Zoomorphology (2008) 127:121–133 DOI 10.1007/s00435-008-0057-5 123 ORIGINAL PAPER Morphological and histological examination of polyphenic wing formation in the pea aphid Acyrthosiphon pisum (Hemiptera, Hexapoda) Asano Ishikawa · Sakiko Hongo · Toru Miura Received: 17 March 2007 / Accepted: 20 January 2008 / Published online: 19 February 2008  Springer-Verlag 2008 Abstract Aphids display divergent adult phenotypes, depending on environmental conditions experienced during their embyonic and nymphal stages in their complex life cycles. The plastic developmental mode is an extreme case of phenotypic plasticity, so-called “polyphenism”, in which discrete multiple phenotypes are produced based on a sin- gle genome. For example, winged and wingless adult females are derived from a single genotype. However, the developmental mechanisms producing these polyphenic traits according to the extrinsic stimuli, such as density con- ditions, still remain unknown. In this study, to analyze the developmental processes underlying the wing polyphenism, we extensively observed and compared wing development in the winged and wingless individuals in parthenogenetic generations of the aphid Acyrthosiphon pisum (Harris), using scanning electron microscopy and histological sec- tioning. At the Wrst-instar stage, the wing primordia were observed both in the future winged (W) and wingless (WL) nymphs. Developmental diVerences can be seen from the second-instar stage, when wing primordia degenerate in the WL nymphs, while they develop and become more thick- ened in the W nymphs, suggesting that the developmental programs should be launched prior to this stage. Further- more, during the third- to Wfth-instar stages, wing buds and Xight muscles were well developed in the W nymphs, while wing primordia completely disappeared in the WL ones. In addition, the observation on the detailed developmental process of wing primordia during the third-instar W nymphs showed that the wing buds become swollen espe- cially at the basal part, even during the intermolt period. This was caused by the development of wing epithelia under the cuticle of this instar nymph. Actually on the sur- face of the cuticle of wing-bud bases, there were numerous furrows, which gradually expand during the intermolt period. The similar situation was also observed at the forth- instar nymphs, in which the wings are formed in the com- plicated manner inside the wing pads. Furthermore, the developmental process of Xight muscles was also described in detail. These dynamic developmental diVerences between the wing morphs should be regulated under the gene expression cascades that switch according to environ- mental stimuli. Keywords Flight muscle · Polyphenism · Postembryonic development · Wing bud · Wing polymorphism Introduction All organisms can Xexibly alter their phenotypes depending on the environmental condition they inhabit. This feature is termed “phenotypic plasticity” (West-Eberhard 2003). When multiple discrete phenotypes are formed, it is called “polyphenism” (Nijhout 1999, 2003). Polyphenisms are observed in various animal species, such as the caste diVer- entiation of social insects (Hymenoptera, Isoptera), the phase polyphenism of locusts (Orthoptera), the seasonal polyphenism of lepidopterans, and the induction of defense morphologies seen in amphibians and daphnids (Gilbert 2001). A. Ishikawa · T. Miura (&) Laboratory of Ecology and Genetics, Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan e-mail: miu@ees.hokudai.ac.jp S. Hongo Department of Biology, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan