the eggs hatch, producing larvae, which feed on organics in the bookbinding
at the appropriate time each larva makes the transition (which ghawd designed because... well, why not?) into a fly
boy flies and girl flies mate and the girl flies lay their eggs
the book is opened and this pushes open a space at the top and bottom of the binding and the flies extant at that moment take their leave
only when the book is opened does that space become available for the available flies to escape
while the book remains closed, the cycle continues within the binding and probably the larvae eat the bodies of the flies that don't get to make the exit
as the lead in one of my favorite movies says after reading his grandfather's diary, "...IT COULD HAPPEN!"
about that same grandfather, a minor character later declares, "YES, Viktor vas my ... BOYFRIEND!"
They were not pastrami caught between the bread slices.
There were no larvae, eggs, or otherwise signs of infestation inside the book, in between its pages, or the binding.
They were no ordinary flies, smaller than houseflies. They looked like Egyptian miniature drones in drag. They shot straight up in the air from the book when it was opened, and hit the air where they fluttered for a few seconds, and then disappeared as mysteriously as they appeared. They left not a trace.
It wasn't one time, but each time I opened that book. Despite my investigating and looking for its source, which I could find none. The book subject matter was as old as time, as was the book resembled a relic from an ancient past. Something about those flies struck me as having an ethereal quality. Which in turn gave the book a certain added weight and dimension.
Leading me to wonder if there was an ancient curse by an overseer, over the geologist who had authored the book, or the book itself. But I'm a romanticist, so I look for subtleties in meaning.