Urbanization contributes to declines in biodiversity of moths, but how it acts in megalopolises is unclear. In this study, I hypothesized that relative abundance of species within the family Saturniidae changes within megalopolises in eastern and midwestern North America. After compiling records from iNaturalist.org, I assessed 3,272 saturniid observations in areas of low and high human population density around East Coast and Midwest megalopolises including New York City, Philadelphia, Baltimore, Washington, and Chicago. From rural counties to cities, observations of Antheraea polyphemus increased by 15% while those of all other saturniids decreased by 63%–99%, depending on species. Species with greatest percentage decrease were Dryocampa rubicunda and Anisota senatoria. In cities, observations of A. polyphemus surpassed those of all other saturniids combined. These observations were nearly five times greater than observations of Actias luna, the saturniid with the next highest number of observations. By contrast, in rural counties, observations of the three saturniids (A. luna, A. polyphemus and Eacles imperialis) with the most observations differed by 25% or less. The proportion of saturniid observations specific to A. polyphemus more than doubled from rural counties to cities in each of the five metropolitan regions and in each of New York City's five boroughs except for the borough of Staten Island. The proportion of saturniid observations specific to A. polyphemus was essentially the same in Staten Island compared to less populous rural counties outside the city. Expansive tracts of natural forest in Staten Island likely blunted increases in abundance of A. polyphemus relative to other saturniids. Relative abundance of A. polyphemus in megalopolises increased in proportion to human population density on a subcontinental scale. On a local scale, environmental conditions potentially moderated this increase.