Which blood-feeding insect is known to affect outdoor enthusiasts and has larvae that develop in fast-moving water?

The correct answer is black fly. Black flies are a significant concern for outdoor enthusiasts because they are known for their painful bites and can be quite aggressive, especially near fast-moving water bodies where they breed. The larvae of black flies develop in clean, fast-flowing water, such as streams and rivers, which makes them an issue for those engaging in outdoor activities near these environments.

In contrast, while mosquitoes also affect outdoor enthusiasts, their larvae typically develop in stagnant or slow-moving water, making them less associated with fast-flowing water habitats. Although horseflies and deerflies can be bothersome to outdoor activities, they also do not have larvae that develop in fast-moving water. Therefore, black flies are the insects that fit both criteria: they affect outdoor enthusiasts with their bites and their larvae develop specifically in fast-moving water.