Tiny sputniks

I was clearing away some hedge trimmings today at the nature reserve and found these tiny structures on a wild rose stem. They are sputnik galls induced by the gall wasp Diplolepis nervosa. Each one of these spiked galls houses a single larva of the wasp, which should have pupated by now and, all being well, will emerge as a new adult wasp next spring. Sometimes the galls of this wasp will be just smooth pea-like structures. Why some of them develop these spectacular spikes is not clearly understood.