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www.thevillagenews.co.za
25 November 2020
MY ENVIRONMENT
Dragonflies – marvels of flight and sight By Dr Anina Lee
B
ack in the 1950s, when we were young, we did not have a swimming pool. But our neighbours did. It was tucked away in a corner of the garden, surrounded by stone walls and pergolas where Catawba grapes twined and dropped their over-ripe fruits on the ground. Bees fed off the spilt juice – and unwary children stepped on them with bare feet. We loved that swimming pool. The pool technology was primitive – a massive sand filter, the size of a small house, which together with the outsize pump, operated from the ‘pool house’. Despite the best efforts of the owners and doses of chlorine, the pool remained obstinately dark green and murky. Contrary to what one would expect, the deep depths of that pool was home to surprising numbers of incredible wildlife. Our favourites were the strange dragon-like creatures with an extendable ‘arm’, with which it caught other creatures. We had no idea what they were, and never thought to ask. There was no internet, no tradition of using libraries, and parents who were educated in the arts and the law and had scant knowledge of nature. It did not occur to us children that there could be any connection between the strange creatures in the water and the dragonflies we saw darting and dancing and dipping their tails into the water. I learned much later that the creatures in the water were dragonfly larvae. Dragonflies are very ancient, and
were the first insects to inhabit our planet, having arrived on the scene around 300 million years ago. They've had a long time to perfect the art of flying, hunting, and just being amazing. It was evident even to us ignorant children, that dragonflies are spectacular flyers. They can catch small insect prey in mid-air. Gnats, mosquitoes, midges, beetles and other ‘pests’ are on the menu. Dragonflies can judge the speed and trajectory of a prey target and adjust their flight to intercept it. They're so skilled they have up to a 95% success rate when hunting. They have two sets of wings with muscles in the thorax that can work each wing independently. This allows them to change the angle of each wing and practice superior agility in the air. Dragonflies can fly sideways and backward, and can hover in a single spot to ambush unsuspecting prey from any direction. Their serrated mandibles (mouthparts) are also impressive. No wonder dragonflies and their close relatives the damselflies are classified in the order Odonata, meaning ‘toothed ones’. When hunting, dragonflies catch prey with their feet, tear off the prey's wings with their sharp jaws so it can't escape, and devour it, all without having to land. To be such a successful hunter, dragonflies also need to see clearly. They have huge compound eyes that allow near-360-degree vision. If you look at a dragonfly's head, you might notice that it's almost all eyes – well, except for the mouthparts. Their compound eyes are made up of around 30 000 facets, each of which brings visual information. Incidentally, the way to distinguish a dragonfly from a damselfly is to look at the eyes. The dragonfly’s two compound eyes meet at the top
of the head, while the damselfly’s eyes are separated on the sides of the head. You can also recognise a damselfly by the fact that it folds its wings down the length of the body in repose, while a dragonfly’s wings point out to the sides. Dragonflies are so skilled on the wing that they also mate and lay eggs in-flight. As you can imagine, it's not easy for two flying insects to hook up without some ‘docking’ mechanism. The male has specialised appendages at the end of his abdomen with which he clasps the female behind the head – so docking successful. Now how to get the sperm to the female when the business end of his abdomen is clutching her neck? Ah, he has a plan. Before he grabbed the female, he had already transferred sperm to his 'secondary accessory genitalia’ positioned below the second and third abdominal segments. These genitalia have a small sac for sperm storage and a penis. After the male has grasped the female, the female bends her abdomen around and under to lock with the male’s genitalia and copulation takes place in this ‘wheel’ position with the transfer of sperm. Soon after mating, the female starts to lay her eggs, often still in the nuptial clasp with the male – which ensures that other males don’t get any favours. Eggs can be deposited on the water surface, on aquatic plants or even underwater, depending on the species. But what about the dragonfly nymphs that so entranced us as kids? They hatch from the eggs underwater. Dragonfly nymphs are also serious predators. They live below the surface for up to two years. They moult as many as 17 times as they grow and get ready to head to the surface where they transform into the drag-
A Blue Emperor Dragonfly. Dragonflies were the first insects to inhabit our planet around 300 million years ago. PHOTO: Karien Jordaan onfly adults we see in the air. The larvae are exceptionally well adapted for aquatic life, with the ability to snag prey at lightning speed. For this, they use an extendible lower jaw or labium (also called a mask). The mask is a hinged structure, unique to dragonfly larvae, which can be extended to catch prey, or withdrawn under the head when not required. They feed on a wide variety of food, including other insect larvae, tadpoles, and even fish! Furthermore, they breathe in a rather unexpected way. They draw water in and out through the anus. There are gills in the rectum that can extract oxygen from the water. However, one thing they cannot adapt to is pollution. You will find no dragonflies in polluted water. For
this reason, dragonflies are excellent indicators of the health of a water body. The presence of dragonflies indicates a happy ecosystem. If there are none, it gives reason to find out why and do what is necessary. As is the case with many of earth’s creatures, dragonflies need protection from the damages humans have wrought, from pollution to habitat loss. Not only are they beautiful aerial artists, but they do a sterling job of controlling populations of pest insects. They also inspire us to create new technology – from drones to artificial visual systems – based on their incredible skills in flight and vision. We need to urgently protect our freshwater systems and support the conservation of habitats – for the dragonflies and us – so that they can survive for another 300 million years.
