About Xanthorhoe fluctuata (Linnaeus, 1758)
This species, Xanthorhoe fluctuata (Linnaeus, 1758), has a wingspan of 27–31 mm (1.1–1.2 in). Its wings are greyish white, with three irregular black blotches along the leading edge (costa) of the forewing; the middle blotch is the largest. Occasional darker, melanic forms of this species exist. Despite its variability, it is easy to recognize. In the typical form, the ground wing color is dirty whitish, and the median wing band is almost or completely absent in its posterior half. Several named aberrations are recognized: ab. ochreata Prout has an ochreous ground color; ab. neapolisata Mill has a ground color heavily darkened to brown-grey; ab. costovata Haw. has a median band that is much narrowed and somewhat shortened, and deleta Ckll. is only a rather extreme development of this form that does not need to be separated as a distinct type; ab. immaculata Tutt is a much rarer aberration that lacks the median band entirely; ab. abstersata H.-Sch. represents the opposite extreme, with the band fully developed and darkened completely across the wing, and the distal area sometimes also darkened. Two named geographic races are also recorded: thules Prout is a dark race from the Shetland Islands. Less extreme forms of this race intergrade into ab. neapolisata, but the name strictly applies to specimens where both forewings and hindwings are entirely darkened (infuscated), so that markings are almost completely obliterated. sempionaria Rätzer is a small race from the Simplon, with a rather purer white ground color than the typical form, and a small but sharply marked median half-band.
