Their bodies are ovoid in shape, aand covered in a segmented carapace of a chitinous material. They are fat in the middle and taper at either end, and can grow up to three feet in length, though the average is about two and a half. At the rear of their bodies are two curved, pointed spines which are used for movement. At the front they have a short, beak shaped maw which comes to sharp points, that they use for breaking of pieces of softened rock.
The carapace of the rumblebug is made up of segmented pieces which can flex and move over each other, this allows the Rumblebug to to contract and extend its body. When it wants to move forward the bug can contract it's body to almost half it's size, it then digs the spines in it's rear into the rock and then extends itself out, sliding it's tough under carapace across the rough stone, it then contracts its rear half into itself again and repeats the process. They aren't the fastest of movers, but considering their diet consists of rock, which also doesn't tend to move very fast, they don't really have to be.
Rumblebugs have a very unusual diet and an even more unusual way of achieving that diet. Rumblebugs have evolved to eat rock which is always in plentiful supply when you live deep underground. Unlike some of their more giant cousins, Rumblebugs are not capable of chewing through raw rock, they have to soften it up a bit first. To do this the Rumblebugs emit low vibrational sounds which softens the rock and makes it much more maleable, more like a hard clay. They then use their hard beaks to break off chunks which they then grind up with rows of powerful teeth with line their mouth and upper throat. It is mostly the minerals inside the rocks that the Rumblebugs are after and they bits they don't digest are excreted as a wet sandy substance.
The Glomes raise Rumblebugs and train them to work together to soften much larger sections of stone, which the Glomes can then shape into buildings and other structures. These groups of Rumblebugs are called Choirs and usually have about twelve members, though it has been known to join join choirs together if the Glomes need to soften large sections of rock for any reason. When a Rumblebug dies the Glomes harvest their carapace and use it for building materials, protection and decoration.