Something that is fleeting or short-lived is ephemeral, like a fly that lives for one day or text messages flitting from cellphone to cellphone.
Ephemeral (ə-FEM-ər-əl) was originally a medical term with the specific meaning "lasting only one day," as a fever or sickness (Hemera means "day" in Greek.) The word became more general, coming to mean "lasting a short time," covering the life spans of plants or insects and then eventually anything that is fleeting or transitory. A related word is the plural noun ephemera, meaning "things that are meant to last for only a short time." Posters for a rock concert are often ephemera, unless the band is so famous that they get saved and sold on eBay.