National Geographic : 2016 Jan
PHOTOS: REBECCA HALE, NGM STAFF, AT DEPARTMENT OF ENTOMOLOGY, NATIONAL MUSEUM OF NATURAL HISTORY, SMITHSONIAN INSTITUTION. GRAPHIC: NGM ART. SOURCE: PHIL DEVRIES Walk through the understory of a neotropical rain forest and you’ll see an unusual sight—brown or clear-winged butterflies gliding just above the forest floor, “like leaves floating on a stream,” says University of New Orleans entomologist Phil DeVries. They’re demonstrating something called the ground effect. It’s an aerodynamic phenomenon that occurs whenever wings are near a fixed surface, which increases lift and decreases drag. If you’ve been on a plane as it lands, you may have felt a brief floating sensation. That’s the ground effect. In a recently published study, DeVries and colleagues found that all but one species of Haeterini butterflies glide near the ground in this energy-efficient manner. Their evolutionary secret: forewings longer than those of their relatives, which favor flapping flight. — Jeremy Berlin To Flap or Glide? Butterfly Forewing Shape A A B. Gliding flight Less than five inches above the ground A. Flapping flight More than five inches above the ground B B B B B Sex, habitat, and wing shape (right) separate the flappers (A group) from the gliders (B group) in Haeterini butterflies and their Satyrini relatives. All butterflies are shown to scale.