Any of numerous kinds of common flying insects with hard wing-covers. Beetles are not mentioned directly in any of Tolkien's tales, but they were evidently well known in Middle-earth. When Samwise Gamgee saw that Gollum had been eating some unknown thing that left him covered in mud, Sam speculated on the wretched creature's diet, and among the unpleasant meals that he conjured to mind were beetles. (Whether Gollum had actually been eating beetles is not confirmed, though it seems at least plausible.)