Final answer:
Flea pupae emerge to become adults primarily due to factors such as a) direct contact and increased temperature. Extreme conditions like harsh frosts and low humidity can impact their survival and development.
Step-by-step explanation:
The factors that would cause flea pupae to emerge from their case to adult include a) direct contact, increased temperature, and possibly humidity.
However, specific to flea pupae, based on the context provided, the emergence is primarily associated with host-dependent offspring quality and environmental conditions such as temperature rather than humidity alone.
Flea pupae can be prompted to emerge by external stimuli such as warmth and direct contact, which would suggest a host is near, prompting the flea to complete its development and emerge in search of a blood meal.
Certain extreme environmental conditions can affect the survival and development of insect pupae.
For example, drier conditions below 60% humidity and harsh frosts can significantly decrease survival chances, indicating that both extremes of temperature and moisture can impact the life cycle of insects, including fleas.