Some have proposed that the evolution of ultrasound as a male courtship cue in milkweed tussocks has arisen stepwise through transitional stages of redundant courtship cues (both pheromone and ultrasound). Furthermore, it has been proposed that ultrasound courtship was evolutionarily linked to a switch to cardenolide-producing larval host plants. To test the hypothesis of a transitional stage and proposed correlation of the behavior with larval hosts, the phylogenetic relationships of the phaegopterines Euchaetes, Cycnia, Ectypia, Pygarctia, and Pareuchaetes were assessed with 88 characters (205 states) derived from adult morphology. The resulting strict consensus placed the Pareuchaetes among the outgroup genera and recovered the remaining four genera as a clade. Euchaetes was not monophyletic; however, removal of Pareuchaetes results in monophyly. Based on this phylogeny, the ancestor of the ingroup clade fed on cardenolide-containing host plants, possessed well developed tymbals and lacked coremata (pheromone distributing structures). Thus, the use of male ultrasound in courtship was correlated with larval feeding on cardenolide hosts. The hypothesis that ultrasound first arose as a redundant courtship cue, with a subsequent loss of the male pheromone, was not supported by our data. Rather, male pheromone courtship was reacquired after ultrasound courtship in two lineages, based on the presence of coremata and behavioral data.
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Vol. 99 • No. 5