Keep finding these guys on my sliding glass door, near my houseplants, which have frequent aphid infestations (correction: I should specify that I don't know they're aphids, just that they're small, flying, and highly annoying insects in large numbers). The smallest ones might be around 0.5cm long; the largest approach 2cm. The insects seem lethargic, moving slowly or not at all; when they do fly, they fly slowly. They appear almost exclusively in the afternoon and evening. I submitted this picture to reddit, where someone suggested that it might be Pemphredoninae, but suggested I post it here for confirmation and/or a more precise ID.