What insect is indicated by the presence of very active larvae in a pool?

The presence of very active larvae in a pool is indicative of mosquito larvae. Mosquito larvae, commonly referred to as "wrigglers," are often found in still water environments such as pools, ponds, and containers. They are characterized by their active movement in the water, where they feed on organic matter and microorganisms.

Mosquito larvae breathe air through a siphon-like structure, emerging to the surface periodically, which contributes to their noticeable and active behavior. This activity is a crucial part of their development before they metamorphose into adult mosquitoes. In contrast, other insects like house flies and fruit flies have different developmental habitats and behaviors, making them less likely to be found in a pool as larvae. House fly larvae typically thrive in decaying organic matter, while fruit fly larvae are usually associated with fermenting fruits and vegetables. Sandflies, which are also a different family, prefer sandy or humid environments and do not have a larval stage that would typically be found in a pool.

Thus, the identification of active larvae in a pool corresponds specifically to mosquito larvae, making this the correct answer.