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”.

This slideshow requires JavaScript.

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

Advertisements

To bee or not to bee – why some insects pretend to be dangerous

I just had my first article published at the Conversation – an excellent online collaboration between journalists and academics. As part of their publishing model, anybody can share any articles. So here’s mine!

To bee or not to bee – why some insects pretend to be dangerous

Christopher Hassall, University of Leeds

In the summer of 2011, panic gripped a small community in Gatineau, Quebec. Hundreds of small, striped insects were buzzing around a children’s playground. The playground was evacuated and entomologists were called in to establish whether or not the animals were dangerous. The answer was no, but it is easy to see why local residents were concerned. The animals that had taken up residence in the playground were hoverflies, a family of harmless fly species that have built up quite an arsenal of tricks to convince would-be predators that they are dangerous.

The panic that a swarm of hoverflies can generate belies the fact that they are immensely beneficial insects. Many of their larvae (the baby hoverflies that look like maggots) crawl around on plants feeding on the aphids that would otherwise eat our flowers and crops. Meanwhile the adults –- the stripy, flying insects that instil such terror –- spend their days pollinating flowers as they feed on nectar and pollen. But flying around in the open leaves hoverflies vulnerable to predators, a problem they have solved by evolving to resemble the stinging, pollinating insects such as bees and wasps with which they share the flowers.

Yet the story is not quite so simple. For every hoverfly that presents an exquisite example of mimicry (like the wonderful Spilomyia longicornis pictured above) there are several that really do not seem to be trying at all. Given that mimicry can obviously benefit hoverflies, why don’t they all evolve such excellent abilities?

Researchers found a potential solution to this Darwinian puzzle in 2012, when they looked into the characteristics of mimicking and non-mimicking hoverflies. You might expect that birds would prefer to eat larger species of hoverfly, since those hoverflies represent a bigger, more rewarding meal. Those larger species would therefore have more to gain from mimicry because they are under greater pressure from predators. Sure enough, it turns out that the colour patterns of the largest hoverflies (which are effectively flying buffets for birds) bear a close resemblance to the yellow, black, and white stripes of wasps and bees. The smallest species (which are barely worth chasing) do not show such similarity.

However, hoverflies have more than just wasp-like costumes. Some species also have considerable acting talents. It has been known for decades that certain hoverflies will pretend to sting when attacked, or hold their dark front legs in front of their heads to make it appear as though their antennae are long like those of wasps.

A recent extensive field survey showed that the species that behaved like wasps and bees were comparatively rare (just like the species that look like wasps and bees). This behavioural mimicry also tended to occur only in those species that already showed a strong visual resemblance to wasps and bees. In other words, those species that had the costumes also had the acting skills.

Insect sound bites

One of the most fascinating aspects of hoverfly mimicry has recently been dissected in great detail. As well as looking like wasps and bees, and acting like wasps and bees, some species also sound like wasps and bees. As part of our most recent project, my colleagues and I caught 172 insects from 13 species of hoverflies and nine species of wasps and bees, and brought them into a soundproofed recording studio. There, they recorded the sounds the insects made during regular flight and when the animal was attacked (simulated using a sharp poke with a pair of tweezers).

When they ran a statistical analysis on these sounds, the researchers found that some species of hoverfly make sounds when they are attacked that are indistinguishable from the high-pitched alarm buzz of bumble bees. The high-pitched buzz that bumble bees make seems to be produced by the bee unhooking its wings from the muscles that drive them, resulting in a completely different sound. This is a bit like what happens when you take your car out of gear and rev the engine – a lot of noise and you don’t go anywhere. It seems that hoverflies are capable of the same behaviour.

A board covered in ‘pastry prey’ used for experiments on wild bird predation.

But just because statistical analysis can’t tell the difference, that doesn’t mean natural predators can’t. To test for the benefits of this sound imitation in the wild, researchers presented pastry models of insects to wild birds with the different sounds. Pastry has approximately the same nutritional content as the insects that the birds forage on naturally, being part fat and part carbohydrate. The pastry can also be painstakingly painted to resemble insects as well, as in the photo to the right. To the surprise of the researchers, the birds only avoided the bee sounds. This was despite the fact that the hoverflies sounded identical to the computer-based analysis.

So we are left with a situation where an animal brain outperforms human researchers and their technical wizardry, which is not altogether surprising. Birds have evolved alongside a host of potential prey, developing the ability to find safe prey while avoiding animals that sting. While the hoverflies have a complex and fascinating suite of acting skills to dissuade would-be predators, they are still part of an evolutionary “arms race” where predators either keep up or starve.

