The Secret World of Dragonflies | Short Film Showcase

Dragonflies are undoubtedly one of the natural world's design classics, with a superbly adapted body plan and behavioural repertoire that has stood the test of time. The earliest fossil specimens date back 300 million years and in their form are virtually indistinguishable from their modern day counterparts. The secret of their survival lies perhaps in the ecological niche that dragonflies occupy. Dragonflies are intimately connected with freshwater ecosystems and it's the strategies and adaptations that dragonflies employ at each phase of their life cycle that make them such a success in this habitat. Although it is usually the brightly coloured aerial hunter that we think of, this phase of the dragonfly's life cycle forms only a small proportion of their life, often less than a month in a total lifespan, that may be two to three years. For the majority of their life a dragonfly exists as a nymph, a fully aquatic predator, superbly adapted to exploit the abundance of food available in freshwater ecosystems. Several species often share the same habitat and have adopted a range of strategies enabling them Many of the larger hawker nymphs are active predators, openly moving through the aquatic vegetation stalking prey, whereas chaser nymphs have specialised as ambush predators, burying themselves in the sediment and waiting for prey to come to them. The smaller and more vulnerable damselfly nymphs rely on concealment, for much of the time remaining sedentary and coloured to merge with the surrounding vegetation. Although their lifestyles may vary, all nymphs come equipped to hunt. With large compound eyes and an extendable lower jaw that they can project out like a hydraulic ram, they are a formidable predator. The nymphs are indiscriminate and voracious hunters, and are not averse to cannibalism. Size matters in this world, so even the nymphs of the largest dragonflies need luck to survive the first few months of their lives, when their diminutive size exposes them to predation by larger nymphs. If the nymph survives the hazards of its early life, it must overcome the limitation placed on it by its rigid exoskeleton in order to grow, and this is achieved through a series of molts. Once free of the old exoskeleton, the nymph puts on an instantaneous growth spurt while the still supple. After an hour or two it will harden, and no further growth will occur until the next molt. The nymph looks almost ghost-like immediately after casting its old skin, and behaves cautiously as it will remain extremely vulnerable while the new exoskeleton hardens. Molting punctuates distinct phases of growth for the nymphs, and these are known as instars, and may be repeated up to 15 or more times before the nymph is finally ready to leave the water. After two and a half years living below the surface, the nymph is ready to undertake a final molt that will see it leaving its aquatic home and embarking on a final journey to pass on its genes to the next generation. Just before dawn, on a warm July day, the nymph climbs up a reed stem and sits motionless. A few minutes later, the carapace above the thorax begins to split, and the dragonfly begins to ease itself out of the nymph's exoskeleton. The dragonfly waits as its legs harden enough for it to support itself. As it reorients itself and begins the slow process of unfolding its wings, the dragonfly flexes, stretching the still supple exoskeleton, steadily elongating and expanding to reach its full adult size. Once the wings are fully extended, the powerful muscles in the thorax reorient the wings into their final horizontal position. The dragonfly is now almost ready to take its maiden flight. Tremors in the flight muscle are the first sign of impending departure, and finally, 12 hours after emerging as a nymph from below the surface of the pond, the flight muscles burst into life, and the dragonfly floats up into the afternoon sunshine. The pale and fragile dragonfly must now find a quiet spot to roost, while its exoskeleton hardens and develops the rich colour scheme of the adult dragonfly. In the spring sunshine, a profusion of damselflies accompany the first of the dragons to emerge from the water, and these are the chasers. Chasers are easy to spot, with their broad tapering abdomens and distinctive colour schemes. This male four-spotted chaser will spend most of his time perched close to the water, hunting and waiting for passing females. Although he's territorial, he can't compete for aggression with his close relative, the broad-bodied chaser. The bright blue males are fiercely territorial, and competition at the best mating pools is fierce and remitting. The males engage in spectacular dogfights, in attempt to keep other males away from a favoured pool. They perch close to the water, ready to dart into the air and drive away a rival, or grab a passing female. Mating only lasts a few seconds, before they break apart and the female heads for the margins of the pond, where she will begin laying almost immediately, dipping the tip of her abdomen in water and scattering her eggs. While the female lays her eggs, the male hovers close by, to prevent her from being grabbed by another male before she's finished ovipositing. There's no time for sentimentality here, and this female will mate with many males and lay many hundreds of eggs in the few short weeks of her adult life. As spring moves into early summer, a steady stream of colourful combatants arrive at the mating pools. The emperor is Britain's largest dragonfly, and the black darter, smaller than many damselflies, is Britain's smallest. In contrast to the chasers, mating for many of the later emerging dragonflies is carried out in less of a rush. The male takes hold of the female behind the head. The male has already transferred his sperm from the tip of the abdomen to a secondary sexual organ situated on the underside of his abdomen at the base of the legs, and as the female curls her abdomen in response to the male's grip, the sexual organs come into contact and fertilisation takes place. The darters remain attached during the process of This is a useful strategy for the male, ensuring that the female isn't hijacked by another male before she's completed laying. It also seems an effective way of scattering eggs, the male seeming to swing the female to and fro as she deposits her eggs. Hawkers take an altogether different approach to egg laying. After fertilisation, the females become searching for suitable locations to lay their eggs in the marginal vegetation at the edge of the pond. This female appears to be concentrating intently as she places individual eggs into the rotting stems and vegetation. Female hawkers are equipped with a sickle-shaped appendage, and this even enables them to drill into rotting wood to place eggs. The females lay in a wide variety of locations, maximising the chances that some of these eggs will hatch successfully. It may seem somewhat random, but it's worth remembering that this pattern of behaviour is tried and tested through countless generations. By mid-October, the pools that were alive with dragonflies a few weeks before are now quiet. The odd male hawker still remains, but the few that have lasted this long will soon succumb, and either fall into the water or fall prey to another predator looking for a meal. It's the end of the dragonfly's breeding season, but down below the surface of the pond, there are dragons alive and well, and looking for a meal.