I have never heard that expression my self...but there are some insects that live for only a "day" as an adult...Because of the short lifespan of the highly visible, winged adult form, the mayfly is called 'one-day' or 'one-day fly' in some languages - http://en.wikipedia.org/wiki/Mayfly#Life...

I kinda read up on this and from my understanding the longest a fruit fly lives is one week. And supposedlythe friut fly is the shortest living fly. So where did the rumor of flies living 24 hours come from?