Our key to Illinois butterflies, exclusive of the skippers (Hesperiidae), was published in 1980. It includes those butterflies listed by Irwin and Downey in their 1973 Illinois checklist plus two species,Anaea aidea (Guerin-Meneville) (Nymphalidae) and Celastrina ebenina Clench (Lycaenidae), added to the state list after 1973.
Sites, R. W. and McPherson, J. E.
"The First Records in Illinois of Heliconius Charitonius (Lepidoptera: Heliconiidae) and Phoebis Agarithe (Lepidoptera: Pieridae),"
The Great Lakes Entomologist, vol 14
Available at: https://scholar.valpo.edu/tgle/vol14/iss4/8