Mystery of Moth Flight Uncovered

Moths often baffle and elude us with their bouncy, seemingly erratic flight paths, especially around outdoor lights, but they have a piloting system that has allowed them to thrive on Earth for hundreds of millions of years.

Now scientists have learned the details of this flight system and find that moths use their antennae as spatial orientation sensors to steady themselves as they fly and hover over flowers.

Most animals possess organ and neurological systems that help them stay oriented and move safely through 3-D space. In humans, the eyes and inner ear canals feed spatial and movement information to the brain. But some insects, like moths, lack ear canals and their eyes often fail to provide sufficient information to help insects stay oriented in space. And they don't have little appendages that two-winged insects use to steer themselves.

"So the key question here is: What are all the inputs that insects require?" study author Sanjay Sane of University of Washington said.

{{ video="moth_flight" title="Moths in Flight" caption="Segment 1: Normal moth flight. Segment 2: A moth flies erratically after its antennae are severed. Credit: Sanjay Sane" }} 

Moths are closely related to butterflies. In fact, scientists think that butterflies evolved from and are a subset of all moths.

Gyroscopic 'wings'

Researchers have known that insects such as dragonflies use visual cues to correct their flight, but night-flying insects, such as moths, can't rely on their visual systems, because they work too slowly in low-light conditions.

Experiments with two-winged insects, such as houseflies and mosquitoes, showed that they used their stunted hind "wings," called halteres, to detect forces that might throw them off-balance. As a fly's body rotates, the same inertial forces that spin a gyroscope cause the halteres to deflect and send signals that allow the fly to straighten up and fly right.

But no one knew how four-winged insects, which lack halteres, corrected their flight paths.

Experiments conducted by Sane on hawk moths, detailed in the Feb. 9 issue of the journal Science, showed that the antennae of moths act it in essentially the same way as is found in two-winged insects.

The antennae vibrate at certain frequencies, and as the moth's body rotates, inertial forces deflect the antennae. Sensors at base of antennae, called mechanosensors, detect these mechanical movements and send signals to the moth's brain so the moth can correct its orientation.

Disoriented and destabilized

The researchers cut off a moth's antennae to test how important they were to stabilizing the moth's flight. As expected, the moth became disoriented and was unable to stabilize itself in the air. The moth flew highly erratically, collided with walls, and crashed to the floor. [video]

"When we did the behavioral experiments, we realized that of course the mechanical information seems extremely necessary for exactly the kinds of things one would expect," Sane said. "It shows us behaviors that one would expect if the insect were disoriented or unable to stabilize its wings."

This erratic behavior is similar to what would happen if the inner ear system in a human was damaged.

"You're constantly working on what your eyes and ears and inner ear system, etc. are telling you, and that is why you're able to maintain your sense of balance and be able to do anything at all," Sane said. "But the moment you cut off one or more of those inputs, it becomes difficult, if not impossible, to maintain that balance."

Stability restored

When the moth's antennae were glued back on, the moth regained some control over its flight.

"So essentially what we're able to show with this is that, look, it is only the mechanosensory information that matters in this case," Sane said.

The mechanosensors can operate again because they are at the base of antennae and were not damaged when the antennae were severed, but the moth did not have total control because other detection systems were damaged.

For instance, the moth's olfactory sensors run along the length of its antennae, and so are damaged when the antennae are severed and won't work when antennae are reattached. This damage prevents the moth from finding flowers, Sane said, or anything else for which they need to use their sense of smell.

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