De Geer was born in Finspang, Sweden, and educated in the classics at the University of Utrecht; he then studied under Linnaeus at Uppsala. His extensive Mémoires pour servir à l'histoire des insectes (7 vols. 1752–78; Contributory Notes on the History of Insects) include excellent drawings and probably the earliest published accounts of the maternal instinct in such nonsocial insects as the earwig Forficula auriculara and the shield bug Elasmucha griseus. He also initiated a system of insect classification based on the wings and mouthparts.