Ok, so maybe a LOT gross. Sorry about that (but you can click on the photos if you'd like to see a bit more detail).
We noticed that the lemon tree was attracting flies for some reason (the husband thinks they're attracted to yellow). There seem to be an awful lot of them this year - we've had alternating wet and warm weather, which probably has a lot to do with it.
There are a few types of flies, including big blowflies (erghhhh), and it's quite close to the front door, so not particularly desirable. We didn't want to use sprays, so we bought a few of these flycatchers from the hardware megastore. And do they work or what! This was about two days worth of flies!
I'm not exactly relishing the task, but I'm going to have to empty the traps out and re-set them with more of the liquid attractant soon (we've also put traps in the pear and apple trees). Or go with the husband's idea and just toss the whole thing and put up a new trap ... tempting ...