With a wingspan of five to six inches, the giant swallowtail is the largest butterfly species in the United States. The base color of the upperside of the wings is dark brown with a series of yellow marks that form lines. The underside of the wings is mainly yellow with some orange and blue on the hindwings. The tail is dark brown with a yellow center.
The caterpillar host plants are wafer ash (Ptelea trifoliata) and prickly ash (Zanthoxylum americanum). The caterpillar’s coloration when it is about half grown allows it to mimic a bird dropping. The adult feeds on flower nectar. Adults fly from April through October. There are two generations per year. The chrysalis stage overwinters. The species is found statewide, but most commonly in central Illinois.