The noun clause in this sentence is Whatever crawls in
A noun clause is used to name something when a single word is not enough. It is a dependent clause and it can not stand alone.
A noun clause usually starts with such keywords as: how, that, what, whatever, who, whoever etc.
A noun clause can function as a subject of a verb : Whatever crawls in is trapped, as an object of a sentence, as a subject complement or as a object preposition.
