Comparative analysis of caste differentiation during embryogenesis of social aphids whose soldier castes evolved independently

We investigated the morphological characters of normal nymphs, soldier nymphs and developing embryos of a social aphid, Colophina arma, which has a sterile soldier caste in the first instar. Morphometric analysis revealed that normal nymphs and soldier nymphs were clearly distinguishable on the basis of several morphological characters. At late embryonic stages, normal embryos and soldier embryos were also distinguishable morphologically. The younger the embryonic stages, the smaller the morphological differences between them. In young embryos of less than 600 μm in body length, normal embryos and soldier embryos were no more distinguishable, suggesting that the onset of soldier differentiation occurs at an early embryonic stage. Through the embryonic development, morphological differentiation of soldier caste proceeded gradually: forelegs and midlegs were exaggerated, and growth of mouthpart was suppressed, which resulted in the soldier morphology specialized for attacking behavior. On the basis of these results, developmental aspects in soldier differentiation of C. arma were compared with those of Pseudoregma bambucicola, a social aphid with a first instar soldier caste of independent evolutionary origin. Ecological and evolutionary relevance of the differences between the two social aphids was discussed.