Wikipedia
Antonia Caenis or Cenide, a former slave and secretary of Antonia Minor (mother of the emperor Claudius), was the mistress of the Roman emperor Vespasian. It could be thought that she had family in Istria, now in Croatia, based on a trip she took there (Suet. Dom. 12.3). In her 30s Caenis, still possibly a slave, was in an unofficial relationship known as contubernium, with Vespasian before his marriage. According to Suetonius, after the death of Vespasian's wife Flavia Domitilla, Vespasian and Caenis, now a freedwoman, resumed their relationship; she was his wife "in all but name" until her death in AD 74. She had a remarkable memory and considerable influence on the emperor's administration, carried out official business on his behalf, and apparently made a lot of money from her position . However, she was treated with disrespect by Vespasian's son Domitian, who refused to greet her as one of the family (Suet. Dom. 12.3).
Caenis is a genus of mayflies. They are very small in size, sometimes with a body of only an 1/8 of an inch .