Revisiting geographic variation in melanism of monarch butterfly larvae in North America using iNaturalist photos.

Monarch butterflies in North America have an exceptionally large breeding distribution, occupying regions west of the Rocky Mountains and throughout the eastern seaboard. An experimental study conducted 17 years ago and published in this journal appeared to show that western monarch larvae tend to have smaller black stripes than those from eastern parents, which at the time was thought to be an adaptation to higher solar exposure in California. Here, we revisit this question by measuring melanism of eastern and western larvae from online photographs submitted to iNaturalist by members of the public.