 Jenny's question reminds me of a previous episode of Crowd Science where we try to work out how many people have ever existed. That episode is still available wherever you get your podcasts. But as hard as that question was, I have a feeling this is going to be even harder. At least humans keep records of themselves. Also, it's arguably a bit clearer what is human and what isn't. But when it comes to flies... How would you define a fly?