Wasps

That's one for @General_Lucifer !

Christ, that's even bigger than the nest I battled!! How many stripy bastards would live in that??

I think a nest of this size could harbour significantly more than 10,000 wasps, but that it may have been added on to over several years and not actually had all of its parts inhabited at any one time. Wasps' nests deteriorate quickly, depending on the conditions surrounding them, and I think they would add on to an existing nest rather than reuse or repair all of it, but I don't know that for certain. Perhaps it was this entrance tunnel that kept wasps coming back and establishing themselves at the end of it.

Wasps don't build as space-efficiently as bees or reproduce as quickly, and much smaller beehives can contain more than 100,000 individuals, but the main limit on how many wasps can be in a colony is to do with their annual cycle. They haven't evolved a mechanism for a whole colony to survive the winter, unlike bees with their honey, so that queens hibernate in solitary. I believe they rarely hibernate in established nests, as those aren't actually very safe places for them, and may well not return to the same nest the following spring. That said, I think common species of wasps regularly produce colonies of tens of thousands of individuals. I seem to remember 30,000 as the rough maximum recorded, but I'd have to look it up. Again, you'd be surprised how many of them can fit into a nest that looks quite small to us. :)