What part of speech is caterpillar?

Answer 1

"Caterpillar" is fundamentally a noun. Like most nouns naming tangible objects, it can also be used to modify other nouns and in that use is often considered a "substantive adjective". "Look at that bright-colored caterpillar!" (simple noun); "Those are caterpillar tracks" (substantive adjective). To help tell the difference, note that the last example sentence could alternatively be written as, "Those are tracks of a caterpillar" or "Those are a caterpillar's tracks". This distinction is especially useful when translating English into some other language that does not allow as much freedom to adapt one word form to another part of speech as English does.