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My first interest in dragonflies came from their often vivid colours, and the sheer difficulty of photographing them in their swiftly-moving, jerking flight patterns. They are one of the very few animals that can fly up, down, backwards, forwards and sideways.
I have become fascinated by the sheer difficulty of distinguishing one species of dragonfly from another. Despite helpful pictures in many books, it can often be hard to make a good ID.
In an earlier article "Oh! So Similar" I talked about the way that many dragonflies at first seem to look so alike. Now I am going to look at two species of dragonfly that are very like each other and show what I feel are good ways of telling them apart. The species is Pincertails (Onychogomphus in Latin).
This commentary covers only Pincertails found in the Mediterranean area of S. France that lies at the foot of Les Albères in the Pyrenees, near the river Tech.
There are two species of Pincertail (Onychogomphus) in this area - Large and Small Pincertails. They are called Pincertails because of the eye-wateringly substantial male appendages; these are used during copulation by the male in grasping the female. Telling the difference between Large and Small can be tricky as the overall sizes of the two are not all that different.
Large Pincertail male (Onychogomphus uncatus)
Small Pincertail male (Onychogomphus forcipatus)
Small Pincertail (O. forcipatus) appendages
Small Pincertail (O. forcipatus) appendages
I have found the Large Pincertail to be the less common of the two, having only seen 2 examples. It appears they are less plentiful in the lower reaches of rivers, existing at heights of up to 800 metres, and as a species is rarer than the Small Pincertail.
The Small Pincertail has three subspecies, of which only one is found in this area (Onychogomphus forcipatus ssp. unguiculatus). All O. forcipatus pictures in this article are of the subspecies.
In telling the difference between Large and Small there are three (fairly) visible markings which are usually straightforward to identify and are diagnostic.
Starting at the front with the first of these diagnostic markings -
In the Small Pincertail (O. forcipatus) the vertex - the black area between the eyes - has got a small yellow mark.
The Large Pincertail (O.uncatus) has an all-black vertex.
Large Pincertail (O. uncatus) all-black vertex.
Moving towards the back of the insect a little for the second diagnostic marking, both species have a yellow collar at the front of the thorax (not always easy to see). In the Large Pincertail this is broken by a black bar.
Large Pincertail (O. uncatus) yellow collar broken.
The Small Pincertail lacks this black break.
Small Pincertail (O. forcipatus) No break in collar.
Finally the third diagnostic marking. In the back wing, both species have got a triangular area close to the body at the rear of the wing called the anal triangle.
In the case of the Large Pincertail male this triangle contains four cells...
Large Pincertail (O.uncatus) Four cells in anal triangle.
....whereas the Small Pincertail male's anal triangle contains only three cells.
Small Pincertail (O. forcipatus) Three cells in anal triangle.
There are other markings that can be used or ID, but in the pincertails I have seen these are either quite variable and therefore not really diagnostic (see below) or require the dragonfly to be caught for detailed observation (for instance tiny differences in the tail pincers).
In the case of the Small Pincertail below there two curving black lines running front to back on each side of the top of the thorax; these should be joined about half way along by another curving black line. On one side the line clearly joins but on the other not very clearly.
Small Pincertail (O.forcipatus) Do the lines join?
A comment on nomenclature.
In the case of many dragonflies there tend to be different names for each species in English. These vernacular names need to be as user-friendly as possible. The names proposed in a classic reference work by Klass-Douwe Dijkstra (K-DD) - "Field Guide to the Dragonflies of Britain and Europe"- are becoming widely preferred, mainly because they are more descriptive. The British Dragonfly Society (BDS) names have a more regional focus.
For example the Small Pincertail (K-DD) is also called Green-eyed Hooktail (BDS); Small Pincertails in our area have blue eyes (see photo above), and the name Hooktail is less descriptive than Pincertail.
Elsewhere in the world (e.g. USA) names can yet again be different. There is a need to standardise international names for European species, and currently we are seeing polarisation rather than consensus.
The definitive name is always the Latin one. Even so, this is not always clear as there is discussion on which species a dragonfly belongs to (especially in the case of sub-species - i.e. is it a variant of an existing species, or an evolved new species).
K-DD BDS French Latin
Pincertails Hooktails Gomphidés Onychogomphus
Large Pincertail Blue-eyed Hooktail Le Gomphe à Crochets Onychogomphus uncatus
Small Pincertail Green-eyed Hooktail Le Gomphe à Pinces Onychogomphus forcipatus
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