Galls as an alternative idea?

I've probably got enough weird interests, but yesterday I think I came up with another one. I was on a fieldtrip to an ancient woodland. Some of you might be going on the trip today. As we squelched and slipped through the mud, I kept seeing what I blithely assumed were tiny crab apples amongst the leaf litter. I was set on collecting lichens and tree buds, so I didn't pay them much attention. But finally I picked one up, only to find it was attached to the bottom of an oak leaf, along with a couple of others:

They weren't apples at all, but galls. How mad are they? I think pretty mad. It turns out they are called 'cherry galls' (only because they look a bit like cherries). You find them only on oak leaves - they are attached to the veins on the underside of the leaf. Here's the instigator, a tiny wasp (about 5 mm long).

CC image by Wofl

As the female lays an egg she injects the leaf with chemicals that induce the tree to grow a gall. Then the little grub lives cosily in the gall and waits until spring to pupate. It's actually even weirder than this - the wasp species has two generations each year, one asexual and one fertilised. I'm a bit unclear on the mechanics of this, but I think when the wasps (both sexes?) emerge in spring from the galls, the females lay unfertilised eggs on the tree trunk. These become protected by another sort of gall (which is small and purple). When those wasps hatch, the sexes mate and eggs are laid to produce the cherry galls.

Anyway. I recall that galls have been a 'taxonomic collection' in the past. Not that they're found on any confined group of plants, and are in fact produced by flies, wasps, aphids, sawflies... many different creatures. So if you're interested it might be worth a discussion with Katy. In any case, I think I might be keeping a closer look-out for them in future, purely out of interest.

Can you believe it, there's a British Plant Gall Society.  Britain's great isn't it.
Also I might buy myself one of these FSC Aidgap guides to galls or maybe the Wildguide. (Oh well, just bought both. Call it an early christmas present. To myself).