Dragonflies and damselflies are predatory flying insects. Dragonflies are robust aerial hunters, capable of performing impressive mid-air acrobatics. Damselflies are related to dragonflies but are smaller and daintier. These insects spend the first part of their lives as aquatic nymphs (also called naiads), hunting for small insects and invertebrates in sediment and clumps of leaves at the bottoms of lakes and rivers. They may also serve as prey for fish. Dragonfly and damselfly nymphs are sensitive to poor water conditions and will not survive in polluted water. Therefore, their presence can be used as an indicator for the health of freshwater ecosystems.