The wingspan is 70–75 mm or more. The forewings are grayish to brownish with prominent black bands above, and whitish below. The upperside of the hindwings is orange red, with two roughly concentric blackish bands on each wing, and lacking hairs at the base. The outer band separates a margin that is lighter in color than the rest of the upper hindwing. The inner band does not reach the trailing edge; its hindward tip forms a hook which points back at the wing base, or almost so. The underside of the hindwings is whitish along the leading edge; the trailing three-quarters of the wing are orange red. The inner black band is present on the underwings also. As in many relatives, the foreleg tibia of this species possess no spines, while the tarsi carry three rows of spines.^[1]

Adults are on the wing from June to September depending on the location. There is probably one generation per year. The caterpillars feed on willow species (Salix) and – in the west of their range – Fremont cottonwood (Populus fremontii).

This moth is placed in the subfamily Catocalinae, either of the owlet moth family, Noctuidae, or – if the Noctuidae are circumscribed more strictly – of family Erebidae. Within the Catocalinae, it belongs to tribe Catocalini and – if the Noctuidae are circumscribed widely – subtribe Catocalina.

This species is probably related to other North American Catocala that feed on willow and cottonwood. If so, it is one of the rather few species of this radiation that ranges east of the Rocky Mountains.^[1]

Synonyms edit

In the past, the name Catocala aspasia had often been used for this species, but actually it seems to refer^{[verification needed]} to the rather similar C. electilis.^[2]

Many taxa described as distinct species or subspecies have recently been merged with C. junctura, and are considered mere forms without taxonomic rank; the alternate vernacular name "Stretch's underwing" in fact refers to one of these, the former C. stretchii. Not all authors have already adopted these changes, which are, however, probably warranted. The junior synonyms and other invalid taxa used for the joined underwing are thus:^[2]

• Nelson, John M. & Loy, Peter W. (1983). "The Underwing Moths (Lepidoptera: Noctuidae) of Oklahoma" Archived 2012-11-02 at the Wayback Machine. Proceedings of the Oklahoma Academy of Science. 63: 60–67.
• Savela, Markku. "Catocala junctura Walker, [1858]". Lepidoptera and Some Other Life Forms. Archived from the original on July 20, 2019. Retrieved February 18, 2019.

• Anweiler, G. G. (September 28, 2004). "Species Details Catocala junctura". University of Alberta Museums. E.H. Strickland Entomological Museum. Retrieved November 8, 2020.
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