Spring brood individuals are the smallest, those which emerge in September or even October are much larger. Developing pupae exposed to cool conditions produce greyer adults than those kept drier and warmer. Imagines from the latter tend to be much paler and browner. The defensive 'glaring eye' posture is more exaggerated in this species than it is in S. ocellatus . S. planus also drops to the ground more readily on being disturbed.
Emergence takes place just after dark, with females calling for a short period around midnight.
Egg-laying often commences on the same night immediately after pairing. Females of this species will readily attract wild males of S. ocellatus in Europe, with the resulting eggs being fertile and the larvae vigourous. However, only male hybrid pupae hatch, usually at the same time as the S. planus parent. Depending on latitude and weather, there are between one and four generations farther east in China, with adults on the wing between April and September. Two are normal in Beijing and Ningxia, three in southern Shaanxi, four in Jiangsu and Jianxi.