Here's why:

* Predicate: The predicate of a sentence tells us what the subject does or is.

* Simple Predicate: This is the verb (or verb phrase) that forms the core of the predicate.

* Subject: The subject of the sentence is "The scientists."

The sentence tells us what the scientists have done: they have learned respect for insects. The verb phrase "have learned" is the simple predicate because it expresses the action of the subject.