Tuesday, 31 July 2018

Egg laying Southern Hawker

On the way back from work I popped in my local wildlife garden. I was looking for dragonfly exuviae in the small pond when I gasped: A female Southern Hawker had materialised like out of nowhere and was hovering inches from my face. This is a large and inquisitive dragonfly, with a striking apple green, yellow and black pattern, which often approaches humans when they walk on their flight path. She took no notice of me and landed just by my feet, on the side of the pond-dipping platform. She was egg laying! I watched, mesmerised, how the tip of her abdomen appeared to direct proceedings, making searching movements on the pond lining, in a caterpillar-like fashion. After a few moments she flew low across the rushes growing in the pond, her wings clashing with the leaves. She landed nearby on the shore and started egg laying again. Taking her time, her abdomen appeared to 'look' for crevices amongst the mosses and litter, just a few centimetres over the pond surface.
 There was no male about. Hawkers oviposit on their own. 

Oviposition sites
Hawkers often lay their eggs endophytically, which means inserted into plants, mud or debris or wood on the pond margin, as opposed to exophytically, meaning dispersed on water. However this species appears to often chose odd places as substrate, and indeed, in the space of 10 minutes the female today had probed or laid in several places in the pond:
Probing on pond liner...
on mosses at the pond margin...
...on the decaying leaves of aquatic plants.
on the wooden panels of the pond-dipping platform.
 Having recently read the New Naturalist volume on Dragonflies, I wasn't too surprised. Philip Corbet and Stephen Brooks compiled a list of totally bizarre substrates chosen by Southern Hawker females to lay eggs. These included:
-A brown woolly jumper
-A wellington boot
-Someones ankle
-A dog's rump
-The skin of a Yellow-bellied toad

Corbet and Brooks remarked that in some of these cases the female might be just investigating the suitability of the particular substrate chosen, as opposed to ovipositing.

The Southern Hawker female hovered a few times by the pond and then moved on, hawking low along the garden path by the hedge a few times and exploring the bushes in the typical fashion of the Southern Hawker. This low flight is a behaviour that unfortunately, makes this species a frequent prey of cats.

More information
Klaas-Douwe B Dijkstra. 2006. Field Guide to the Dragonflies of Britain and Europe. Illustrated by Richard Lewington. British Wildlife Publishing. 320 pp.

Philip Corbet and Stephen Brooks. 2008. Dragonflies.  The New Naturalist Series. harper Collins Publishers, London. 454 pp.

Monday, 30 July 2018

Small Red-eyed Damselfly range expansion

This morning I watched several males of the Small Red-eyed damselfly at North Cave Wetlands, a YWT nature reserve with a diversity of large lakes, shallow reed bed lakes and dragonfly ponds (top shot). It was the first time I see this species in this site, where is now established. The Small Red-eyed is quite a distinctive damselfly, but it can be confused with its relative the Red-eyed Damselfly. The Small Red-eyed has an later flight season (end of July-August, although they do overlap), is smaller, has more blue in the tip of the abdomen and tends to sit with the abdomen curved upwards. Both species like to sit away from the shore, on floating aquatic plants, and individuals come back to the same spot after hunting. Given their habits and small size binoculars are a must to identify it!
Female Small Red-eyed Damselfly at Clubley's Scrapes (Spurn NNR, 21/07/18)
 The Small Red-eyed has been a British species for less than two decades. After a range expansion within Europe culminating with the colonisation of the Netherlands and Belgium, the species was first recorded in coastal sites in the south of the UK in 1999 and rapidly expanded north and west at a pace of 28 km per year. Today it is present up to North Yorkshire, but the rate of expansion is reducing. In 2006 it was first found in East Yorkshire, in a fishing lake in Hull, Oak Road Lake, and now it is present in several East Yorkshire sites.
 Colonisation can be associated with loss of genetic diversity, especially if the species in question is a poor disperser or population growth is slow after establishment. Given the speed and recency of colonisation of the Small Red-eyed Phillips Watts and colleagues investigated the genetic relationship and genetic diversity of nearby European populations and UK ones. It was presumed that the waves of migration came from NW France and Belgium, but there was another colonisation centre in the UK around the Isle of Wight, which hadn't expanded as much. Watts screened Small Red-eyed populations with 10 very variable molecular markers, similar to the markers used in forensics. The Isle of Wight population had less diversity than the remaining British populations, but the populations involved in the main expansion had a similar diversity to European populations investigated, with no evidence of population bottlenecks. This indicated that the waves of colonisation from the continent likely involved large numbers of individuals, and or colonising populations grew rapidly after establishment precluding losses of genetic diversity. Although it might appears surprising that this tiny insect can be capable of long distance dispersal and rapid colonisation, but migration is a increasingly acknowledged feature of many insects.
Small Red-eyed in flight.
Spot the tiny Small Red-eyed underneath an ovipositing pair of Common Darters and a Common Blue Damselfly.

