"Dang" is english slang equivalent to "darn" or a way of expressing exasperation with something. "Dang beetle" implies that the writer is annoyed at the beetle for some reason, which sets up the story and makes you want to know why.
English is an interesting language in that it is rather "sloppy" about its structure, and it accumulates many expressions that convey meaning beyond the words used. The meanings also evolve with time, so the same phrase changes its meaning with the years. Each generation and subculture also evolve their own vocabulary to create a distinction of belonging and differentiation. By reading a text, it is often possible to identify when it was written and by who, just from the expressions used and the implied meanings associated with them.
English is not my mother tongue, and I am very well aware of how the formal structure is often at odds with its colloquial usage. However, it has an adaptability which is remarkable, and is one reason why other languages often import english words to express new ideas or concepts. Sometimes a phrase in english is much more concise than the equivalent phrase in (say) French, or Spanish.