Social aphids produce different morphs, which are genetically identical but morphologically different. Each morph performs a different duty in its community. Social aphids usually produce morphologically distinct soldiers to protect their colonies. The social aphid Pseudoregma bambucicola
produces sterile first instar soldiers with specialized body parts and unique defensive behaviors, such as hind leg waving. By using this species as a research model, this study tested the assumption that the functional morphological basis of defensive behaviors of soldiers is related to specialized body parts. Field observations and a comprehensive morphometric analysis were carried out for natural populations. The results showed significant differences in functional morphology between soldiers, first instar nymphs, and adults. Elongated hind legs in soldiers are an important functional morphological basis for the deimatic behavior of hind leg waving, while sclerotized front legs and head horns are related to the function of directly attacking natural enemies. The size variation of different body parts among different morphs also indicates a cost–benefit trade-off in the evolution of the social aphid species.
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