concept

Wake recapture

The trick a four-winged flyer uses to take back some of the energy its front pair just threw away. Every wingbeat leaves a mass of swirling air behind it, and that swirl carries real kinetic energy. If the rear pair beats at the right lag - measured on dragonflies at roughly a quarter of a cycle - it passes through that swirl and harvests part of it, cutting the total power needed by up to about a fifth compared with beating independently. This is biology's answer to the tandem-wing penalty: not a different wing layout, but different timing.