Dragonfly or Damselfly?

I was pointed to this lovely story the other day (thanks James):
http://focusonfauna.wordpress.com/2013/03/21/when-beliefs-are-shattered/ 
(please read the story before continuing here)

The story reads "My Rule 649 – Dragonflies and Damselflies says one of the differentiating features between dragonflies and damselflies is that dragonflies rest with their wings flat (open) and damselflies rest with their wings closed".

Here is an example of a damselfly that fits the rule (Common Bluetail): http://natureshare.org.au/observation/4669/ (NB. NatureShare is best viewed in Firefox, Chrome or Safari, not Microsoft Internet Explorer)


... and here is an example of a dragonfly that fits the rule (Southern Tigertail):http://natureshare.org.au/observation/1309/


The story then expresses great pain on discovering there are species that don't fit this rule. I share the pain! It is a well used 'rule' but the more I see, the more I realise there are quite a few species that don't fit the 'rule'. Here is an example of a damselfly that holds it wings open (Common Flatwing): http://natureshare.org.au/observation/7851/


But it is more painful – recently I came across a tiny dragonfly that holds it wings closed, the Common Shutwing (Cordulephya pygmaea) – it is apparently only seen in Autumn but fairly common (flying now in fact): http://natureshare.org.au/observation/8701/


Rather than a rule, wings being held open or closed is in fact a ‘rule of thumb’. A rule of thumb is nicely defined by Wikipedia as “A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for making some determination.” A bit like a ‘non-core’ rule. A good example of a rule of thumb we were all taught at school is ‘i before e except after c’ (as in receive). As it happens this rule has now been dropped by most educators because there are more breaches of the rule than there are examples of the rule working; ie. it is a very inefficient rule of thumb!

So there are breaches of the dragonfly-damselfly rule of thumb on both sides – most breaches occur for damselflies (there are quite a few that break the rule and hold their wings open), whereas I think there is only one family with one genus of dragonfly in Australia with wings held closed (Cordulephya – and only the abovementioned species occurs in Vic – I think). Despite this, the rule of thumb does work much more often than not so it isn’t too bad (as a rule of thumb).

As it happens there is another rule of thumb that works better for damselflies vs dragonflies. It’s the eyes. In one respect this isn’t as good as a ‘rule of thumb’ because it is harder to see with the naked eye (in comparison to wings held open of closed) but I can only ID these creatures from a photo and the eyes are easily seen from photos.

So how does the ‘eyes rule of thumb’ work? All damselflies have a large gap between the eyes (they are held a bit like a hammerhead shark):


Dragonflies have eyes that are touching (or nearly so):


But, of course, being a rule of thumb there are exceptions. The exceptions though are only the family Gomphidae, which are dragonflies with an obvious gap between the eyes. However, with Gomphidae species, the eyes generally aren’t separated as much as damselflies, they are medium-large dragonflies and they are quite distinctive in their own right (whenever I’ve seen a Gomphidae species it has been obvious they are dragonflies and not damselflies). Here is a member of the Gomphidae family (Yellow-striped Hunter - Austrogomphus guerini):
 

The good thing is at least one part of this ‘eyes rule of thumb’ is absolute – here is how to apply it:
1. Eyes touching or nearly so = always a dragonfly
2. Obvious gap between eyes = damselfly, or a dragonfly in the Gomphidae family

Finally, is there an actual ‘rule’ that works for dragonflies vs damselflies? The answer is yes. Taxonomically speaking all dragonflies and damselflies form the insect order called Odonata (Order: Odonata). Confusingly the common name for the order Odonata is ‘dragonflies’ (noting that it contains damselflies too). The order Odonata comprises two sub-orders: 1) damselflies (sub-order Zygoptera); 2) dragonflies (sub-order Anisoptera – sometimes called ‘dragonflies proper’). According to Theischinger and Hawking (Dragonflies of Australia) damselflies and dragonflies can be separated by wing venation, thus:

1. Damselflies – Discoidal cell is a simple quadrilateral (sometimes traversed by crossveins, occasionally open at base).


2. Dragonflies – Discoidal cell is divided into a hypertriangle and triangle (often differing in shape in fore- and hindwing, and often traversed by crossveins).


Here are all the damselfly and dragonfly species we’ve found (so far) in Riddells Creek
http://natureshare.org.au/collection/28/
Using this list, the wing venation isn’t always easily visible but you’ll be surprised how many times you can see it. (NB. once in a specific observation in NatureShare – click on the largest square beneath a photo to see the photo in its largest resolution).