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Dragonflies They are not difficult to observe, and are visually very attractive, many exhibiting bright colours ranging through red, blue, bronze, yellow and green. They are creatures of movement, with very efficient wing design displaying amazing powers of flight, capable of movement upwards, forwards, backwards and also able to hover. Having compound eyes, large in relation to body size, vision dominates their behaviour. Heavily dependent on warmth and sunshine for their activities, they can regulate body temperature by behavioural and physiological means. Displaying much complex behaviour, they have a fascinating life style divided into two distinct and very different phases. A large proportion of a dragonfly's life is lived as an aquatic larva, having hatched from an egg which may have been inserted by a female into plant tissue, or simply dropped into the water of a lake, pond, river or stream, dependent on the species. They are opportunistic feeders using specialised mouth parts known as the labium mask which can be shot out rapidly to pierce, or grasp a wide range of water living prey. They moult their skins as they increase in size, development time varying amongst the species, roughly from 1 to 2 years to as much as 5 to 6 years, largely dependent upon the nature of the water site occupied, its situation, latitude, warmth and availability of food. Emergence can be during the summer months from May onwards, spread over a few weeks, or in some species en masse. Many select suitable emergence sites e.g. vertical plant stems, emerge from the water and commence a drastic change in life style, from a water living larva to flying insect. With considerable variation amongst species, and in very brief simple terms, the larval skin splits open and the insect emerges, hangs from a support, pumps fluid through its body and wings and gradually expands, dries off and makes its maiden flight. Life on the wing may be relatively short, a few weeks or a month or two. Beginning with very little colour the dragonfly soon assumes bright colours, and enters a period where feeding and reproduction are major requirements. Dragonflies are carnivorous feeding voraciously on other flying insects e.g. midges, other small flies etc. capturing prey on the wing and consuming much of it whilst in flight, but sometimes landing to eat whilst perched. Such behaviour varies between species, for example the large Anisoptera species e.g. Brown Hawker Aeshna grandis spend much time in flight patrolling suitable habitat where prey may be found, whilst others such as Libellulids e.g. Common Darter Sympetrum striolatum will perch on regularly used prominent sites and dash out to catch passing prey. In some species males are territorial, regularly patrolling an area, keeping out other males of the same species, and returning again and again to the same perch. Reproduction activity is instigated by the male, and in for example the Brown Hawker Aeshna grandis, the male begins by transferring sperm from the genitalia at the end of his abdomen to an accessory genitalia on the underside of his thorax. He then flies off in search of a mate and having found one grasps her and places abdominal claspers on the back of the females' eyes. The male then lifts the female with him into the air and if willing the female curls her abdomen up bringing her genitalia into position with the males' accessory genitalia, in so doing forming what is known as the wheel position, and thus being able to receive the male's sperm to fertilise her eggs. The situation is similar with the Zygoptera Damselflies, but throughout the range of dragonflies there are many detail variations. Indeed some males remove from the female sperm which she has received during a previous mating before replacing it with their own. The time taken over copulation varies greatly from a few seconds in flight for some species to many minutes, perhaps hours spent perched on vegetation by others. The subsequent egg laying methods and associated activity also have great variability and may be by the female alone or whilst she is still accompanied by the male. In some Damselfly species e.g. The Emerald Damselfly Lestes sponsa the male may accompany the female below the water surface whilst still in tandem. In some species females alone beneath the water surface whilst laying eggs have remained submerged between one and two hours, having taken down a layer of air and it is believed being also able to absorb oxygen from the water. In the Wharfedale Naturalists Society area about 18 species can be seen, of which about 15 are regular breeders. The following have been recorded: ANISOPTERA Hawkers Skimmers Chasers Darters ZYGOPTERA Demoiselles Damselflies Amongst sites in the area particularly good for seeing dragonflies
are: The fascinating life styles and beautiful colours of dragonflies
make them well worth seeking out. They are most interesting insects
to observe and study and give great satisfaction and enjoyment to
those who know them well. |