How are the hind legs of powerful swimming predaceous diving beetles specialized?

Flattened into broad, oar-like structures fringed with long, stiff hairs

The leg adaptations in water beetles are strongly correlated with their primary mode of life, whether swimming rapidly or dwelling on the substrate. For species identified as powerful swimmers, exemplified by the predaceous diving beetles (or 'water tigers' in larval stage), the hind legs have evolved into highly efficient locomotive instruments. These legs are flattened substantially, creating a broad surface area similar to a paddle or oar. To maximize propulsion, these flattened surfaces are further equipped with numerous long, stiff hairs, or fringes. When these specialized hind legs work in coordination during swimming, they generate powerful thrust, enabling strong movement through the water column necessary for pursuit predation.