After reading his article, I took a look on the ground when I walked under an oak tree in the field behind the Church – and found these knopper galls.
The galls are fluted, distorted acorns, which have been hijacked by a parasitic wasp, to become the nursery of its larva.
The wasp – which is not the one that plagues picnics, is apparently, black and only around 2mm long, would have laid its eggs in the oak catkins early in the summer. The larva then feeds on the acorn, and will overwinter in it, before emerging next spring.
There are many different types of oak galls, and a few days later, under a different tree on the path down to the river,I found these spangle galls.
Again, these are caused by a small parasitic wasp – a different one to the knopper galls. These ones feed on the leaf, rather than the acorn.
Both these wasps lead – to us – strange lifestyles, with alternating sexual and non-sexual generations.