it was on hn a while back. the link is dead now. https://news.ycombinator.com/item?id=14146776
it’s on wikipedia, with the same dead source https://en.wikipedia.org/wiki/Mirror_test?wprov=sfti1
EDIT: Working link to the paper (in some journal of science): https://www.udocz.com/read/are-ants-capable-of-self-recognit...
http://difusion.ulb.ac.be/vufind/Record/ULB-DIPOT:oai:dipot....