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Dragonflies are a well known and fascinating order of insects; you will likely see plenty of them as you go out into the field in late summer, normally near water. They are more common in warmer parts of the world than in temperate areas like the UK and of the 5 300 named species world-wide, only 38 live and breed in Britain. They are conveniently divided up into two groups Anisoptera the true Dragonflies which rest with their wings out from their body in a cross shape and Zygopteran or Damselflies who hold their wings above their body. In this article when I say Dragonflies I will mean both Anisoptera and Zygoptera, but I will use these terms separately when talking about the individual groups.
Dragonflies have strongly biting mouthparts and are active and aggressive carnivores, both as adults and as young (called nymphs), preying mostly on other insects. The adults have massively large eyes, often meeting at the top of the head in Anisopterans. These eyes may each contain as many as 30 000 individual lenses or ommatidia (your eyes have only one lens each). Because of this Dragonflies have exceptionally good eyesight and have been known to respond to stimuli from more than 40 feet away. They have very small and poorly developed antennae though.
They have two pairs of almost equally sized long thin membranous wings; both pairs of wings usually have a stigma (a dark or coloured patch near the middle of the leading edge) and a mass of cross veins giving them the appearance of being a mesh. Unlike most insects, which either flap both pairs of wings in unison (i.e. Bees and Butterflies), or only flap the hind pair (i.e. Beetles), or only have one pair (i.e. Flies), Dragonflies can flap or beat their wings independently. This means the front wings can be going down while the back ones are coming up. You can see this happening if you watch closely. Dragonflies are excellent fliers, particularly the Anisopterans and can loop-the-loop, hover and fly backwards quite easily. It is not unusual for the larger species to reach 30kph and the Australian Austrophlebia costalis has been clocked in at an impressive 58kph or 36 mph for short bursts. They flap their wings relatively slowly though, at less than 30 beats per second. Compare this with 200 bps for a hoverfly or 300 bps for a honey bee.
Dragonflies are a very ancient order of insects and fossils exist from more than 300 million years ago. Dragonflies are also relatively large insects, even now, but in they past they were much larger. Fossil remains of some of the largest flying insects to have ever existed are Dragonflies, one species Meganeura monyi had a wingspan up to 75 cms. The largest Dragonfly in the world now is actually a Damselfly (Zygoptera) Megaloprepus caerulata from Costa Rica with a wingspan of 19.1 cm or 7.52 ins and a body length of 12 cm or 4.72 ins. Tetracanthagyna plagiata from Borneo is the largest Anisopteran or true Dragonfly. The smallest is probably Agriocnemis naia from Burma with a wingspan of just 1.76 cm or .69 ins.
Dragonflies are unique in the insect world in that the male possess a set of secondary sexual organs on the 2nd abdominal segments as well as his primary sexual apparatus on the 9th segment at the end of his abdomen. Before mating can occur the male Dragonfly must charge his secondary copulatory apparatus with sperm from his primary copulatory apparatus. Mating commences with the male grasping the female with his abdominal claspers. The pair then assume the wheel position with the tip of the females abdomen and thus her sexual apparatus engaging the males secondary copulatory apparatus. The male first uses his penis to remove any sperm left by a previous male before inseminating her himself.
Copulation can take from several minutes to several hours depending on species. The male stays in tandem with the female in many species while she lays her eggs. In those species which lay endophytically some lay below the water line, and in some cases both the male and the female may become fully submerged. In other species the male stays close to the female guarding her while she lays, while in those strongly territorial species the male may be satisfied by continuing to expel all other males from his territory allowing the female to lay within the territory at her leisure.
Eggs are laid either endophytically (inside the living tissue of a plant) or exophytically (into or onto the water or the mud of the bank). Eggs are normally laid in batches (one at a time in quick succession), endophytically laying species tend to be limited to several hundred or less eggs per day whereas exophytically laying species can lay several thousands per laying episode. Some temperate Dragonflies overwinter in the egg stage and thus the eggs do not hatch for several months, however with tropical species the eggs can hatch in as little as 5 days.
Larvae are aquatic, normally in rivers, streams, ponds and lakes but some species such as Podopteryx selysi make use of plant trapped water, such as water filled tree boles. The exception to this rule is the Australian Antipodophlebia asthenes whose larvae may be terrestrial in subtropical rainforests. Zygopteran larvae swim by flexing their abdomen from side to side, but Anisopterans tend to walk though they can turn on the speed with jet propulsion by expelling water from their anal respiratory (breathing) orifice. Zygopteran larvae, like other aquatic insects breath through caudal gills (their tails) but Anisopteran larvae breath through their anus. Which is an enlarged cavity with special internal folds to increase the surface area, water is pumped in and out of this muscularly to increase water flow across the respiratory membranes. Most larvae are free ranging, though they tend to hunt by stealth often sitting waiting for their prey to walk by, but some live in burrows in the mud. Larvae are carnivorous detecting their prey by sight in most cases and catching it by means of a rapidly extensible grasping modification of the labium, (you have to watch this to believe how fast it moves), they eat mostly other invertebrates.
