Dragonfly's Eyes

 

Blue Dasher

Dragonflies have two huge compound eyes and three small simple eyes on their forehead in between. They are the most effective predator alive today, hunting and eating in mid flight. In the last posting we looked at their remarkable aerial ability. It's how well they see the world that makes all that agility so effective.

One of the thousands of cells in a compound eye

Dragonflies have incredible vision. Their characteristic compound eyes are the largest in the insect kingdom. Each is made of over 30,000 individual facets or ommatidia, which are like mini-telescopes but detect light from the direction in which they point it. 

Spotted Spreadwing

As their eyes wrap around their head almost entirely, their field of vision is almost 360 degrees, perfect for surveying their surroundings for potential targets, without having to reorient themselves on their perch. 

Calico Pennant

With 30,000 ommatidia spanning a full 360 degrees, dragonflies do not have the same visual acuity as a human or a bird, but are much more sensitive to movement than we are. A common prey target in the distance will rarely span more than 1 to 3 across their field of vision before they decide to pursue it, which equates to movement across 2 to 3 ommatidia at most, meaning their brain is capable of detecting and processing movement incredibly quickly, so they can immediately perceive prey or avoid a threat.

Ruby or Cherry-faced Meadowhawk

Their eyes are also specialized based on the region. The upper or dorsal region of their eyes is most sensitive to shorter wavelengths of light, that is, ultraviolet and blue. Many dragonflies fly low above the surface of the water when searching for prey, as this allows potential targets to contrast as much as possible with the blue sky. On the other hand the middle and lower, or ventral parts of their eyes are sensitive to a wider range of wavelengths. 

Northern Spreadwing

It's not just the wings or the eyes that makes the dragonfly so successful, but it's the brain The dragonfly's brain uses a highly optimized hunting strategy that allows it to predict where the prey is going and to quickly maneuver toward where they predict the prey will be. This is more difficult than simple tracking, as the locations and speeds of the prey and predator must be processed together to make such a prediction. 

Twelve-spotted Skimmer

Even though dragonflies have the speed and agility to engage in tracking if they so desire, they primarily use interception as a form of capture. In humans interception is a learned behavior but in dragonflies it's hypothesized that such behavior must be hard-wired into their nervous system, linking their visual system and motor system directly.

Violet Dancer

However, an interception model of pursuit means that the prey being pursued is less likely to remain in the same spot within the dragonfly's field of view, which can make the prey more difficult to track. Studies have shown that dragonflies seemingly adjust their head in near synchrony with their prey, fixing their target within their areas of higher visual acuity for tracking. In fact this synchronization is so perfect that even the head movements appear not to be reactionary but based on prediction.

Wandering Glider

All of this mind and body integration have given the dragonfly the best hunting record that researchers have ever found, catching around 95% of the prey they chased. This compares very favorably with the success rate of peregrine falcons, at 23%, and lions, at 30%.

_____

* These text excerpts  are transcribed from the YouTube video "The Insane Biology of: The Dragonfly."