In the eye of the dragonfly
New insights into visual control of flight
Have you ever marvelled at the dazzling aerial control exhibited by dragonflies? One moment they hang mid air perfectly stationary, and in the next they dart away like a lightning bolt. Part of their aerial mastery relates to how they ‘see’ their surrounds and scientists at the Research School of Biological Sciences (RSBS) have discovered that the middle eye of the dragonfly is a lot more sophisticated than previously believed.
All insects have two large compound eyes on the sides of their heads, and three simple lens eyes or ocelli in the centre. There are two small lateral ocelli and a larger middle one called the median ocellus. In the dragonfly, the median ocellus is curved.
“Ocelli are generally thought to have poor resolution,” says Josh van Kleef, a PhD student at RSBS who is leading the research on dragonfly vision. “The general belief has been that they’re there to measure general light levels which can be used to stabilise their flight. The idea is that if they detect that light levels are falling they’re heading down, and if the levels are rising they’re heading up. This information is then used to guide their movement.”
However, Mr van Kleef’s research is revealing that a dragonfly’s middle eye does a lot more than simply pick up on general light levels.
“The median ocellus is a simple lens containing about 1500 photoreceptors,” he explains. “These photoreceptors are connected to 11 large neurons (nerve cells) which go from the retina all the way down the neck of the dragonfly to the insect’s motor centres. It’s a very direct route from a visual response to a change in movement, and allows for a very rapid response to their environment.
“Now, we’ve demonstrated that the neurons in the middle ocellus have a much better resolution than was previously believed. They seem to be focussing on the horizon rather than average light levels.
“We created a special display that’s able to produce UV and green images. It’s long been known that large ocellar neurons respond to UV and green light but no other researchers have tested for both wavelengths of light in the same test rig.
“We showed that if you move images of bars (rows of lights) in front of the dragonflies, that they can detect them. What’s more, we discovered that when you switch from green to UV these neurons become very directionally selective. This demonstrates that dragonfly image perception through its median ocellus is much more sophisticated than was previously appreciated.”
Mr van Kleef’s tests and analysis showed that the optimal speed for the directional responses was about 750 degrees per second.
“That’s very fast,” he says. “But it makes sense because dragonflies move quickly so they’re looking for very fast movements. We think it’s crucial for them in stabilising their flight to be able to pick up on sudden movements where they’re pitching up and pitching down very quickly.
“These types of movements are not picked up quickly by their large compound eyes. The other thing about the compound eyes is the information takes a longer time to go through the brain; it’s processed a lot, and therefore at each synapse there’s another delay, whereas the neurones in the median ocellus go from photoreceptor to the motor centre and they’re able to do the calculation in one step.”
Understanding how dragonflies fly and interpret their environment is far from a purely academic question. It’s one that is of particular interest to those building small flying robots called micro-aerial vehicles which typically have wingspans of less than 30 cm.
“A dragonfly’s capacity for rapid darting and precision hovering is of great interest to designers of micro-air vehicles, and the research we’re doing is in part supported by the US Air Force. Stabilizing small air vehicles is a really big problem in terms of control and the forces at play on the vehicle. It’s much harder than for larger, more stable aircraft.
“Dragonflies are supreme fliers. If we can work out how they can control their flight movement we’ll have some good ideas on how to effectively program micro-air vehicles.”
And where does Mr van Kleef source the dragonflies for his research?
“The dragonflies we work on are a local species that we catch down on the banks of Sullivans Creek,” he explains. “It’s great, I feel like Huckleberry Finn; we go down with big butterfly nets and catch our dragonflies.”
Which begs a question: If dragonflies are so perceptive and such great movers, how do they let themselves get caught by a slow human with a butterfly net?
“Dragonflies might be great fliers but when there are lots of others around they tend to be more worried about these other dragonflies than me,” laughs Mr van Kleef. “They get caught up in this mating game where the males patrol a bit of real estate down on the creek –they zoom back and forth along a track around 10 metres or so in length. If another male comes they fight it off, and if a female comes they try and mate with it. They get quite crazy when the numbers are up, and they’re all focussed on each other rather than a big human standing nearby with a butterfly net.”