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A medium-size (approx. 3 cm wingspan) diurnal jet black and cream moth. The forewings are black with a wide curved subterminal band and a large triangular basal patch of light cream. There is also a smaller square reniform spot, also cream. The pale areas are divided by a fine line of dark scales along each vein. Hindwings black with a large oblong cream patch in the center divided by a thick vertical black line. The black lining of the veins crossing the cream patches will separate ridingsii from all our other foresters except for the very similar but smaller (approx. 2.5 cm wingspan) Androloma maccullochii. Androloma males have a modified inflated costa containing a strip of membrane with vertical striations, giving the forewing costa a slightly convex shape. This is entirely lacking in ridingsii, which thus has a straight or slightly concave costa. The basal cream patch on the forewing of ridingsii extends only about halfway to the outer margin, but extends much further and almost meets the lower end of the subterminal band in maccullochii. The cream patches on the hindwings and in particular the basal patch is also larger in maccullochii, and the veins in the hindwing patch of maccullochii are more strongly lined with dark scales. Females are similar but lack the modified costa found in maccullochii.

A western species, found at higher elevations in the mountains, north to Alaska and south through BC at least into the pacific northwestern states, west to Vancouver Island. There is a single Alberta record based on a specimen found in the Shigematsu collection, collected at Highwood Pass July 7, 1968.