The best part of this particular story is that it is possible to watch it unfold in your back garden. Next time you see or hear an animal that makes you reach instinctively for the rolled up newspaper, take a minute to check that it isn’t one of nature’s great actors.

The Conversation

Christopher Hassall, Lecturer in Animal Biology, University of Leeds

This article was originally published on The Conversation. Read the original article.

Damselflies change shape as they move north

Background: It has been proposed that animals and plants of the same species vary in their shape and size depending on where they live. Individuals living close to the cooler, northern range boundary might possess traits that increase their ability to deal with cooler temperatures, for example. However, under climate change the places where animals can live are expected to move as warmer temperatures expand the areas where climate is suitable for different species.

What we did: This study was part of my doctoral research and compared populations of three species between their range core and their range margins.  The three species varied in the degree to which they were expanding their ranges under climate change: Pyrrhosoma nymphula (the large red damselfly) is not expanding in the UK and is found all the way to the northern coast of Scotland, Erythromma najas (the red-eyed damselfly) is found as far north as Cheshire and is not expanding its range margin, and Calopteryx splendens (the banded demoiselle) is found as far north as Northumbria and is expanding rapidly.  The results showed that there was greater variation between the core and range margins in C. splendens, the species which was expanding, less difference in E. najas which is barely expanding, and almost no difference in P. nymphula, which has expanded its range as far as it can.

Importance: In order to respond to climate change, species will likely need to shift their geographical ranges.  This involves being able to colonise new habitats which are currently outside of their range. The detection of variation in morphology such as in this study suggests that there might be traits that would facilitate this colonisation at range margins. If it could be demonstrated that the variation in morphology was evolutionary and not the result of phenotypic plasticity, then this would provide important evidence of adaptation to coping with climate change.


This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “Variation in morphology between core and marginal populations of three British damselflies”, was published in the journal Aquatic Insects in 2009. You can find this paper online at the publisher, or on Figshare.

Image credit: Jean-Daniel Echenard, CC BY-ND 2.0, http://bit.ly/1AHimY5

Blood-sucking mites are worse in mid-summer for damselflies

Background: Parasites drain resources from their hosts in order to survive and reproduce.  The effects that this has on the host have been shown to be substantial in some species of dragonfly and damselfly. However, in order to assess how serious these effects are, we need to know something about patterns of parasitism: how many parasites does an animal carry and how does that number vary throughout the year?

What we did: We had a two year study looking at a single population of the azure damselfly, Coenagrion puella, at a single site in southern England.  All the damselflies (1036 in total) emerging from the pond were caught, marked individually, and the number of parasitic mites that were clinging to them were counted. Technically these mites don’t suck blood, but they do feed on the “haemolymph” of the insects, which is the insect equivalent.  We had a number of hypotheses as to what might drive variations in parasitism: higher temperatures might increase the effectiveness of mites at finding and latching-on to hosts, larger animals might have more parasites, or there might be a difference between sexes in parasitism. We found that most of the variation in parasitism was related to the animals emerging in the middle of the season having the most parasites, while animals emerging early or late had fewer parasites.

Importance: The seasonal pattern suggests that variation in parasitism is the result of ecological interactions where parasites have evolved to take advantage of their hosts’ patterns of development. Given that dragonflies and damselflies have been shown to be emerging at different times in response to climate change, it remains to be seen whether mites will be able to track these changes.


This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “Phenology determines seasonal variation in ectoparasite loads in a natural insect population”, was published in the journal Ecological Entomology in 2010. You can find this paper online at the publisher, or on Figshare.

Image credit: Brad Smith, CC BY-NC 2.0, http://bit.ly/1q6YTeA

Damselfly sex doesn’t always produce children, and that’s a problem for evolutionary biologists!

Background:  At the core of ecology and evolutionary biology is the concept of “fitness”, broadly defined as the number of copies of an animal’s genes it manages to leave in subsequent generations. However, biologist rarely measure this genetic fitness.  Instead, we use proxies such as the number of times an animal mated or the number of eggs an animal laid. Sometimes, we use proxies that are even further removed, such as body size (under the assumption that larger females lay more eggs).

