Morphological differences between wing morphs of two macrosiphini aphid species, Acyrthosiphon pisum and Megoura crassicauda (Hemiptera, Aphididae)

In addition to caste polyphenism in social insects, aphids display one of the widest ranges of polyphenism, which produces several distinct phenotypes in response to environmental stimuli, e.g., daylength and density conditions. However, the mechanisms underlying aphid polyphenism remain poorly understood. This study was designed to evaluate the developmental impact of density conditions. We compared the morphologies of winged and wingless parthenogenetic females in the aphids Acyrthosiphon pisum (Harris) and Megoura crassicauda Mordvilko, based on body-part measurements and observations by scanning electron microscopy. The earliest and the most remarkable differences between winged and wingless morphs appeared in the mesothoracic parts from the third instar, where wings are formed in the winged morphs. Winged adults also have narrower heads, well-developed compound eyes and well-sclerotized thoracic cuticles, compared with wingless adults. Although differences in hind-tibia length were identified in both species, the relationships between the two morphs were reversed in the two species, probably because the ecological significances of the winged/wingless morphs are different for the two species.