You find them everywhere – small, apple-shaped balls on oak leaves, fat, swollen stalks on thistles, or things that look like small hedgehogs on rose stems. Despite the differences in appearance, they are all varieties of the same thing. They are galls – an entomological scam performed by many different insects and other animals, with the goal of tricking innocent plants into providing both food and shelter to the gall-maker’s eggs and larvae.

There are several hundred different kinds of galls, and they all start with a gall-making insect, for example a gall wasp, that lays an egg on, or in, a plant. The egg is covered with something similar to artificial growth hormone that makes the plant react and produce extra tissue around the egg. The galls can be quite small but can also be as large as potatoes. All the extra plant tissue that grows around the egg protects it, and when the larva hatches, it is completely surrounded by food. All it has to do is start eating.

You would think that galls just look like random lumps, but they are actually so characteristic, that you can work out what kind of insect made them.