More information
Watts, P. C., Keat, S. & Thompson, D. J. Patterns of spatial genetic structure and diversity at the onset of a rapid range expansion: colonisation of the UK by the small red-eyed damselfly Erythromma viridulum. Biol. Invasions 12, 3887–3903 (2010).

Tuesday, 17 July 2018

How do female dragonflies avoid male harassment?

After egg laying, female damselflies and dragonflies can be exposed to males trying to mate. Females will then be unreceptive and in order to avoid a lengthly and potentially costly copulation, they have evolved different strategies to avoid male harassment. These are only some of these strategies:

1) Avoiding water. This strategy appears to be quite general in the group. Males will return to the breeding ponds first after they mature and will be very obvious as they spend a lot of time patrolling the pond or sat on prominent perches, lake or river or hanging around near the water. In contrast, females often stay well away from the water, only returning to the breeding site to mate and oviposit. Staying away from the water allows females some control about when to mate.

2) Sneaking in. Females can be quite secretive when approaching an oviposition site, or they may lay during cloudy or cold weather - especially the larger species which can generate heat by whirring their flight muscles. Males are more likely to be roosting during dull weather or early in the morning, so females might be able to oviposit uninterrupted if they time their visit to the pond well.

3) Adopting an oviposition posture. If a male flies overhead, a female may try and repel him curving her abdomen down, like she was ovipositing. A great photo showing this behaviour in an Emperor female is here.

4) Looping the loop. Female dragonflies can fly faster than a chasing male, or do a loop the loop or even dive under water to avoid harassing males!

5) Androchromes. In many damselflies, such as the Blue-tailed and the Common Blue damselflies there is genetic variation in female colour with some of the colour forms strongly resembling males. I have covered this topic recently in Blue-tailed Damselflies.
A female Emperor with a very blue abdomen (16/07/2018). 

6) Age-related male mimicry? Females have an ability to store sperm and the sperm from a single mating should be enough to fertilise all her eggs for two weeks. In some species of dragonflies, females change as they age to resemble males (e.g. Emperor, Common Darter). The abdomen of mature female emperors is green, but it may turn blue - like a male's- when they are about 2 weeks old (it is unclear, however, it this is a purely age effect or a temperature response to warmer weather as the season progresses). If this was age-related, then male mimicry might reflect a different evolutionary response to the same selective factor than the genetic androchromes. When the female is young it is in her interest to attract males and mate, but as she ages she is likely to have already mated and the colour change might make it easier to avoid male attention. More research is definitely needed!

7) Playing dead. One of the most striking strategy of male avoidance is that of the Common (or Moorland) Hawker Aeshna cyanea (top shot). Ovipositing females often chose sheltered spots with denser vegetation in ponds to avoid male detection. As females leave ponds after ovipositing, males chase them. Rassim Kheliffa carefully documented that in such occasions, females dived into the tall grass surrounding the pond, staying motionless, often upside down, and so avoiding being grabbed by the males. The females were alert and responsive, and 87% (out of 31 attempts) where able to successfully avoid being picked by Rassim. He hypothesised that death feigning has evolved by females co-opting a pre-existing antipredator behaviour into a male avoidance strategy to avoid undesired mating attempts

More information
Corbet, P. S. The Life-History of the Emperor Dragonfly Anax imperator Leach (Odonata: Aeshnidae). J. Anim. Ecol. 26, 1–69 (1957).
Khelifa, R. Faking death to avoid male coercion: extreme sexual conflict resolution in a dragonfly. Ecology 98, 1724–1726 (2017).

Credit: Top photo of Common Hawker by Robert, used with permission.