Monday, 4 June 2018

Emerging damselfly

I went to the wildlife garden on my walk to the school run. I was lucky to find a just emerged damselfly. It was sitting on top of its larval skin (top shot) on a leaf. There were other dry exuviae on the same leaf and an immature Azure Damselfly, so I guess the newly emerged individual is this species too. I was surprised to find this emergence as it has been a dull, cold day with a bit of a drizzle at times.
A teneral (recently emerge immature) Azure Damselfly on the same leaf.
A photo of the leaf with the teneral individual on top, its exuviae and the newly emerged one at the bottom.
The wings have started to expand. Note the relatively thick abdomen.

The fully extended wings. The individual is still very green and soft, it will slowly acquire its black markings and later the mature colours. 

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).

Thursday, 31 May 2018

Chasing the climate: Broad-bodied chaser and the dragonflies of East Yorkshire

There are 45 dragonfly and damselfly species in the UK, of which 23 breed in East Yorkshire. I was amazed to find that 15 of these have established themselves in the county only after WWII. The recent colonists include now common and widespread species like the Common Darter, the Blue-tailed Damselfly and the Emperor Dragonfly.
Cumulative number of East Yorkshire dragonfly and damselfly species and the first record of each colonist species (data compiled from Paul Ashton's Dragonflies of South East Yorkshire, 2013).

 Recent studies taking advantage of the wealth of records held by the British Dragonfly Society indicate that warming climate largely explains northward range shifts, range expansions and earlier emergence observed in most British dragonflies and damselflies. In contrast, the few northerly species have also shifted their range north, and their ranges remained stable or slightly contracted in size. Some extreme examples of range expansions are the Common Darter, with a northern distribution range that moved north 346 km in 40 years, the Keeled Skimmer shifted 190 km.

  One of the recent East Yorkshire colonists is the Broad-bodied Chaser, with a first record in 1995. It is now quite widespread but still establishing and expanding its range.
Today I visited a pond in a site in the outskirts of the city, which has maintained water until now this year probably due to the higher than usual rainfall this winter and spring. I took advantage of a short sunny spell this afternoon in an altogether warm muggy day to try and see the Broad-bodied Chaser I had seen there before in mid May. This is a large, striking dragonfly, which readily colonises new ponds, including garden ponds. They have a flattened abdomen and dark spots on the base of the wings. Males have a powdery blue abdomen and yellow spots on the sides (top shot), the females a yellow/brown abdomen (below). Males maintain territories in ponds, where females may only approach the pond to mate and oviposit. As an adaptation to the shallow, small ponds where they develop, their larvae are able to move over damp, but otherwise dry terrain if the pools where they are developing are dry.
A female Broad-bodied Chaser (22nd May 2017, Leven Canal)

  Not long after I arrived, a male passed flying by hunting and patrolling over the pond, about half a meter above the water. I briefly saw a female too, which didn't settle. This species flies in a zigzagging way which makes it hard to follow, but it is a large animal, and has a habit of perching in favourite spots regularly, which more than compensates for it. The male settled on the most inaccessible part of the pond a few times, but finally it alighted on a large clump of Yellow Flag irises near me, where I could watch and photograph it easily.

More information
Ashton, P. (2013) Dragonflies of South-east Yorkshire. 105 pp.

Hassall, C., Thompson, D. J., French, G. C. & Harvey, I. F. Historical changes in the phenology of British Odonata are related to climate. Glob. Chang. Biol. 13, 933–941 (2007).

Hickling, R., Roy, D. B., Hill, J. K. & Thomas, C. D. A northward shift of range margins in British Odonata. Glob. Chang. Biol. 11, 502–506 (2005).

Piersanti, S., Rebora, M., Salerno, G. & Gaino, E. Behaviour of the larval dragonfly Libellula depressa (Odonata Libellulidae) in drying pools. Ethol. Ecol. Evol. 19, 127–136 (2007).

Wednesday, 30 May 2018

Azure Damselflies ovipositing

On Monday, I went to my local wildlife garden to check on dragonflies and damselflies. There were only Azure damselflies, Coenagrion puella. The Azure Damselfly is a common species, frequent in small ponds. Males are blue and black and females greenish and black. It can be distinguished from the Common Blue Damselfly by a forward pointing sput on the sides of the thorax. I walked around the pond and counted three mating pairs in tandem, and many individuals, including two teneral ones - just emerged and still not showing their full colour. The females were egg laying, with the males clinging to their thorax, guarding them as they lay from the attentions of other males. The females insert their eggs into stems of submerged aquatic plants. Females can lay over 4,000 eggs in their lifetime. I noticed that the pairs were not randomly assorted around the pond, but tended to gather in the same spot (top shot). Indeed, ingenious field experiments by A Martens showed that this species form aggregations to oviposit, the pairs particularly attracted to the vertical posture of the male in a tandem. Pinned models of a male in vertical position attracted tandems, but pinned models of females did not. Two model tandems were more attractive to pairs than single tandems, which will start an aggregation.
An adult male resting.
A teneral male, recently emerged.

Two pairs egg laying. 
The exuviae of three damselflies still stuck to the stem of a plant. 

More information
Martens, A. Field experiments on aggregation behaviour and oviposition in Coenagrion puella (L.)(Zygoptera: Coenagrionidae). Advances in odonatology 6, 49–58 (1994).

Monday, 18 September 2017

Speckled Wood feeding on ripe plum

When I took this photo I didn't realise that this Speckled Wood had landed by one of the few over ripe plums left on the tree. This butterfly is rarely seen nectaring on flowers, they feed instead on honeydew, but they appear to be partial to the sweet juice of plums.