LAUSR

Abstract Novel host plants are incorporated into the diets of phytophagous insects when females oviposit and juveniles feed and survive on them. A change in diet, however, can have morphological consequences. We recently found a population of the leaf‐footed cactus bug, Narnia femorata (Hemiptera: Coreidae), a historical cactus specialist, living and feeding on Cirsium thistle. We also found adults breeding and males using their enlarged hind legs (i.e., weapons) in male–male combat on thistle. When we compared this thistle population with a population feeding on cactus, we found that both populations had similar body and weapon sizes as well as weapon composition. However, the population living on thistle had longer mouthparts than the population found on cactus, although this difference only occurred at larger body sizes. This difference in adult mouthpart size is likely a result of the juvenile rearing environment (i.e., thistle or cactus). However, genetic differences may also affect this trait. Our results provide some interesting avenues for future research (e.g., a reciprocal transplant experiment) in a species with a recent host range expansion.