Can you recognise individual dragonflies from their faces?

bug-189906_1920Dragonflies are beautiful, alien-looking animals. They have bits that move and bend in ways that you wouldn’t expect, enormous eyes, and intricately patterned wings. I have written about the hydraulic gill system of dragonfly larvae, which powers both their jet propulsion and their “mask” that grabs prey. Meanwhile, dragonfly adults have basket-like legs to ensnare prey, as well as flexible abdomens which they use to form mating “hearts”. I’ve been interested in why dragonflies look the way they do, and that that means for their evolution, for a number of years.

I was intrigued, therefore, to read a paper that described how a pair of scientists had been able to tell dragonflies apart just by looking at the markings on their bodies. I do not remember how I first came across it, but the work is described in this German paper published in 2009 in the journal Entomo Helvetica by Schneider and Wildermuth*. The paper described a population of the southern hawker (Aeshna cyanea) in which a substantial number of animals could be identified from their facial markings. The paper is not creative commons so I can’t share the document, but you can see for yourself if you download the manuscript from the public link above and look at Figure 2 (it’s worth it – the pictures are stunning!). The title of the paper translates as “Dragonflies as individuals: the example of Aeshna cyanea“. So why might these markings occur?

There are lots of reasons why it might be advantageous for animals to be able to identify individuals. You might be trying to identify mates of high quality to increase your chances of reproduction. Many social animals (including humans, but also ants, meerkats, and molerats) distinguish relatives from non-relatives or friend from foe using sight or smell. Many theories of how cooperation evolved rely on animals having repeated interactions with one another, and remembering who has scratched whose back so that the favour can be repaid in the future. However, none of this applies to dragonflies. Dragonflies rarely have any structure to their mating (it’s usually first-come-first-served, and a mad scramble if many males are involved), they are not social (while they live in groups they do not necessarily act together), and they do not cooperate (apart from mobbing of predators such as hawks, but that’s probably not true cooperation).

san_marco_spandrel
A spandrel Photo by Maria Schnitzmeier, CC-BY-SA, http://bit.ly/2czvygt

More likely what we are seeing is not the evolution of a trait, but the by-product of another trait. In a provocative article written in 1979, Stephen Jay Gould and Richard Lewontin wrote about this idea**: that some things we observe in nature are not the product of evolution directly, but occur as a result of some adaptation. Gould and Lewontin gave the example of “spandrels” from Rennaisance architecture. Spandrels (like the example on the right, from the Basilica de San Marco in Venice) were the accidental byproduct of the way that arches were designed – a small curved area was left in the corner of the arch, and this was often filled with artistic renderings. However, the spandrel itself was never the focus of the design.

In the case of dragonfly faces, the same is likely true. Dark patches on insects are usually caused by a substance called “melanin” (which is the same pigment that produces darker skin in humans). Melanin is involved when insects fight off infections or heal injuries. It is most likely that the patterns on the faces of the dragonflies are due to some kind of damage, perhaps during emergence from the water, or perhaps as a result of conflict between territorial individuals. What is most interesting, though, is that Schneider and Wildermuth seem to have found a population in Switzerland that has an unusually high number of animals with such markings. When I went to Flickr to look through other photographs of this species, I found very very few examples. Below is a gallery of some of the creative commons photos, and there are many more if you go to Flickr yourself and search for “aeshna cyanea”.

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That’s not to say there are no other examples. See here and here for examples of the markings in other photographs (but note that many of the most striking examples are taken by the same photographer).

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Photo by Zak Mitchell.

The researchers who published the original paper offered an interesting addition to the literature on understanding individual insects. Usually, we do this by marking the animals (with dragonflies you can write on their wings, for instance, as you can see on the right) or more recently by attaching radio transmitters. There are some species that use natural markings to identify individual animals, including work on whales, dolphins, and killer whales. The technique is also used for some amphibians where the underside of the animals is often mottled in unique ways. However, given the fact that the markings are not always present, that we don’t know how long they last, and that the method requires some very specific (and challenging!) photography, it is unlikely that this particular method will be used widely in insect ecology. Instead, the study highlights an interesting example of unexplained variation in dragonflies, which deserves more study in its own right.

References

*Schneider, B. and Wildermuth, H. (2009) Libellen als Individuen – zum Beispiel Aeshna cyanea (Odonata: Aeshnidae), Entomo Helvetica, 2: 185-199.

*Gould, S.J. and Lewontin, R.C. (1979) The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme” Proc. Roy. Soc. London B, 205: 581–598

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Do dragonflies give birth to live young?

