Readers’ wildlife photos

Today we have a photo-and-text submission from Athayde Tonhasca Júnior on fly migration. It’s a subject dear to my heart as I used to work on it, publishing three papers on migration in Drosophila.  Athayde’s subject, though, is hoverflies, not fruit flies. His captions are indented, and you can enlarge his photos by clicking on them. Note: I changed Athayde’s words “hover flies” to the more common usage “hoverflies,” but Athayde notes that most entomologists use the two-word rather than one-word description.

On the road again, goin’ places that I’ve never been

Sometime between 1400 and 1200 BC, Yahweh (aka God) decided it was time to nudge the Egyptians to let their captive Israelites go. Yahweh could have tried diplomacy, but in his infinite wisdom he concluded that “The Egyptians shall know that I am the LORD”. And there was no better way to let the Pharaoh and his people know who the bigwig was around there than by punishing them with a series of plagues. Of the ten celestial disasters inflicted upon the Egyptians, two involved mosquitoes (or midges) and flies, which probably were also the agents behind another two plagues manifested as infectious diseases of people and livestock. Yahweh understood very well the efficacy of some flies (order Diptera) and pathogens to wreck revenge – after all, he created them.

Fig 1. The Third Plague of Egypt, by William de Brailes, circa 1250. Aaron strikes his rod on the ground, transforming dust into gnats (kinnim in Hebrew). In the King James version of the Bible, lice are the culprits, but today most scholars accept that kinnim should be translated as ‘gnats’ or ‘mosquitoes’ © Jan Luyken, 1712, Wikimedia Commons:

The tales of pestilent flies depicted in the book of Exodus could have been inspired by real events, as pest infestations and epidemics were recurrent in the ancient world. Fly outbreaks are facilitated by these insects’ capability to disperse for long distances and arrive at new locations suddenly and in massive numbers. There are no better examples of these efficient colonisers than hoverflies or syrphid flies (family Syrphidae) such as the marmalade (Episyrphus balteatus) and the migrant (Eupeodes corollae) hoverflies. Each autumn, they leave Britain and head south to spend the winter in southern Europe and the Mediterranean. Their offspring move northwards in the spring, lay eggs, and the new generation sets out on the cycle again. Researchers have estimated that up to four billion marmalade and migrant hoverflies cross the English Channel to and from Great Britain every year. This represents 80 tons of biomass. If you are impressed by these figures, you should know that hoverflies account for a fraction of insects’ latitudinal migrations known as ‘bioflows’: about 3.5 trillion insects, or 3200 tons of biomass, migrate into southern Britain annually (Wotton et al., 2019). Insect bioflows pour vast amounts of nutrients (particularly nitrogen and phosphorus) and countless prey, predators, parasites and herbivores into ecosystems, but we have only a vague understanding of their impact on food webs and local species.

Fig 2. A female marmalade hoverfly, a long distance frequent flier © Guido Gerding, Wikimedia Commons:

These hardy wanderers have another particularity of significant ecological importance: they transport pollen grains.

Most flies have no pollen-collecting structures and have few ‘hairs’ (setae), which are important pollen gatherers. These are negative marks for candidates to the pollinators’ club, but some flies compensate their shortcomings by their massive numbers. Each marmalade and migrant hoverfly carries an average of 10 pollen grains from up to three plant species on their journey into Britain. That’s paltry compared to a bee, but altogether, those flies bring in 3 to 8 billion pollen grains on each inward journey.

Pollen importation via flies is a recurrent phenomenon. In Cyprus, warm temperatures and favourable winds bring millions of insect migrants from the Middle East region, more than 100 km to the east. Flies make up nearly 90% of these bioflows, and many of them are loaded with pollen (Hawkes et al., 2022).

Fig 3. A common drone fly (Eristalis tenax) (A) and a blowfly (Calliphora sp.) (B) with orchid pollinia attached to their heads after a > 100-km sea crossing to Cyprus © Hawkes et al., 2022:

Pollen-loaded flies can turn up anywhere the wind takes them, even to specks of dry ground in the middle of nowhere. Over a two-month period, 121 marmaladehover flies reached a North Sea oil rig approximately 200 km from Aberdeen, UK. Over 90% of these flies had pollen attached to them, sometimes from eight plant species. Based on pollen barcoding and wind trajectory modelling, it was estimated that these flies traversed from 265 to 500 km of open water in a single journey, probably from the Netherlands, Germany and Denmark (Doyle et al., 2025).

Fig 4. (a) Location of an oil rig visited by hoverflies (b), and its aerial view © Doyle et al., 2025:

Flies’ long-distance pollen transfers may help connect isolated plant populations, such as in fragmented habitats, but we don’t know much about the ecological implications. However we do know that their contribution can be important. In continental Europe, wild carrot (Daucus carota) depends on a range of insects for pollinators, especially bees. But bees are absent from La Foradada, a 1,6 ha Mediterranean islet about 50 km off the Spanish coast. In this solitary spot of land, D. carota subsp. commutatus relies on the accidental arrival of common drone flies for its pollination (Pérez-Bañón et al., 2007).

Fi 5. La Foradada, devoid of bees and humans, is visited by pollinating drone flies © JavierValencia2005  Wikimedia Commons:

Butterflies, bumble bees, moths and dragonflies are known travellers, but we know much less about migrant flies, which may have significant roles in pollination ecology. We just have to pay more attention to these unpretentious pilgrims.

References

Doyle, T.D. et al. 2025. Long-range pollen transport across the North Sea: Insights from migratory hoverflies landing on a remote oil rig. Journal of Animal Ecology 94: 2267–2281.
Hawkes, W.S.L. et al. 2022. Huge spring migrations of insects from the Middle East to Europe: quantifying the migratory assemblage and ecosystem services. Ecography e06288.
Pérez-Bañón, C. et al., 2007. Pollination in small islands by occasional visitors: The case of Daucus carota subsp. commutatus (Apiaceae) in the Columbretes archipelago, Spain. Plant Ecology 192: 133-151.
Wotton, K.R. et al. 2019. Mass seasonal migrations of hoverflies provide extensive pollination and crop protection services. Current Biology 29: 2167–2173.