The long-legged drifter

If you have been walking through the woods lately, you may have noticed clouds of jet-black, shiny flies with dangling long legs, drifting slowly upwards and downwards, writes Athayde Tonhasca.  These are St Mark’s flies.

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A St Mark’s fly (Bibio marci) © James K. Lindsey, Wikipedia Creative Commons

This fly is so-named because it appears around 25 April, which is St Mark’s day. Swarms of St Mark’s flies are found throughout the UK on woodland edges, grassland and wetlands. The adults feed on nectar, but they don’t live very long: only about two weeks. After mating, females lay their eggs in the soil and die soon afterwards. Their larvae feed on dead vegetation and living plant roots through the autumn and winter. They are important recycling agents, accelerating the decomposition of organic matter.

As is the case for many of our insects, we don’t know much about the ecology of St Mark’s flies. They are regular flower visitors, and the density of hairs on their bodies suggests they do pollinate plants.


A hairy St Mark’s fly male. © Rolf Dietrich Brecher, Wikipedia Creative Commons

Hairiness is related to pollen load, which is the quantity of free pollen that is carried on the body of an insect and available for transfer to flowers. The pollen load of mining bees, bumble bees and honey bees visiting rapeseed fields range from around 1,900 to 5,800 grains – it sounds like a lot, but a single rapeseed flower can produce over 100,000 pollen grains.

Bibionidae flies, the group St Mark’s flies belong to, carry a load of around 900 grains of rapeseed pollen. This is a modest contribution when compared to bees and other flies, but it is far from negligible. Considering that massive numbers of these flies visit rapeseed as well as fruit crops, each one transporting a few grains of pollen, their contribution to pollination may be substantial. The true impact of the ephemeral and sluggish St Mark’s fly remains to be discovered.

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St. Mark’s fly feeding © Jeanette Hall

The elusive pollinator

In 1862, James Bateman, a well-known British orchid grower, sent Charles Darwin a box of orchids from Madagascar. Among the exotic and expensive plants, there was Angraecum sesquipedale, with its beautiful star-shaped flower. But Darwin’s attention was drawn to its spur, a tubular projection where nectar is produced and stored. The origin of the plant’s Latin name explains Darwin’s amazement: sesqui, “one and half times”, and pedalis, “measuring a foot”. So we may forgive the taxonomist who described the species for being sesquipedalian, i.e., fond of long words.

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Angraecum sesquipedale, lithograph by W. Fitch, c. 1862 © Wellcome Image, Wikipedia Creative Commons.

Darwin wrote to his friend Joseph Hooker, director of the Royal Botanic Gardens: “Bateman has just sent me a lot of orchids with the Angraecum sesquipedale: do you know its marvellous nectary 11½ inches [29.2 cm] long, with nectar only at the extremity. What a proboscis the moth that sucks it, must have! It is a very pretty case.”

In those lines, Darwin’s predicted the existence of a moth in Madagascar with a proboscis (a feeding tube) long enough to reach the nectar at the end of the orchid’s spur. His hypothesis was based on his understanding of the evolution and ecology of orchids and insects. More importantly, in a later book on orchid pollination, Darwin suggested that the lengths of the flower’s spur and the moth’s proboscis evolved together. Co-evolution is well known and documented today, but it was a novel concept during Darwin’s time and one of his greatest contributions to evolutionary biology.

The idea of a moth with a 30 cm proboscis sounded far-fetched to some until 1907, more than 20 years after Darwin’s death, when Xanthopan morganii was found in Madagascar. This moth, known previously from the Congo region, has an impressive proboscis which is kept coiled like a fire hose when not in use.

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Xanthopan morganii © Esculapio, Wikipedia Creative Commons.

A possible match had been found, but it wasn’t sufficient to corroborate Darwin. Then in 1992, 130 years after his prediction, X. morganii was seen hovering over A. sesquipedale, which is known today as Darwin’s orchid or the Christmas orchid. And in 2004, evidence of the moth feeding on the flower was registered for the first time in this captivating video.

Here in the UK and other temperate regions, moths contribute to the pollination of non-crop plants, which are crucial for maintaining the biodiversity of the wider countryside. They are particularly important for pollinating some orchids and plants in the family Caryophyllaceae (carnations and campions). However, the role of moths as pollinators is poorly understood. This should surprise no one, as it is not easy to study animals at night. As nocturnal moths comprise around 90% of all Lepidoptera species, there is much to learn about these secretive flower visitors.

For another interesting example of orchid pollination see The trickster.

By Athayde Tonhasca

Which is which ?

Some sayings stick in your mind. When I was a schoolboy, studying for my geography ‘O’ level, I struggled to remember in which order the Great Lakes of North America ran. This was important because I was pretty sure I would have to name them in the forthcoming exam.  I needn’t have worried. My geography teacher had an easy solution. “All you need to do is recite this little saying ‘Some men hate eating onions’”, which he explained, if you take the first letters, gives you Superior, Michigan, Huron, Erie and Ontario running west to east.

