January 21, 2013 | 1
One of the things I love most about science blogging is the opportunity to learn about entirely new things. Of course, we all have that opportunity on most days, but having to find something to blog about three times a week definitely keeps me on my toes. And what I learn can be so fascinating! Often it’s about barnacle sperm or the evo psych of romance novels, but there are other, safe for work kinds of fascination, too!
And today, my fascination is with dragonflies. These little guys are amazing. I’m ashamed to say that until today, I had NO IDEA that dragonflies were predators. I don’t know what I thought they ate, but other bugs was definitely not on the list (kind of like butterflies, it’s my own personal belief that butterflies live on sunshine. I can’t see their mouths, you know*). But in fact, dragonflies are some pretty impressive predators, eating ants and mosquitoes and even wasps. They have independent control over their front and back wings, which means they can even fly backward. And the vast majority of their life cycle (up to five YEARS) is spent living underwater as nymphs. Dragonflies. They are cooler than you.
And my fascination with them grew even more as I read this paper. You see, not only do they have independent control over each pair of wings, they have only eight pairs of cells doing the primary coordination, resulting in an incredibly complex series of behaviors, from very, very few cells.
Gonzalez-Bellido et al. “Eight pairs of descending visual neurons in the dragon!y give wing motor centers accurate population vector of prey direction” PNAS, 2013.
The authors recorded from the visual neurons of a large number of dragonflies as they were presented with prey moving across the visual field. This particular species perches and waits for prey to fly above it.
These dragonflies have a set of neurons called small target movement detectors, which specifically detect the small and speedy movements of potential prey. The authors hypothesized that these neurons led directly to target-selective motor neurons, which can take charge of the dragonfly’s trajectory during attacks. These are only eight pairs of these target-selective motor neurons. If they did associate directly with the visual information from the small target movement detectors, then the authors hypothesized that they would react very quickly, and also that each motor neuron would correspond with a part of the visual field.
They recorded from the motor neurons, and saw that prep movement very quickly activated the descending motor neurons, and that the activation was associated with the fast interception of the prey. They were also able to show that each descending motor neuron corresponded with activation of a very specific area of the dragonfly’s visual field, showing a direct, and very fast, line from the visual field to motor control of the wings.
The visual fields were all partially overlapping, creating an area of very high sensitivity right at the midline. With such direct activation of motor neurons heading to the wings, the dragonfly can correct its movements and stay on target with lightning-fast effectiveness.
And with the overlapping fields from these neurons, not only is the response fast, it’s also highly accurate. Movement on one side of the visual field will result in activation of a small, but very specific, subset of the neurons, and this population level activity allows the dragonfly to head accurately in the direction of prey. It’s a small population of cells (only 8 pairs total) that act together with incredibly high accuracy. And the processing for this does not even occur in the brain! It occurs in the thoracic ganglia, smaller groupings of neurons outside the brain itself.
Not only is this impressive on its own (and also important for people trying to develop things like robots based on dragonfly morphology), it’s also astonishing to realize just how OLD this visual to motor circuit could be. Giant dragonflies roamed the Carboniferous period, around 300 million years ago. It’s really mind-boggling to realize that such an elegant system may have been around all this time. But it’s both simple and efficient, so why change?
Gonzalez-Bellido PT, Peng H, Yang J, Georgopoulos AP, & Olberg RM (2013). Eight pairs of descending visual neurons in the dragonfly give wing motor centers accurate population vector of prey direction. Proceedings of the National Academy of Sciences of the United States of America, 110 (2), 696-701 PMID: 23213224