Butterflies are a diverse and charismatic insect group that are thought to have evolved with plants and dispersed throughout the world in response to key geological events. However, these hypotheses have not been extensively tested because a comprehensive phylogenetic framework and datasets for butterfly larval hosts and global distributions are lacking. We sequenced 391 genes from nearly 2,300 butterfly species, sampled from 90 countries and 28 specimen collections, to reconstruct a new phylogenomic tree of butterflies representing 92% of all genera. Our phylogeny has strong support for nearly all nodes and demonstrates that at least 36 butterfly tribes require reclassification. Divergence time analyses imply an origin ~100 million years ago for butterflies and indicate that all but one family were present before the K/Pg extinction event. We aggregated larval host datasets and global distribution records and found that butterflies are likely to have first fed on Fabaceae and originated in what is now the Americas. Soon after the Cretaceous Thermal Maximum, butterflies crossed Beringia and diversified in the Palaeotropics. Our results also reveal that most butterfly species are specialists that feed on only one larval host plant family. However, generalist butterflies that consume two or more plant families usually feed on closely related plants.

As the number of observations submitted to the citizen science platform iNaturalist continues to grow, it is increasingly important that these observations can be identified to the finest taxonomic level, maximizing their value for biodiversity research. Here, we explore the benefits of acting as an identifier on iNaturalist.

The caterpillars of many Lepidoptera are neither attacked nor tended by ants but nevertheless appear to be obligately ant-associated and benefit from the enemy-free space created by ants. Obligate myrmecophiles that do not attract ants through stridulatory or chemical signaling are limited to habitats where ants are reliably present for other reasons, either among ant-attended hemipterans, on ant-plants, or around ant nests. Particularly in the tropics, obligate ant associates that passively coexist with ants are more diverse than previously recognized, including, for example, hundreds of African species in the lycaenid subfamily Poritiinae. Mutualists and parasites of ants have been reported in eleven families: Tineidae, Tortricidae, Cyclotornidae, Coleophoridae, Crambidae, Erebidae, Notodontidae, Hesperiidae, Pieridae, Lycaenidae, and Riodinidae. Altogether, myrmecophily has originated at least 30 times in Lepidoptera, and many groups may remain undiscovered. The butterfly families Lycaenidae and Riodinidae contain the vast majority of ant-associated species: larvae of at least 3841 (71%) of the ~5390 described Lycaenidae and 308 (20%) of the ~1562 described Riodinidae are known or inferred to be ant-associated, and both families possess specialized, convergently developed exocrine glands and stridulatory devices to communicate with ants. Many caterpillar-ant relationships previously characterized as mutualisms may actually be parasitic, as caterpillars can manipulate ants and ultimately exert a fitness cost. In the family Lycaenidae, highly specialized and obligate ant associations are found largely in the Old World tropics, Australia, and Southern Africa, where the stoichiometry of soil micronutrients, particularly sodium and phosphorus, climate, host plants, and geography may all selectively shape caterpillar-ant associations.

Butterflies are a diverse and charismatic insect group that are thought to have diversified via coevolution with plants and in response to dispersals following key geological events. These hypotheses have been poorly tested at the macroevolutionary scale because a comprehensive phylogenetic framework and datasets on global distributions and larval hosts of butterflies are lacking. We sequenced 391 genes from nearly 2,000 butterfly species to construct a new, phylogenomic tree of butterflies representing 92% of all genera and aggregated global distribution records and larval host datasets. We found that butterflies likely originated in what is now the Americas, ~100 Ma, shortly before the Cretaceous Thermal Maximum, then crossed Beringia and diversified in the Paleotropics. The ancestor of modern butterflies likely fed on Fabaceae, and most extant families were present before the K/Pg extinction. The majority of butterfly dispersals occurred from the tropics (especially the Neotropics) to temperate zones, largely supporting a "cradle" pattern of diversification. Surprisingly, host breadth changes and shifts to novel host plants had only modest impacts.