Juvenile animals are often less successful in acquiring nutrition than adults as their feeding traits are still developing. Furthermore, in most animal species, adults do not provide food to their offspring, and therefore, juveniles must compete with adults for food. Developmental plasticity in feeding traits may be an important adaptation promoting juvenile survival in competitive environments, but social impacts on feeding morphology have rarely been investigated. Here we examine how adult presence affects morphological development in the leaf-footed cactus bug Narnia femorata (Hemiptera: Coreidae). Juveniles of this species are limited in their feeding potential as their piercing-sucking mouthparts are shorter than those of adults, but whether feeding near adults can impact their developing mouthparts is unknown. To investigate this, we raised juveniles in groups containing either an unrelated adult or no adult. Juveniles reared with an adult nearby developed longer mouthparts relative to their body size than those raised without adults. This suggests that plasticity in mouthpart length is a potential adaptation that juveniles use to compete effectively with larger adults. Our study aids in our understanding of animal development and the strategies used by juvenile animals to overcome environmental challenges.