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Blackthorn blossom

If, like me, you wonder about the difference between hawthorn and blackthorn then there is an equally easy way to remember which is which. Blackthorn blossoms before its leaves show, whereas hawthorn flowers after its leaves have opened.


Blackthorn - 1Both are great sources of food for pollinators. Blackthorn generally flowers first, and is a welcome and wonderful early source of nectar and pollen for emerging bumblebees. Solitary bees and honey bees will join the feasting too and, with hawthorn not too far behind in the calendar, these popular hedgerow shrubs serve our pollinators particularly well.

Hedgerows can offer more than food for pollinators, their root systems are popular nesting sites for mice, and abandoned rodent holes and crevices between the roots can be appealing sites for bumblebees to set up home.

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A hawthorn bush on the banks of the River Tay

Of course, blackthorn and hawthorn are good for nature in general. They provide excellent safe nesting sites for a variety of birds and act as corridors which small mammals and birds can use to move cautiously across the countryside. Pollination transforms the flowers into fruit and this in turn is a valuable autumn food source for many birds. Humans too have exploited these shrubs. The sloes (‘sloeberries’) of the blackthorn are synonymous with gins and jams, whilst the hawthorn was traditionally the way in which many fields were separated.

Autumn will see blackthorn and hawthorn fruiting. Their berries are distinctively different. Alas I don’t have an equivalent of the neat ‘Some men hate eating onions’ to give you, but given that the hawthorn berries are bright red, unlike the bluey-black sloe fruits, I don’t suspect there is quite the same need.

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Hawthorn blossom

Brooms, baskets and happy accidents

When we watch a bee heavy with pollen lumbering away from a flower, we may be tempted to think it is purposely doing the plant a favour. But there is nothing altruistic here; the bee will try to keep for itself as many pollen grains as possible to provide the proteins that are essential for the development of its eggs and larvae, writes Athayde Tonhasca.

Bees may take 95 to 99% of pollen back to their nests, leaving the remainder – unintentionally – for pollination. But in this liaison it is tit for tat: plants have developed adaptations to minimise pollen harvesting, such as inconspicuous anthers, narrow floral tubes, difficult flower structures, or progressive pollen release to force pollinators to make repeated visits. Some plants like orchids also cheat by attracting pollinators with scent but not giving any nectar or pollen in return (see The Trickster).

Rather than collaborating, then, bees and flowers are taking advantage of each other. Granted, this mutual exploitation has been fine-tuned by natural selection in order to avoid disastrous imbalances: overly rapacious bees and pollen-stingy plants would just collapse the pollination relationship.

Right now, chocolate mining bees (Andrena scotica) are well into the business of collecting pollen from hawthorns, blackthorns, willows, dandelions, brambles, fruit trees, and many other spring-blossoming plants throughout UK – see solitary success story. The red mason bee (Osmia bicornis) will soon be out in number too, pollinating apple trees and several other plants. Follow this link for more on the red mason bee.

These bees take away pollen attached to their scopa (Latin for “broom”) which is an area of dense, stiff hairs used specifically for this purpose. The scopa of the chocolate mining bee is on its hind legs, and in the case of the red mason bee, on the underside of the abdomen.


A mining bee scopa ©Allan Smith-Pardo, Bees of the United States, USDA APHIS PPQ,, Wikipedia Creative Commons.


A chocolate mining bee with pollen attached to its legs © Athayde Tonhasca.


A red mason bee loaded with pollen in its abdominal scopa ©Jeremy Early, Nature Imaging Conservation,

Other bees such as the honey bee and bumble bees carry almost all their pollen in their corbiculae or pollen baskets. From the Latin diminutive of corbis (basket), the corbicula is a shallow leg cavity surrounded by a fringe of hairs (a modified scopa). These bees, not surprisingly called corbiculate bees, moisten the pollen with regurgitated nectar and saliva, so that it can be conveniently packed for transport and easily unloaded once the bee reaches the nest.Corbiculate bees are also good at self-grooming to remove pollen attached to their hairs.


The hind leg of a honey bee. © Gilles San Martin, Wikipedia Creative Commons.

Carrying pollen in the corbiculae or the scopa makes a world of difference for pollination. Pollen tightly packed in the corbiculae is not easily stripped off by floral structures when the bee visits another plant, and it quickly loses its reproductive viability because it has been wetted. Pollen attached to the scopa on the other hand is dry and loosely attached to the bee, so it has a much greater probability of being dislodged and resulting in plant fertilisation.


Bumblebee with a loaded pollen basket © Beatriz Moisset, Wikipedia Creative Commons.

So paradoxically, bees that are very good pollen harvesters are not necessarily the best for plants, although they may compensate their over-tidiness with their large numbers and high frequency of flower visitation. Bees that are sloppy with their pollen such as the chocolate mining bee and the mason bee are more prone to the happy accident of pollination. It is not surprising then that we keep finding evidence of the importance of solitary bees for the pollination of crops and wildflowers.


