In some species of aphids, the formation of tubercles (bumps between the antennae) is used to assist with identification. These tubercles are located on the inside of each antenna near the base, producing an apparent indentation in the aphid head between them. Note in this photo and line drawing that there is a pronounced indentation between the antennal bases, but not as straight as the foxglove aphid. The sides of this indentation converge away from the head.
Line drawing from Integrated Pest Management for Bedding Plants: A Scouting and Pest Management Guide, Cornell University.