Adult dragonfly life spans are short, usually only a few weeks. But the young dragonflies have lived in aquatic habitats for several years, so the total life span in fact is rather long for an insect: three or more years from egg to death. This short life span strategy, as for most other insect species, has survival value.
The large mammalian strategy is to have long life spans and few young. Insects have opted for short life spans and many young. Which works better? They both work, since both insects and mammals are among the great survivors in the lottery of life on earth. Everything that is alive today is a survivor species and these only represent about 10% of all the life forms that have been around in the last billion years. While the trilobites and the dinosaurs are gone, along with millions of other life forms, other vertebrates and arthropods have not only survived to the present, but have diversified into thousands of species: about 40,000 vertebrates and a whopping 879,000 described arthropod species. Among the different animal phyla, they are the winners in the diversity of species category.
One of the oldest groups of living insects are the Dragonflies and their smaller cousins, Damselflies (order Odonata). They appear in the fossil record in the Carboniferous Period, when our ancestors were still fish. They had a short but probably pleasant time as the top predators in newly evolving terrestrial ecosystems. First, plants conquered the land: mosses, ferns, lichens. Then the animals could follow and insects got there early. Giant dragonflies with two meter wing spans buzzed over the swamps that would eventually become the fossil fuels that we are currently burning up. These dragonflies were one of the first insects to take to the air and this enabled them to become top predators.
They have big eyes to see prey with, strong mandibles to bite them and six powerful legs with grasping claws to catch their food. They can't fold their wings back like more advanced insects and their immature stages are still aquatic. Eventually when land vertebrates evolved, dragonflies and the other predatory insects were displaced as the top predators. This was primarily due to the difference between internal and exoskeletons which allowed vertebrates to get much bigger than insects. Yet the dragonflies have survived anyway. Both nymph and adult stages have had to be successful in their respective habitats for the dragonflies and damselflies to survive 400 million years essentially unchanged.
In my part of the world, brilliant velvety red species and bright blue species vie for the air space on transparent,irridescent wings. The waters are inhabited by many nymphs which provide a food source for fish. The nymphs are also predators, they are brown for camouflage, have six strong legs plus biting mandibles and gills so they can survive underwater. They stalk the local shrimp, tadpoles and small fish or hide from bigger fish. This phase lasts several years.
Eventually, the nymphs get big enough that, as the days get longer in spring, internal changes occur. The nymph gills wither away while spiracles appear and the nymph must leave the water or drown. Up it crawls on the stalk of some swamp plant until it stops to moult. It has moulted before, but this time is so different. What emerges is the adult form, with wings on the thorax and a long thin abdomen with reproductive organs inside. The wings must be pumped up hydraulically to unfold. Once they unfold and harden, they will never fold again.
For adult dragonflies the enemies are birds and bats. But adult dragonflies are just as likely to exhaust themselves holding territories, fighting for sexual favours and laying eggs. The adult males in my streams flash their bright colors and defy the birds to attack them while they chase off other males and court the females. Their sexual life is complex and interesting. In territorial species, females coming into a male's territory will be grasped and held while he inserts his organ in her receptacle. In nonterritorial species, copulation is preceded by the males courting the female and convincing her who is worthy. This may involve showing the female his chosen oviposition or egg-laying site. Females show their readiness to mate by flexing their abdomen. Mating can take place in the air and they may go on flying in tandem for part or all of the sequence while the male grasps the female and then mates.
After a few weeks of these activites, the poor old battered males die, especially when the nights go cooler at the end of summer. The females also eventually exhauste themselves. After mating with the males of their choice, they spend their body's energy making lots of eggs and flying about, delicately depositing them on suitable sites. The eggs carry the species over the winter months when food is short and conditions unfavourable for ectothermic hunters.
When the eggs hatch, the longest part of a dragonfly's life begins: life in the water, hunting for prey and being hunted by fish and other predators and trying to get enough food to grow, moult and eventually win the lottery, grow wings and emerge to a lovely summer in the sunshine, winging it, buzzing over their little kingdoms and apparently 'enjoying' their little lives.
Every year, a new generation, for 400 million years: that's 400 million generations of Dragonflies living, flying, mating, dying so that this summer more bright velvety red dragonflies will continue to enliven the banks of my river. It's an awesome thought.