Beetles are my specialist subject, and you find lots of different species carrying out different roles in even the smallest compost heaps. The most common you may find are from two families: Carabidae, the ground beetles, and Staphylinidae, the rove beetles.
Ground beetles are generally robust-looking insects that are mostly shiny black, or various metallic colours, although some have spots, stripes and markings of many colours. They have a large size range (2mm to 30mm) and – as the family name suggests – can be found most frequently on the ground. Ground beetles are predators, feeding on pretty much anything they can get hold of, relative to their size. Some are specialist hunters, for example, Cychrus caraboides, the snail hunter. Some have larger eyes to enable them to hunt fast-moving prey, like the springtail-stalker Notiophilus biguttatus.