Heliocypha perforataSomething strange seems to be happening in one particular species of damselfly, the common blue jewel Rhinocypha perforata (pictured right). Or at least it has been caught on video for the first time… Aside from being a particularly attractive species of damselfly found in China, Thailand, Laos, Malaysia and Vietnam, the common blue jewel seems to adopt a rather unusual form of reproduction (for an insect, at least). Read More »

How big is a damselfly, and why?

Calopteryx_maculata_mating_(crop)Background: Body size is among the most important characteristics of animals and plants. Larger animals are capable of buffering against their environment (think big polar bear vs tiny chihuahua in the snow!) so that they can survive in a wider range of locations, are capable of eating a wider range of prey, and consume more prey than smaller animals leading to a stronger impact on ecosystems. However, we are still trying to understand the factors that influence body size, both ecologically and evolutionarily.

What I did: A number of previous studies have compared body size in particular animals across different locations to see whether or not there are consistent patterns in that variability. I wanted to collect specimens of a single species (the ebony jewelwing damselfly, Calopteryx maculata) for analysis from across its entire range in North America, but the range is so large (Florida to Ontario, and New York to Nebraska) that I wouldn’t have been able to travel to sufficient sites within the one season that I have available.  Instead, I asked a lot of local dragonfly enthusiasts to catch and send me specimens from their local sites. I am extremely grateful to all of them for helping, as this could not have been done without their kind volunteering of time and energy.  I ended up with a substantial dataset of animals from 49 sites across the range.  I showed that there was a general increase in size further north, but that this was not a simple increase. Instead, there was a U-shaped relationship between latitude and size with larger animals in the south and the north with an intermediate size in the middle. When I looked at the drivers of this trend, it appeared that warm temperatures resulted in higher body sizes in the south. In the north, the animals use shortening days as a signal to accelerate their development and so in the most northern regions animals were developing very quickly despite the cold.

Importance: Large scale (across the whole of an animal’s range) measurements of body size are essential to provide an ecologically relevant test of explanations for changing body size. These findings support previous laboratory work which suggested a twinned role for temperature and photoperiod in driving development in damselflies.

This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “Time stress and temperature explain continental variation in damselfly body size”, was published in the journal Ecography in 2013. You can find this paper at the publisher’s website or for free at Figshare.

Image credit: Kevin Payravi, http://bit.ly/1q7B2Ph, CC BY-SA 3.0

Invasive damselflies get bigger as they move through the UK

Exif_JPEG_PICTUREBackground: A large number of species are expanding their ranges in response to climate change. This is also true in the damselflies, where the small red-eyed damselfly (Erythromma viridulum) has recently (around 1998) crossed the sea from France to England.  Since then, the species has moved hundreds of kilometres north in an unprecedented range expansion (at least as far as European dragonflies and damselflies are concerned). What is less clear is what impact this expansion has had on the species. Are the newly-founded populations the same as those that are resident in France? Can we trace the arrival and expansion of the species through genetic techniques?

What we did: Simon Keat was a PhD student at the University of Liverpool who was lucky enough to be just beginning his PhD when the small red-eyed damselfly first established in the UK.  Simon surveyed a number of populations around Europe and in the UK, collecting animals to measure them and extract DNA. With the body size measurements we showed that animals tend to show a strong relationship with latitude: populations further north were much larger and this held for both the older populations in France, Belgium and Germany as well as the newer populations in the UK. Looking at the genetics, we had expected to see declining genetic diversity further north, as a small number of individuals led the charge up the country. However, instead of a decline in diversity in the UK we saw an almost complete lack of genetic pattern.  This suggests that the animals were moving in such great numbers that there was not the time for any local patterns to develop.

Importance: Range expansions have important consequences for many aspects of human life: agricultural pests shift and threaten crops, diseases and their vectors shift and threaten human health, and endangered species shift and potentially move out of protected areas. We have shown that during this particular range expansion there has been negligible change in genetic structure but that newly-invaded areas contain relatively large damselflies.  Since damselflies are voracious predators, this could have substantial implications of local ecosystems.

This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “Bergmann’s rule is maintained during a rapid range expansion in a damselfly”, was published in the journal Global Change Biology in 2014. You can find this paper at the publisher or archived at figshare.

Image credit: Quartl, http://bit.ly/1uX3BOA, CC BY-SA 3.0.