Garnock’s Buzzing Wobbly Apple Experiment

I’m Lorna Cole an ecologist and Wildlife and Conservation Management lecturer at SRUC. Like many parents, I have suddenly found myself plunged into unchartered territory – working from home whilst trying to home-school two teenagers. All this whilst coping with the global pandemic that surrounds us all. Clearly as an ecologist my potential to help fight ‘the virus’ is limited … let’s say non-existent. This made me think – what can I do to help others? 


Eventually I came up with the idea of bringing an experiment on apple pollination to the hordes of bored children (and adults!) stuck in the house. This experiment, originally designed by Koos Biesmeijer from Naturalis, explores how insect pollinators influence the quality and taste of apples. This experiment gives us huge potential to educate youngsters of the value of insect pollinators as well as generate data on apple pollination from across the globe.

In the UK apples are an important crop with production contributing approximately £100 million annually to our economy (Vanbergen et al. 2014). While some plants can pollinate themselves, apples rely heavily on cross pollination needing insects to transfer pollen from one tree to another. Bumblebees, honeybees, solitary bees and hoverflies all visit apple blossoms increasing both yield and quality (Garratt et al. 2014). The diverse range of insects that pollinate apples contribute over £88 million annually to apple production (Vanbergen et al. 2014).

When an insect successfully pollinates apple blossom, a seed is produced. The number of seeds in an apple therefore provides a measure of pollination services. Apple trees that are not adequately pollinated (i.e. experiencing a pollination deficit) produce fewer apples and fruit that is smaller and misshapen (Garratt et al. 2014). There is even evidence that well pollinated apples are sweeter!

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With insect pollinators struggling to survive in many parts of the world, concerns are growing that pollinator declines could influence food security.  Using seed count as an indicator of pollination services, we are hoping our experiment will shed light on pollination deficits in apples and how these vary across the world. The experiment also aims to determine how pollination influences the quality of fruit – which is why we measure how wobbly the apple is and how good it tastes. By using a citizen science approach, we are hoping to be able to get a vast amount of data – without a huge research budget!

Managing orchards in a pollinator-friendly way is likely to result in higher apple yields and better-quality fruit that achieves a higher market value. Thus providing habitat and forage for pollinators is likely to benefit apple growers and biodiversity. Apple trees tend to flower in spring and their blossom provide important early season forage. However, with blossoms quickly disappearing, orchards can lack forage in summer and autumn. To combat this growers should consider how they increase forage to sustain pollinator assemblages throughout the season. This may be through including later flowering crops such as raspberries, planting flower rich areas or simply reducing mowing to encourage wildflowers to bloom.

The work is being carried out by SRUC as part of the Garnock’s Buzzing project. Garnock’s Buzzing is one of 25 projects being undertaken by Garnock Connections, a landscape partnership funded by the National Lottery Heritage Fund.

Watch the video @


The Pollination Party Poopers

This rarely seen lichen crab spider, aka running crab spider (Philodromus margaritatus), was spotted recently on a bench near Loch Ness. It lives on trunks of trees or even electricity pylons covered with lichens. It has also been found on garden apple trees. If you can imagine it standing on a patch of lichen, it will be obvious why it is rarely seen: it would almost disappear.


Philodromus margaritatus © Rachel Taylor.

While running crab spiders are relatively rare, “true” crab spiders or flower spiders (Family Thomisidae) are common garden inhabitants. Many of them show some degree of crypsis, which is the ability to avoid detection by other animals by blending in with their environment (whereas mimicry is disguising by resemblance to another organism). We can just say that running crab spiders and crab spiders are very good at camouflage.

You may wonder what spiders are doing in a blog about pollinators. Well, they and other predators are an important and often overlooked aspect of pollination ecology. Flowers are predictable food sources for pollinators, and so too for predators such as wasps, bugs and spiders. So many unsuspecting flower visitors are seized by opportunistic ambush predators, who sit and wait for lunch to fly in.

Interestingly, flies are less susceptible to spider predation than bees, possibly because they have better vision and can avoid or dodge attackers. Bumble bees are also less likely to become prey than solitary bees and honey bees, just because they are larger and bulkier, so more difficult to capture. It has been suggested that the long proboscis and the swing-hovering flying pattern of some moths have evolved as predator avoidance mechanisms: the further from the flower and less static, the better chance of escaping a lurking predator.


A crab spider (Thomisus onustus) capturing a bee © Alves Gaspar, Wikipedia Creative Commons.

But it’s not only through killing that predators disrupt pollination: in some cases, their mere presence scares pollinators away. There are fewer flower visitations and the time pollinators spend on flowers is reduced. As a result, pollination rates and therefore seed production may be lower.

Predation has enormous influence on the balance of ecological communities. It is beneficial to biodiversity by preventing a few species from taking over, and it helps stabilize ecosystems by keeping prey abundance in check. It has been estimated that the global spider community kills the equivalent of 400 to 800 million metric tons of prey annually. More than 90% of this biomass comprises insects and springtails, including a vast number of agricultural pests. For comparison, the annual food consumption of all the world’s seabirds is estimated at 70 million tons.

We don’t have a clear understanding of the impact of spiders on pollinators and plants. But it is quite possible that the characteristics of our gardens, crops and wider flora are to some degree shaped by these itsy bitsy predators.