I took this picture one evening this summer. Could be a good one for the ‘love among the bugs’ page… Are these more ‘modest sphinxes’?
Your Sphinxes are definitely immodest, and they are not Modest Sphinxes either. These are Poplar Hawkmoths, Laothoe populi. As might be expected, the larvae feed on poplar trees. When alarmed the moths reveal a red patch on the lower wings to frighten predators, as depicted in this image we located online.