The Insane Biology of: The Dragonfly

When we think of aerial predators our mind may go to fierce birds of prey like the peregrine falcon who have an overall hunting success rate of 23.1 percent where other aerial predators like hawks single out individual prey birds from their flock with a similar success rate of 22.5 percent where birds like owls hunt by pinning their prey to the ground lacking well-developed aerial hunting techniques but still only have a success rate of around 25 percent where other dominant predators on Earth rarely exceed these numbers where lions succeed less than 30 percent of the time where African dogs are perhaps the best land-based hunters with a success rate of up to 67 percent when working together but this number still pales in comparison to dragonflies at 95 percent.

In 2012 researchers at Harvard found that dragonflies caught around 95 percent of the prey they chased where their hunting success rate pushes toward 100 percent making them the greatest hunter on Earth where this is impressive for any animal let alone a flying one where dragonflies will consume any flying prey roughly the size of their head or smaller like bees moths flies mosquitoes or gnats where larger species will even catch and eat other dragonflies demonstrating unparalleled predatory efficiency.

Dragonflies belong to one of the oldest insect orders the Odonata which have been around for close to 320 million years where their presence in the fossil record is relatively complete and shows that their morphology has remained mostly unchanged through time where their size on the other hand has changed dramatically demonstrating that the dragonfly body plan was optimized early in evolution and has persisted with minimal modification for hundreds of millions of years.

The ancient dragonflies were giants compared to the ones that exist today where some Paleozoic species were known to have wingspans exceeding 70 centimeters whereas the largest existing species barely exceed a wingspan of 16 centimeters where the only difference between extinct and existing dragonflies is their size demonstrating a dramatic miniaturization event where Evolution robbed us of a world where almost 1 meter long aerial beasts ruled the skies creating a stark contrast in dragonfly ecology across geological time.

Odonata means toothed ones in Greek and these fearsome predators’ namesake comes from their prominent chewing mouth parts with sharp teeth found on the underside of their head where odonates are also characterized for having a freely movable head very short antennae large and highly developed compound eyes three simple eyes and two pairs of long wings where these anatomical features collectively enable their exceptional predatory capabilities and aerial dominance creating a body plan optimized for hunting flying insects.

Dragonflies possess direct flight muscles where most winged insects have indirect flight muscles which attach to the walls of the thorax making the thorax deform as they expand and contract where dragonflies instead have individual muscles that attach directly to the base of each of their four wings where being able to tug on each wing individually enables them to move and direct each of their four wings independently in all three axes meaning they can control their horizontal vertical and torsional motion granting them precision and agility that is unparalleled by most insects with indirect flight muscles.

Thanks to this independent control dragonflies are one of the few insects that can propel themselves in all six directions up down left right forwards and even backwards where dragonflies fly backwards by tilting their body upwards at 90 degrees to change the direction of the force generated by the wings where this flight maneuver is not possible for almost any other animal except a few other small types of fly and the hummingbird demonstrating exceptional aerial versatility.

Dragonfly flight differs from other insects in their ability to flap their fore and hind wings in different phases and can vary that phase depending on the maneuvers they wish to perform where counter-stroking flaps wings 180 degrees out of phase allowing hover or very slow forward flight generating large amounts of lift where for fast forward flight they switch to phased-stroking where hind wings beat 90 degrees ahead of fore wings generating far less lift but lots more thrust with some species reaching well over 50 kilometers an hour making them the fastest flying insects where synchronized-stroking flaps all wings together for huge lift enabling acceleration and turning.

Dragonfly wings have a prominent pterostigma which is a small pigmented spot on the leading edge of the wing that is heavier than the rest of the wing where it acts as a counterweight from a biomechanics perspective where the weight of the pterostigma displaces the chordwise center of mass ahead of the torsion axis preventing the outer leading edge from self-excitatory flapping and feathering vibrations known as flutter which would rapidly stall flight when gliding at high speed where this small spot accounting for less than 0.1 percent of total body weight allows them to safely reach a 10 to 25 percent higher flight speed.

A dragonfly’s wings are not perfectly flat plates where their veins form three-dimensional corrugations that make the wing’s performance more efficient where from a structural perspective the patterns traced by the veins prevent the wing from warping and deforming as a result of resonance which enables them to easily alter their angle of attack and generate higher amounts of lift where this three-dimensional structure provides both structural integrity and aerodynamic performance creating an optimized wing design that has inspired biomimetic engineering applications.

Dragonflies have incredible vision where their characteristic compound eyes are the largest in the insect kingdom made of over 30,000 individual facets or ommatidia which are like many tiny telescopes that detect light from the direction in which they point where as their eyes wrap around their head almost entirely their field of vision is nearly 360 degrees perfect for surveying surroundings for potential targets without having to reorient themselves where each ommatidia also has four to five opsins light-sensitive proteins that absorb different colors of light in the spectrum allowing dragonflies to perceive color from orange to ultraviolet.

Dragonfly eyes are specialized based on region where the upper or dorsal region of their eye is most sensitive to shorter wavelengths of light that is ultraviolet and blues where 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 where on the other hand the middle and lower or ventral parts of their eye are sensitive to a wider range of wavelengths where this is likely because color detection is important when identifying and pursuing fellow dragonflies either as potential mates or rivals.

The dragonfly’s brain uses a highly optimized hunting strategy that allows it to predict where the prey is going and to quickly maneuver to intercept it where there are two main strategies used by predators tracking and interception where tracking involves maintaining the target in the same spot in their field of vision and accelerating towards them spiraling in until natural speed advantage allows capture where interception flies in an intercepting path towards where they predict the prey will be where this is more difficult requiring processing locations and speeds together but is less likely to depend on absolute speed advantage helping save power as fast forward flight is energetically taxing.

An interception model of pursuit means the prey being hunted is less likely to remain in the same spot within the dragonfly’s field of view which can make the prey more difficult to catch where studies have shown that dragonflies seemingly adjust their head in near synchrony with their prey fixing their target in areas of higher visual acuity for tracking where in fact this synchronization is so perfect that even the head movements appear not to be reactionary but based on predictions demonstrating sophisticated neural processing that anticipates prey movement rather than simply responding to it.

There are two leading hypotheses for the miniaturization of dragonflies where the first is that 300 million years ago there was much more oxygen in the atmosphere where it’s thought that this huge amount of oxygen was able to support larger insect bodies which rely on passive and active diffusion through holes in their cuticle for respiration where as these oxygen levels decreased to those of the present day so did the maximum size an insect could reach demonstrating the respiratory constraints that limit insect body size.

The other hypothesis for dragonfly miniaturization is that small flying dinosaurs and birds which began evolving 150 million years ago may have outcompeted larger dragonflies for the same ecological niche where since there is less competition if birds hunt larger organisms and dragonflies hunt smaller ones smaller dragonflies were more likely to not go extinct where these hypotheses are not mutually exclusive and both atmospheric oxygen decline and avian competition may have jointly driven dragonfly size reduction demonstrating multiple selective pressures acting on body size evolution.