Figure 2

External morphology in winged males of A. pisum . Ordinal numbers indicate the nymphal stadia (instars). Meso- and metathoraces where wings are produced are indicated in red. Arrowheads indicate wings and wing primordia (third to fifth instars). Although no obvious external structures (bulges) are observed in first and second instar nymphs, they possess wing primordia underneath the cuticle (see Results for detail). No apparent differences of wing developmental process are found between the winged males and viviparous females. See the Additional file 1 for the methods of SEM observation.