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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. We downloaded over 500 photos of larvae from 10 different states (4 in the west, 6 in the east) and used image analysis to quantify the size of each larva's black stripes (% of surface covered by black). We compared average melanism level between east and west, while also accounting for temperature (at the photo location), seasonal effects, and latitude. Results showed larvae tended to be darker with lower development temperatures, and later in the season, though there was no significant difference between eastern and western larvae in their degree of melanism. It is not yet clear why findings here were different from the prior experimental study. Also, the wild larvae appeared to be notably lighter in color than the captive-reared larvae of the original study, suggesting that the captive environment does not truly replicate the developmental experience of wild larvae, at least in terms of cuticular development. Thus, if there truly are innate tendencies for western larvae to differ (morphologically) from eastern, they are not observable in nature. • Monarch butterflies in North America were once thought to have two distinct eastern and western populations. • Early laboratory work published in this journal showed western larvae were lighter in color than eastern. • Melanism levels of 500+ online photos of wild larvae showed no difference between western and eastern subpopulation larvae. • Monarch larvae tend to be darker with lower temperatures, and, later in the season. • Findings are consistent with more recent evidence for a single population in North America. [ABSTRACT FROM AUTHOR]

Abstract: [1.] As ectotherms, insects often experience varying temperatures throughout their life cycle, and some respond by becoming more or less melanistic (dark coloring) during development to increase or decrease thermal energy absorption as larvae or adults. [2.] Monarch butterflies (Danaus plexippus) breed in temperate and tropical environments worldwide and are exposed to different average and extreme temperatures in different parts of their geographic range. In this study, we compared variation in thermally induced melanism among monarch butterflies from eastern and western North America and from South Florida. [3.] We raised the progeny of wild-captured adult butterflies from these populations in a common garden experiment, rearing individuals in cold (19°C), moderate (26°C), and hot (32°C) temperatures to examine population variation in larval and adult pigmentation. [4.] Across all populations, monarch larvae developed the darkest coloration in the cold treatment and were lightest when reared in hot temperatures. Similar results were observed for measures of adult wing melanism, with the exception of adult females, which developed darker colored wings in warmer temperatures. [5.] Significant population-level differences in average measures of melanism among larvae and adult butterflies were observed. Larvae from the eastern population became substantially darker in colder temperatures than S. Florida or western larvae. Western larvae were lightest overall, which might be adaptive to high temperatures experienced throughout portions of their summer breeding range. S. Florida larvae showed a lower response to cold temperatures relative to monarchs from either migratory population. [6.] Population level differences were also observed for thermal responses in wing melanism, particularly among adult females. Moreover, we found significant family level effects for each measure of larval and adult melanism, pointing to a genetic basis or strong maternal effects influencing these traits in monarch butterflies. [Copyright &y& Elsevier]