A flower-loving gutter fly

Not five feet from an Indian Almond (Terminalia catappa) tree abuzz with insects is an open gutter. The sewer runs along the corners of houses, its soiled waters shadowed by an avenue of jamun (Syzygium cumini), kadam (Neolamarckia cadamba), and Indian almond trees. The almond and jamun trees are blossoming, their pale, snowy-flowers arranged as a whorl around slender, soft-green branchlets stick out from under a flush of broad dark-green leaves, liberating a strong sweetish aroma into the heavy summer air. This is in the middle of the city. Every time I enter or leave my office, I hear the trees abuzz with insects. Sharp, short buzz of insects hopping from one flower to another, lapping up the extremely sweet nectar contained in bowl-like flowers.

The gutter is riddled with tubiflex worms. When I was a kid, my father would purchase live tubifex worms from aquarium stores for his fishes, and occasionally weird worms would come with it. The one I have a distinct memory of was the rat-tailed maggot. Aptly named for what it resembles and what it is, they thrive in organic-rich gutters that run along every road or house in a central Indian town. Here they feed on the organic matter in the water, and use their rat-like tail, which is a siphon, to breathe from the surface in otherwise hypoxic waters.
A fly among bees, you say? An Eristalinus fly nonchalantly visited Tridax procumbens while we were watching and discussing bees on a walk around the IISc campus.

It took me nearly two decades to see the connection between the gutter-dwelling maggots and the flower-loving flies. They are one and the same: flies in the family Syrphidae. These Eristaline flies (of the subfamily Eristalinae) thrive in waters – generally flowing – that are rich in organic content, natural, such as water sepages in marshes, as well as sewers.

And here is a conundrum.

On the one hand, we have a garbage-eating maggot that symbolizes a polluted environment, and on the other a fly which visits flowers and often aids in pollination. This ‘flies’ conundrum, as I call it, signifies a basic bias in our understanding of this diverse group of insects. A little search of terms on Google Scholar makes this bias clear. Searching for terms “Diptera, loss, pest” yields 61,500 results and “Diptera, benefits, ecosystem services” yields 22,300 results (depends on where you search from?) – but anything in the latter category, such as “Diptera, benefits, pollination/pollinator” gives below 20,000 results. I am not even going to get into the economic losses versus benefits in economic terms flies provide – for the former the numbers go in millions per year, and the latter is an unknown dimension.
The 'flies' conundrum, illustrated.

The major reason for this is that we have been focusing only on the ecological functions that flies play in an ecosystem that affect us, as pests on our crops, parasites on our livestock, and vectors of diseases in humans.

There is only one colony of Apis cerana close to my workplace, on a fragile peepul (Ficus religiosa) tree that arches over the main city road. So far, I haven’t seen them visit jamun, kadam, and Indian almond trees, and I wonder why. But this niche appears to have been filled by the flower-loving gutter flies that tolerate the pollution of the city.

This brings me to the second form of the conundrum: What we often see is what we believe is important. In the case of Eristalinus macrocephalus, among the most common hoverflies of India, the adults are recognized for their flower-visiting habit. To watch this hoverfly dusted by pollen brush it off from its large-eyed head fills me with joy. Its larval food – the gutters, however, distances me from it. In other words, this species symbolizes another plague in our argument: what we study is what we think is important. We place it at the top of everything else. Tigers on the top, elephants on the top, birds on the top, snakes and frogs on the top, bees on the top.
Another Eristalinus fly visiting Dipcadi saxorum, a rather uncommon ephemeral of the northern plateaus of Western Ghats.

We make arguments for the sake of protecting our vested interest in what we think is important. In case of this fly, we may try to exterminate its larva, but at what cost to the ecosystem, we may fail to answer. Bees generally do not function well under the blaring 45°C of central Indian cities. In such instances, it is flies that take up the task. These Eristalinus flies, fortunately, are local – they breed under the shade of the avenue trees which need to be pollinated by insects in order to fruit. 

Temperatures at micro-level are generally favourable, for instance, if the ambient temperature is 45°C, the gutters and the shade under the tree, even its leaf surface or on the underside, are at much lower temperatures – this shelters locally bred insects which purely feed on nectar and pollen for nutrition and in turn pollinate.

To view ecosystem services such as pollination as an ecological function undertaken “by many” is more relevant in today’s world than ever before. Bees are important pollinators, but an ecosystem will be able to maintain itself so long as it is resilient. In case of a fall in bee populations, we should be aware of other pollinators which also carry out the functions. For this, a broader approach to studying pollination is required, that of co-pollinators. It is not only honeybees that add resilience to an ecosystem, it is a community of co-pollinators, that increase the resilience.

Like the fly born in human sewage pollinating the Indian Almond that is now ripening, we must metamorphose from a hole-in-the-wall view of what we study, embed our subject into the scheme of nature – not make it ascend above it, or let it sink far below it – but to see it as a part of it, one among the many, many among one.
A talk on flies. What could be better. Another Eristaline fly, Phytomia errans, checks Lantana flowers to sip from.

This, and fly taxonomy (based on morphology), was the topic of my talk at IISc Bengaluru for the workshop “Climate change and the essential service of pollination”, organised by CES between 17 and 19 July, 2019. I will try to make the presentation available online once I have sorted the copyright permissions (up on Google Drive now).

The workshop was a fascinating and informative gathering of eminent scientists and ecologists who focused on non-honeybee pollinators, such as solitary bees, nocturnal pollinator species of bees and moths, on the art of studying pollen grains in a lab, their germination (artificial and on the stigma – the female receptacle of the male pollen), the different techniques of measuring frequency of flower visits, pollen transfer study (that is, extracting the flower to check if it has been pollinated), and finally the many problems that we are to face with the warming climate. There are studies that show that rise in temperatures affect the physiology of many insects – at separate stages of their lives, and this not only endangers the pollinators, but also plants which rely on insect pollinators and finally us, humans, whose yields of many fruits are because of the ecosystem services of many bees and flies and other pollinators.
A Phthiria sp. of Bombyliidae takes a break from sipping nectar. This is but one tiny flower-visitor that may also
aid in pollination that we don't know of. Only one species recorded in India so far.

What we need in the beginning is an atlas of flower visitors/pollinators of India from all taxa of pollinating animals, not only insects, and assess their distributions and populations along with the plants, and make this a baseline for any all-encompassing conservation initiatives from protection of pockets of forests to restoration of degraded areas.

Here's the talk on Google Drive! It can be downloaded.


  1. Hey Aniruddha, I enjoyed reading your blogpost above. I recently discovered gutter flies (or hover flies) visit Indian Almond trees at our community, Rustomjee Urbania at Thane. And I also found that blue bottle flies and house flies also pollinate the Indian Almond Flowers. I agree with you. We should also pay attention to non-honey-bee pollinators and focus more on community of pollinators and not just one insect like the honeybee.

    1. Thank you for reading! Definitely, pollinators should be looked at as a whole community - the more diverse they are, the more resilient to changes. Yes, certain blue bottle flies and house flies are opportunistic - their generalistic feeding habits help them survive on literally everything.


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