you may be entitled to a free will
About 80% of the plums I picked in August looked like this. When you cut the fruit open you can see that the stone is rather neatly broken close to its top, and there's a horizontal scar passing across the inside of the plum, but not reaching the edges. I didn't see any creatures, but I suppose the damage is done by some larva a bit like the plum-fruit sawfly, whose eggs are laid in the plum ovary at a very early stage. Unlike the plum-fruit sawfly, however: 1. this one appears to feed only on the kernel and is not interested in the flesh, 2. the fruit do not fall prematurely but remain on the tree and mature normally, 3. There is no visible exit channel. All rather puzzling. Anyway, it didn't stop me using the fruit, so I guess that's a good pest to have. Plum pie, anyone?
That was a month ago, but I never got round to photographing it, so today I went to see if there were any left, and as you see I did get one, but it wasn't easy. The area is shortly to be made into homes, so has been fenced off, but that wasn't my main problem. (I regret the many small ecosystems I have watched on this waste land over the last eight years, - most will now disappear.) The plums were nearly all gone and rotted, but eventually I found a few that were still just about OK, very high up. Eventually I managed to beat this one down with a stick - you can see the bruise where it hit the ground.
Since I can't be bothered to start a new entry, I'm also posting a couple of photos of rosebay willowherb (Chamerion angustifolium), one from July and one from November.