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Dragonfly Wing Sizes
When Steve Houghton mentioned in the news column on 24th August 2003 that he had measured a male Southern Hawker Aeshna cyanea and found it had a wingspan of 14.2 cm, I looked it up in the Brooks/Lewington Field Guide and found that the range of sizes for a single hind wing was 43 – 51 mm. As the hind and forewings are the same length, double this figure would be the wingspan i.e. 8.6 to 10.2cm. Steve had discovered a giant, almost 40% above the higher limit! I have a good eye for sizes and it seemed to me that some of the Hawkers I was seeing were also somewhat larger than those in the book. Having recently taken a photograph of a male Southern Hawker perched on the end of a broken branch (below), I returned to the site and measured the diameter of the branch. By taking the same measurement and the wingspan from a print I was able to calculate the actual wingspan at 11.61cm. This was not quite as large as that found by Steve, but was still a significant amount above the range in the book. Was there a general trend for growth, was this a seasonal phenomenon due to the weather, or did the guide not cover the full range of sizes?
This gave me the idea of measuring all the Hawkers I came across that were docile enough to allow it. (I don’t like the thought of catching them as I am becoming a bit ham-fisted and would hate to damage them.) I believe the single hind wing measurement was chosen because it is the most accurate way to compare sizes. If the dragonfly’s wings are at an angle the distance between the wingtips will vary accordingly. It should be noted that an error of 6% can be incurred if the wings are held at an angle of 20 degrees in any direction. Most Hawkers appear to hold their forewings fairly flat and perpendicular to the body when settled so it seems to be reasonable to measure wingspan. In any case if the wings are held at an appreciable angle the half wing measurement can be taken. So far I have managed to measure one Southern Hawker at 10.2cm which is the upper limit in the guide. This does not support the other two findings so there is insufficient data to point to a general increase in size of the Southern Hawker. However I measured 4 Migrant Hawkers Aeshna mixta at 8.5, 8.8, 8.8 and 8.6cm. This is fairly consistent and amounts to an average of over 8% above the maximum in the field guide. It is just about the end of the season and I am unlikely to obtain many more measurements this year (2003). I wrote this in the hope that others will decide to make their own observations next season giving more data to work from.
If any one is wondering about getting near enough to make measurements may I suggest a procedure which could be of help? This could also assist those who are taking up insect photography or just want to sort out those Blue damselflies. The only equipment necessary is a pocket ruler or a steel tape measure calibrated in centimetres. A pair of callipers might prove useful but not essential. Another requirement is plenty of patience. Dragonflies as with all sighted creatures are sensitive to sideways movement across the field of vision but are less sensitive to direct movement towards them. They have good all round vision and can see for a considerable distance. It seems to be pointless in trying to creep up unseen so it is better to approach slowly without sudden movement in order to show that you mean no harm. The target dragonfly will usually be sighted as it settles several metres away. Give it time to settle in and meanwhile secure any loose items on your person such as binoculars which could flap about when you close in. Have your measuring device in your hand to avoid scaring the target by the sudden movement of reaching into your pocket at close quarters. It might think you are reaching for a gun! While still at a distance, position yourself at the angle you wish to approach from, taking care not to have the sun directly behind you to avoid casting your shadow on the target. Look out for vegetation you might disturb, especially low runners from bramble bushes which are favourite perches. Try to approach in a straight line, moving slowly and steadily. If the target is much below head height, start to crouch progressively at about 3 metres, keeping your head moving towards it in a straight line so as not to loom over it or to have to bend over it suddenly. If you hold your ruler forwards as you do this, you may be allowed to take your measurement with a great amount of goodwill on the part of the dragonfly so don’t forget to express your thanks by departing as gently as you came. Naturally any onlooker who has observed this whole performance will probably regard you as a complete idiot! Unfortunately the usual result is that just as you are about to take the measurement the dragonfly decides it would like to be elsewhere. The first reaction is to want to swat the thing and measure its corpse, but this isn’t fair on an insect merely trying to escape from what it sees as a threatening situation. The best course of action is to follow in an unhurried fashion, watch where it settles and repeat the procedure. I usually find that after two or three tries, the insect seems to realise that I mean it no harm and allows me to measure it. If it does not, there will be other opportunities. On the other hand it may be that a bold approach would be just as likely to produce the same number of successful encounters. I wonder if I should investigate that……? I will be happy to keep a record of the results via e-mail and publish them in another article and I invite any comments or criticisms via the Forum. If sufficient people are interested I will put out a reminder next season. |
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