Bunkers for wasps

In spring and early summer you find lots of strange looking objects on the leaves of many trees. They are galls of so called gall wasps. The wasps lay an egg onto a leaf. The egg induces the growth of the gall. How it does this, is unkown to science. The gall serves as a tiny bunker for the developing larvae. Here, I've cut one open with a pocket knife and measured its dimensions.

By Atlant Bieri