GLIDERS AND THE EVOLUTION OF FLIGHT

Last week, Coturnix linked to a NY Times article about Cephalotes atratus, the gliding Neotropical ant, and the research being done by Stephen Yanoviak and Robert Dudley. The next day, Neil Kelley started up his brand new blog with more thoughts on the subject and a great head-shot of the rudder-faced Daceton. Yanoviak and Dudley are hoping their work will shed new light on the evolution of insect flight, and it got me to thinking about the subject of flight in general.

Animal flight has evolved a number of times, producing three types of wing. The most obvious category is the unique bird wing, a powerful foreleg with degenerate hand bones, and a flying surface composed of feathers: complicated integumentary derivatives found in no other animal group. It is likely that all modern birds are descended from a group of small Jurassic feathered dinosaurs. Various creative intermediate uses for flight feathers have been proposed, including bipedal Coelophysis-like dinosaurs with feathered “butterfly nets” for hands. It seems likely, though, that the first winged birds were arboreal creatures that glided feebly from one tree to the next, much like some of the Madagascan couas (Coua spp.) do today.
Forest trees tend to exhibit a phenomenon known as “crown shyness,” where canopy growth stops before a tree reaches its neighbor. This not only slows the spread of tree pests and pathogens, but complicates the lives of arboreal animals. The ability to move from tree to tree without coming to the ground is of great benefit, and skills at leaping and gliding are a dime a dozen among tree dwellers. Most flying animals probably began their phylogenic careers as arboreal gliders. Among gliders, lift is provided most commonly by the second type of wing, with a patagium, or skin membrane. A number of tropical treefrogs of the families Rhacophoridae and Hylidae use gliding patagia that are nothing more than exaggerated webbed feet. Several geckos can parachute to some degree, using webbed feet and flattened tails. These are most developed in the Asian genus Ptychozoon, which also sports skin flaps along the sides of the head and body. The 28 or so lizards of the Asian genus Draco have five to six pairs of elongated false ribs supporting colorful patagia that give them unparalleled gliding skills. The active Asian tree snakes of the genus Chrysopelea can expand their ribs and flatten their bodies to extend their leaps across tree gaps. Patagia stretched from wrists to ankles have developed independently in at least three mammal groups, the squirrels, possums and colugos. The Mesozoic pterosaurs flew on patagia supported by elongated pinkie fingers, and Matt over at the Hairy Museum of Natural History just alerted me to Sharovipteryx, an amazing Triassic gliding reptile with a "delta-wing" patagium supported by elongated hind legs. Modern bats, with patagia supported by all four limbs and four pairs of fingers, are the most accomplished patagium flyers of all.

Insect wings are quite different from the others. In the typical insect design, two pairs of wings sprout from the thoracic segment, above three pairs of legs. This is not much of a departure from the basic segmented arthropod pattern seen in shrimp and lobsters, where each leg is paired with a gill. Legs and gills are modified to form other structures like mouthparts and swimerettes. It seems quite likely that insect wings are homologous to, and evolved from, gills. If this is truly the case, I can't see much chance of insect wings first evolving in arboreal insects. One of the hallmarks of terrestrial arthropod morphology is a marked simplification of those compound limbs, and I would expect to see modified gills being put to other uses if they had survived long enough to follow their bearers into the trees (beetle elytra, the halteres of flies, and other structures clearly derived from wings don't count).
I remember reading a paper some fifteen years ago about stoneflies (order Plecoptera)sculling across the water's surface on cold days with their wings. The authors snipped away various pieces of wing and found the insects could scull quite well with wings that were far too short to allow flight. They suggested that the first flying insects might have evolved from such surface scullers. This is a possibility, even though there isn't much evidence to support it. Thanks to their small mass, the aerodynamic strictures on insect flight are much more lenient than they are for vertebrates. A stiff breeze can carry small wingless insects astonishing distances. It could very well be that vertebrate flight never evolved in non-arboreal animals, but in searching for the first flying insects my instincts would lead me not into the trees, but to the edge of the nearest pond.
___________________
upper: CROWN CROSSING--PARADISE TREE SNAKE & COPPERY FLYINGLIZARD (1990) acrylic 15" x 40"
center: WALLACE'S FLYING FROG (1995) acrylic 11" x 8"
lower: GREAT MINDANAO HORNBILL & RIZAL'S FLYING LIZARD (1997) acrylic 20" x 15"