What we did: This study compared two traditional forms of fitness measurement, daily mating rate and lifetime mating success, with a genetic measure of fitness based on finding the number of offspring each individual produced in the next generation.  We monitored a single, isolated pond over two years and individually identified all damselflies of the species Coenagrion puella, the azure damselfly.  Each individual also had a genetic sample taken and we used genetic markers called “microsatellites” to identify each individual.  When we came back the next year, we did the same thing.  This species goes through one generation per year so we knew that all the animals in the second year were the offspring of those in the first.  By comparing the genetics of the potential parents with those of the potential offspring we were able to assign offspring to parents to produce a much more accurate picture of this concept of “fitness”.  Unfortunately, what we found was that our behavioural measurements did not reflect this more accurate measure of fitness.

Importance: Since the concept of fitness is so important to evolutionary biology, it is important to test the assumptions of the studies that have sought to measure it.  We have demonstrated that some of those previous studies were not using particularly reliable proxies for fitness.  However, we have provided a case study of a potential method for avoiding these problems: by directly genotyping and assigning parents to offspring in the field we can get a much clearer picture of what “fitness” really means.


This is part of a series of short lay summaries that describe the technical publications I have authored.  This paper, entitled “Field estimates of reproductive success in a model insect: behavioural surrogates are poor predictors of fitness”, was published in the journal Ecology Letters in 2011. You can find this paper online at the publisher, or on Figshare.

Image credit: One of mine, CC-BY 3.0

The first female entomologist: Maria Sibylla Merian

Maria Sibylla Merian (1647-1717)

I recently visited Amsterdam, where I came across the work of a German naturalist of whom I had not previously been aware.  The Rijksmuseum contains a book that dates back to 1730 and was written by (according to the museum plaque) the “first female entomologist”, Maria Sibylla Merian (1647-1717). The book, entitled De Europische Insecten (available online through the Biodiversity Heritage Library) contains hundreds of illustrations of species made by the author (who also happened to be an extremely talented artist).  You can see some of the detailed illustrations from the book at the Sotheby’s auction page for a copy that is for sale (at £25,000-30,000 it’s a bit out of my price range…) and an example of a page below.

A plate from De Europische Insecten

Merian’s story is an interesting one.  Born into a famous publishing family, her father passed away when she was three years old.  Her mother later married an artist, thus combining the literary and artistic aspects of Merian’s upbringing that would determine her career.  She began at the age of 13 by drawing and painting the silk worms that she caught around her home in Frankfurt.  As a young female artist, she was a popular tutor for the daughters of local wealthy families and this allowed her to both earn a good living and gain access to influential people (and their extensive gardens with all those wonderful insects!).  It was as a result of watching the development of caterpillars into butterflies that she became interested in metamorphosis, and this eventually led to her publication of Metamorphosis insectorum Surinamensium (also available online in its entirety).  This volume, drafted by Merian after a two year visit to the Dutch colony of Surinam in South America (which was cut short after she caught malaria), provided European scientists with some of the first full-colour images of the South American flora and fauna.  Merian undertook that trip at the age of 52 with her daughter, Dorothea Maria, and documented many new species of Lepidoptera, including all stages of the life cycle and the host plant on which the caterpillar lives – a wonderful resource for naturalists back home.  During her visit to Surinam, Merian spoke out against the mistreatment of slaves by Dutch plantation owners and took note of the names that indigenous peoples gave to the species she encountered.

Merian’s work was extremely valuable to Carl Linnaeus, who published in 1735 his Systema Naturae (also available online, but nowhere near as aesthetically appealing at Merian’s work) which laid-out the biological nomenclature that we use today.  In particular, the focus on metamorphosis has led to her being listed among the most influential entomologists of all time.  Merian was honoured with a Google Doodle to commemorate what would have been her 366th birthday on 3rd April 2013.  However, despite all this there is a pretty good chance that Merian died penniless in 1717 a few years after suffering a stroke.  But that is all the more reason to appreciate her work today, which is still among some of the most-highly valued and collectable natural history artwork in the world.

2013-05-03 16.15.00
The copy of De Europische Insecten in the Rijksmuseum, Amsterdam

Beetles on flowers

Image

I took this photo while I was teaching on a field course in Spain earlier this month (harder work than it sounds).  It was a nice opportunity to try out my camera (which I have been trying and failing to do on this blog) as spring is in full swing over there.  I was amazed by the diversity of animals that I only found on flowers (although part of that might have been that the flower-dwellers were more noticeable…), but I was surprised to see what look like two different life history stages of potentially the same species on a single flower.  Does anybody know what this beetle is…?

UPDATE 21/7/13: My friend Patrick suggests that it could be Cryptocephalus rugicollis.  Looks like a good call to me!