In the photo above you can see a stag beetle larva, a rhinoceros beetle larva and a rose beetle larva. All three always take the typical C -shape, as does the cockchafer. So not every cockchafer is a cockchafer!
May beetle larvae prefer to feed on living roots of various species.
Rhinoceros beetle larvae live on dead, decomposing plant material, preferably woody. E.g. compost piles, shredder piles, very musty tree stumps.
Rose beetle larvae also feed on dead plant/woody substrate and are sometimes found in potting soil with coarser humus contents.
Stag beetle larvae prefer perennially dead moderating tree stumps. They always need decaying wood (white rot), but also contact with the soil. They can be found up to 50 cm deep in the soil on or in the tree stump. It can also be a tree that is still alive, if it has a larger amount of rotting wood in the root zone.
However, as you can imagine, there are transitions between these demarcations. An old beam or thick fence post can sometimes be colonized with stag beetles. Likewise, in the same tree stump can be stag beetle larvae and rose beetle larvae.
In addition, all larvae start out very small, making it harder to distinguish between them. Beamed beetle larvae look confusingly similar to stag beetle larvae, and both can also be in the same tree stump. However, these larvae do not grow as large.
Take a photo of the larva as shown in the photo, if possible add one of the head and rump when the larva is on its back (hold it appropriately) and send us these photos. We will help or confirm your report. And give you advice, because if you have found a larva, you have found and opened a nest, now you have to see how to proceed. We would like to help you with that.