Friday, 1 June 2018

Female polymorphism in blue-tailed Damselflies


It's 30 days wild! I am joining again this June, trying to see, record and find out about as much wildlife as I can - and reviving BugBlog, which has been dormant far too long. I will make a especial effort with dragonflies and damselflies, writing a post on each species I see, but I will also be blogging in my other blogs Wild about Hull and The Rattling Crow.
 Adult dragonflies and damselflies tend to be active in warm weather. Today it was a warm, but cloudy and humid day with a few brief sunny spells. I went to Oak Road Lake to try and see some. This site is well known by odonatologists (yes, that is the name for people who study damselflies and dragonflies, which are a group called Odonata) in Yorkshire as the northernmost site for the Small Red-eye Damselfly in 2006. Thirteen species have been recorded at the site.
 I walked around the lake and there was not much activity, probably due to the weather. On the northern site I found a few Blue-tailed Damselflies, a common and widespread species that is often active in this kind of weather. This species is interesting as the females come in a wide array of colour morphs, one of the morphs has the looks of a male: blue and black. The colour pattern changes also markedly by age, but adult females can be either looking like a male or more brownish green, although in the process they can look pink or purple. Why would females look like males? won't makes get confused? This actually appears to be the reason behind the females attire: male mimicry. Males actually were more attracted to the brown females than to the blue ones. Females look like males because they are more likely to be left alone, and not harassed by males into further matings. Mating is very long in the species, usually 3-4, but up to 8 hours (which is why it is relatively easy to observe). In addition, females only need to mate once to fertilise all their eggs. Therefore, females could benefit from avoiding unnecessary long matings as they could use that time feeding to produce more eggs, and possibly being less exposed to predation when mating as well. Experiments showed that androchrome females were often found alone, while the other female morphs were more frequently found mating. Further, androchrome females had less sperm in their storage organs and some ended up not mating at all. Another hypothesis, which might not be exclusive, is that when males are very common in comparison to females it pays to look like a male to avoid frequent mating attempts, while when the sex ratio is more even then the androchromes may not benefit as much as they might not be able to mate at all.
 Androchromes trick me as well, but you can see in the top shot that when you find a pair mating the male and the female look the same (top shot, 21/07/2012 at Tophill Low). I only found a mating pair today, on the grass, and the female was brownish/green (below).
The following photos show some of the various colour morphs of Blue-tailed Damselflies
This is female form violacea, which matures into an androchrome. 
This is female form rufescens, which matures into a pale brown form 
This is the olive green form infuscans.
A male or a female androchrome?

More information
Cordero, A., Carbone, S. & Utzeri, C. Mating opportunities and mating costs are reduced in androchrome female damselflies, Ischnura elegans (Odonata). Anim. Behav. 55, 185–197 (1998).

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