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).

img_0224_2
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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Dragonfly mind control?

For those of you not familiar with Cordyceps fungus, that’s the one that attacks insects (and other arthropods) by infecting and then spreading through the whole body. The result is something like what you see below:

Dragonfly with Cordyceps infection (Ophiocordyceps odonatae)

Each one of those little growths is a “fruiting body” and that is where the fungus releases its spores in order to found new patches of fungus. The most famous of these kinds of fungi is perhaps Ophiocordyceps unilateralis, which infects ants and influences their behaviour. The fungus forces an ant to climb a blade of grass or a twig and then attach there until it dies. Meanwhile the fungus produces a series of fruiting bodies that release spores from the new vantage point – the height helps those spores to disperse a greater distance. Apparently fossilised plants from 50m years ago also bear the marks of these Cordyceps-related attachments by insects, suggesting that this is an old battle.

What we don’t know is the extent to which Cordyceps influences the behaviour of other hosts. I posted the image above because it is the first time that I have seen a dragonfly infected in this way. It would make more sense (to me, at least!) for Cordyceps infecting a dragonfly to make it fly upwards while the fruiting bodies are releasing spores to broadcast those offspring as far as possible. However, the only image I have seen is this one where the animal is firmly rooted to the perch.

If it was a parasite that affected dragonfly flight then it wouldn’t be the first. A few recent studies (e.g. Suhonen et al. 2010) have suggested that dragonflies infected with parasitic mites that cling to the outside of the animal result in greater movement. It has been suggested that this could be an attempt to get out of an area with a high parasite population – after all, that’s not a great place to raise your little dragonfly family. However, we think this response has evolved to help the host and not the parasite, which is the opposite to the response elicited by the manipulative Cordyceps.

References

Suhonen, J., Honkavaara, J., Rantala, M.J. (2010) Activation of the immune system promotes insect dispersal in the wild, Oecologia, 162 (3): 541-547.


Image credit: Paul Bertner, https://flic.kr/p/qodUNR, all rights reserved, used with permission.

PhD funding for biological research at the University of Leeds

book-631748_1280If you are interested in doing a PhD but are struggling to find funding that fits your project or have been unsuccessful in applications to the funding schemes that are scattered around (e.g. the NERC DTP schemes that are interviewing at the moment) then don’t despair! There are always funny little pots of money that you can apply to.  The University of Leeds has three such scholarships available that can be used to fund PhD research in biological sciences (and some other areas). These all close on 1st June but if you are interested in applying please do get in touch with me (or one of my colleagues in the Ecology and Evolution Research Group) to discuss a potential project.  The sooner the better!Read More »

Is it a bird? Is it a plane? No, it’s a monk!

I am fascinated by the field of “biomimetics” – attempting to find solutions to problems by looking to the natural world. Sometimes this involves buildings that work like termite nests, swimsuits that use tiny hooks like those found on shark skin, or Velcro that uses the same principles as seed burrs as an inspiration. However, among the most celebrated examples of biomimetics are those involving flight.

Charles_Le_Brun_-_Daedalus_and_Icarus_-_WGA12535There have been a great many legends describing early attempts at flight, with perhaps the most famous being that of Daedalus and his son Icarus. Daedalus created the Labyrinth on Crete for King Minos and the king imprisoned Daedalus in a tower so that he could not spread the knowledge of labyrinth-building to other kingdoms. Daedalus escapes with Icarus, but Icarus flies too close to the sun causing the wax holding his feathers melts and he falls into the sea and drowns. Daedalus, meanwhile, reaches Sicily (750km away). Ovid’s description of the myth states that Daedalus “…flexed each [feather] into a gentle curve, so that they imitated real bird’s wings”, and so this is clearly a calculated (if legendary) attempt to mimic bird flight.Read More »

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.