays from Belt's book The Naturalist in Nicaragua (1874): "Army Ants," and "The Ant and the Acacia Tree."
Thomas Belt (1832-1878) was a mining engineer by profession, but he was also a superb naturalist. When his work took him to places like Nicaragua he was able to put his keen interest and observation skills to good use. When examining leafcutter ants, Belt discussed the likelihood that many local trees and plants were probably less suitable for leafcutter ant gardens than undefended crop plants brought in from other areas. In addition to noting the leafcutters' use of fungal gardens, Belt also first recognized the nature of the relationship between "bull's-horn" acacias and ants, and that Cecropia stems provided homes for ants.
The electronic versions of his book The Naturalist in Nicaragua available for free at Project Gutenberg have a wonderful introduction from the Life and Letters of Charles Darwin. If you are interested in the "history" aspect of natural history, then you might want to take a look.
The Naturalist in Nicaragua is also available on Amazon for the Kindle