Exactly how insects evolved flight is a heated issue, in part because the fossil evidence for winged insects remains full of gaps. But living insects that are similar to ancestral species could also shed light on the origins of insect flight. In a study reported online this week in Biology Letters, researchers report that bristletails, primitive, wingless insects that live in the tropical forests of Peru, can use long antennae-like filaments extending from their rear ends to help them glide to tree trunks as they jump or fall from forest canopies. These observations suggest that winged insects evolved on land, rather than from aquatic habitats, the authors conclude.
According to the fossil record, before about 390 million years ago, the planet was populated by six-legged wingless creatures resembling bristletails and similar living insects called silverfish—those early creatures are considered to be the ancestors to all current-day insects. There’s then about a 65-million-year gap in the insect fossil record. The next known fossils date to about 325 million years ago, and they include insects with and without wings. Given the lack of a fossil showing an intermediate stage of an insect wing, scientists have been left debating two primary theories: that wings developed either from the gills of insects living in water or from extensions on the sides of a terrestrial insect.
To address this debate, tropical insect ecologist Stephen Yanoviak and his team turned to a species of wingless bristletails called Archaeognatha Meinertellidae. Genetic phylogenies suggest they are closely related to the species at the root of the evolutionary tree of insects, and they also lack any aquatic form to help rule out wing development in water. The researchers were interested in whether these bristletails could use their filaments or antennae to help maneuver, or glide, during a fall. Yanoviak says that this ability would have been useful about 400 million years ago when the first trees appeared and wingless insects began to feed on lichens and debris in tree bark. If those insects encountered a spider or other predator while up in a tree, they would need a good escape route. But they couldn't just jump off the tree if they couldn’t guide their fall away from nearby quagmires and to a safe landing point instead.
To explore what aerial control these insects have, Yanoviak and his colleagues collected nearly 200 Peruvian arboreal bristletails and separated them into six different groups. One was left untouched for a comparison, and for each of the other five groups, researchers removed the lateral filament, the medial filament, the medial and one of the lateral filaments, half of all the rear filaments, or the insects’ antennae. When Yanoviak dropped the insects from the top of a forest canopy, he found that 90% of the untouched bristletails successfully glided to a nearby tree before reaching the ground. The insects with either their antennae or their rear lateral filaments removed had about the same overall success rate, but it took them longer to get to their target tree. And those lacking their middle bristle, a filament extending from the insect thoracic segment, often lost the ability to glide under control and wound up on the forest floor. (Here's a video from Yanoviak showing normal bristletails gliding from a tree.)
A common objection to the proposal that insect wings evolved from stubby outgrowths on terrestrial insects has been that half a wing, or a protowing, would do the animal little good. Yanoviak says that his group’s study supports the idea that a mobile protowinglet emerging from a thoracic or leg segment could improve gliding enough to provide a survival advantage, eventually leading to more fully developed wings that flap.
This study "makes a compelling story for how [insect wing evolution] would occur in a terrestrial situation," says evolutionary ecologist Joel Kingsolver of the University of North Carolina, Chapel Hill. But that doesn’t mean that’s how it happened. Kingsolver notes that the earliest known forms of winged insects, which resemble modern dragonflies, all had aquatic stages as they matured, which makes it challenging to rule out the theory that wings evolved in a water setting.
Biologist Jim Marden of Pennsylvania State University, University Park, who supports the aquatic theory of insect wing evolution, isn’t persuaded by the new study, even though he considers it well done. It’s “difficult to make the jump” from the filaments Yanoviak’s team studied to a real wing, he says. The fight about the origins of insect flight apparently soars on.
Photo credit: Steve Yanoviak