Larvae generally go through 12 - 15 instars depending on species and some bivoltine (having 2 generations in one year) tropical species can develop in as little as 60 days. In colder climes species tend to be univoltine (having one generation per year), semivoltine (guess) or even multivoltine, taking up to 6 years to develop in some near arctic species. Most temperate species therefore overwinter at least once as a larvae as well as an egg.
In any given set of climatic conditions most species emerge at well synchronised times of year over a period of about a month. Many species emerge at night to avoid predation during this vulnerable time but in colder environments some species wait for sunrise before emerging.
After emerging most dragonflies leave the vicinity of the water and go through a period of maturation, this generally lasts about one month during which time the gonads finish developing, and the body colour brightens; for many species this is also a time of dispersal. Adults generally feed on flying insects which they catch on the wing, either by flying around constantly, or by sitting perched on a lookout post and sallying forth to catch passing insects; most species use one tactic or the other preferentially.
In some species the males are territorial, i.e. they stake out and defend a territory from all other males, mating with any mature female that enters the territory. Territories may be held for only a few hour, for several days, or even longer in exceptional cases.
Apart from mankind, see below Dragonflies are eaten as larvae by fish (particularly Bass), water shrews, water beetles, water bugs, and birds particularly diving ducks. In one study it was found that Dragonflies made up 14 percent of the food eaten by Ring-necked Ducks in America. Dragonfly nymphs will also happily eat each other. As adults they are also eaten by Birds, with Blackbirds and other Thrushes specialising in taking them as they are emerging from their larval skins and therefore helpless. Another bird that eats a lot of Dragonflies is the Hobby a small hawk which catches them in mid-air. Hobbies are relatively rare though and they do not hurt the population levels.
The order Odonata is divided into 3 Suborders; the Zygoptera or Damselflies which can fold their wings over their abdomen, the Anisoptera or Dragonflies which can't, and thus hold their wings straight out from their thorax, and the Anisozygoptera an ancient suborder possible once containing the seeds of both the other 2 more modern suborders but now containing only two species from Japan. Anisoptera means 'unequal wings' and generally speaking the members of this suborder have their hindwings broader than their forewings. They are also normally stouter, larger and much more acrobatic in their flight, Zygopterans tend to fly slowly and leisurely, it is the Anisopterans that swoop around grandly.
A Cladistic Representation of Odonatal Taxonomy at the Tree of Life
Mankind has long appreciated Dragonflies, in a number of Asian countries both the adults and the larvae are eaten much in the way we eat prawns and in Japan and China they have been popular subjects for poetry and paintings. In fact in Japan the Tombo or Dragonfly is a national emblem and Japan itself is often referred to as Akitsushima (the Dragonfly Island). This is because the first emperor Jimmu Tenno thought that looked at from the top of a mountain Japan looked like a Dragonfly liking its tail. Dragonflies also appear in Japanese mythology, 'Shoryo tombo' is the Dragonfly of the Dead whose job it is to carry the spirits of the families ancestors to the family during the festival of Bon. Dragonflies are still much respected in Japan where they are a symbol of playfulness and victory in war. Japan has far more species of Dragonfly than the UK and was the first country in the world to create a special Dragonfly Nature Reserve.
|Lonesomely clings the Dragonfly to the
underside of the leaf
Ah! The Autumn rain.
Dragonflies have also been given many strange names in the UK where it was wrongly believed that their long tail was a huge sting. Some of the names they were called are Horse-stinger, Horse-adders, Snake-doctors and Devils Darning-needles. We now know ofcourse that dragonflies have no sting, though they will give you a nip if you pick one up with your hands.
Mankind has a diverse effect on Dragonflies, for many his building of ponds and farm ditches supplies much welcome additional breeding habitat but in other ways mankind has destroyed some species habitats entirely and 2 species are known to have gone extinct as a result of mankind's changes to the environment. These are Megalagrion jugarum and from the USA and Sympetrum dilatatum from St. Helena. According to the IUCN (International Union for the Conservation of Nature) there are between 100 and 140 species threatened with extinction at the moment.
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