Damselflies change the colour of their wings when other species are around

Calopteryx maculata M (Four Mile Creek)Background:  Animals and plants have a wide range of colours, and these different colours play different roles in different species. Some species might be signalling to potential predators that they are toxic (like a wasp’s stripes), others might be trying to hide (like a moth’s speckled grey wings), and others might be trying to signal to the opposite sex that they are high quality mates (like a peacock’s train). However, while there are clear functions in principle, the relative importance of different signals might vary depending on the context within which the animal or plant finds itself.  For example, male ebony jewelwing damselflies (Calopteryx maculata) have very dark wings and this is thought to allow females of the same species to choose appropriate mates (i.e. to avoid mating with the wrong species). However, the dark pigment can also play a role in temperature regulation. Damselflies cannot generate their own heat and so rely on absorbing heat from the sun, which is helped by the dark pigment. I was interested in how the darkness of the wings varied between locations which experience different temperatures.

What I did: I wanted to collect specimens of this species for analysis from across its entire range in North America, but the range is so large (Florida to Ontario, and New York to Nebraska) that I wouldn’t have been able to travel to sufficient sites within the one season that I have available.  Instead, I asked a lot of local dragonfly enthusiasts to catch and send me specimens from their local sites. I am extremely grateful to all of them for helping, as this could not have been done without their kind volunteering of time and energy.  I ended up with a substantial dataset of animals from 49 sites across the range. The wings of the animals were clipped from the bodies and scanned using a flatbed scanner, and then the amount of pigment was calculated from the image. I showed that the amount of pigment was pretty constant across the range apart from when the species was found with a similar species: the river jewelwing damselfly (Calopteryx aequabilis). This suggests that there might be an optimal level of pigmentation that is independent of temperature, but that if females start to struggle to identify males of their own species there might be an advantage to changing the levels of pigment.

Importance: There have been a lot of local experiments on the benefits and costs of pigment in animals (including damselflies) but there have been far fewer studies that have looked at large scale patterns in pigmentation.  These sorts of studies are essential to describe biological phenomena in the field and to reveal initial patterns in nature that might indicate interesting or novel evolutionary processes.

This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “Continental variation in wing pigmentation in Calopteryx damselflies is related to the presence of heterospecifics”, was published in the journal PeerJ in 2014. You can find this paper for free at the publisher.

Image credit: That’s one of mine!

Damselfly wings change shape in harsher habitats

Background: Odonata (dragonflies and damselflies) are an ancient order of insects. By this, I mean that they have remained largely unchanged since their ancestors evolved 500 million years ago.  They have a fairly unique flight style which is a product of the configuration and use of their wings. Wing length has been used as a measure of odonate body size for many years, but wing shape has received less attention.

What we did: I was interested in whether wing shape varied with latitude in the UK.  The populations living in habitat in the UK are exposed to a range of temperatures depending on location and it might be that certain wing shapes confer advantages in certain habitats. Based on a survey of seven populations of Coenagrion puella, I compared wing shape using a method called “geometric morphometrics”.  This allowed me to look at shape independently of the size of the wing.  I found that the wing shape in the majority of populations was very similar.  All populations in the south of England were comparable, but the populations in the south of Scotland showed a progressive shift away from this “typical” wing shape until a site near Edinburgh which was significantly (if subtly) different.

Importance: Wing shape has been highly conserved throughout odonate evolution (i.e. ancient odonates are similar in shape to present-day odonates). Because even small variations between species are consistent, wing shape and patterns of wing veins have been used to identify species. My study showed that these wing shapes were not as consistent as people had previously thought and that there might be ecological or evolutionary processes that can cause significant variation.


This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “Wings of Coenagrion puella vary in shape at the northern range margin (Odonata: Coenagrionidae)”, was published in the International Journal of Odonatology in 2008. You can find this paper online at the publisher, or on Figshare.

Image credit: Lauri, CC BY-NC-SA 2.0, http://bit.ly/1zicIZC

The impact of environmental warming on Odonata – a review [paper summary]

This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “The impact of environmental warming on Odonata – a review”, was published in the International Journal of Odonatology in 2012. You can find this paper online at the publisher, or on Figshare.

Background: Odonata (dragonflies and damselflies) are thought to have evolved in the tropics and possess a number of adaptations that allow them to exist at higher latitudes.  This makes them interesting to investigate in the context of climate change, since these adaptations might facilitate a response to increasing temperatures.

What we did: This paper is a review of the literature looking at the ecology and evolution of Odonata in the context of climate change.  A number of areas are discussed including distributional changes, phenological shifts, evolutionary responses, the effects of drought and the physiological effects of temperature.

Importance: A large amount of work has been carried out on the influence of temperature on the biology of Odonata over the past 50-60 years.  This has come from a variety of loosely-related fields and our review brings this together to provide an overview of the state-of-play concerning our understanding of the topic.


Image credit: Patricia H Schuette, CC BY-NC-ND 2.0, http://bit.ly/1BO5i4r