Final answer:
The term that best describes the period in the life cycle of larvae transitioning from 3rd instar to pupa at 70°F is metamorphosis, a process where larvae undergo significant transformation to reach the pupal stage.
Step-by-step explanation:
The period in the life cycle of larvae going from 3rd instar to pupa is best described as metamorphosis. During metamorphosis, the larval tissue and structures are broken down and reorganized into the adult form. Most insects undergo this type of life cycle, which includes distinct stages: egg, larva, pupa, and adult, each differing significantly from the others.