Three Flies Over a Flower - Observation of the Week, 1/30/24

Our  Observation of the Week is this trio of Orange-spined Drone Flies (Eristalis nemorum, Eristale interrompue in French), seen in France by @nicolashelitas!

Nicolas Helitas tells me he was raised by his grandparents in the southwest of France. His father, who lived near Paris, would spend a lot of time fishing when on holiday and took Nicolas with him. “I think the hours I spent quietly on the bank of ponds gave me the love of being in nature,” he says, which led him to study biology. 

I made an herbarium, and I started to look more closely at plants - especially orchids. Those plants are often rare, so I started to take photos instead of collecting them. Then I looked at the butterflies which were visiting the plants, then dragonflies after that, while a passion for photography started to grow in me. Now my main interests are still botany and entomology, but I'm potentially interested in everything in nature!

I currently have more than 200k images on my hard drive. Even if lots of them are redundant (I don't like selecting and deleting!), that represents a lot of data. I offer some images to nature protection associations who look for illustrations, but most of those data have not been used. A friend of mine told me about iNaturalist some time after we had discovered GBIF. We had found an excellent way to give value to years of observations! The link between the two sites was the key for our interest in iNaturalist: the information is stored and shared together with all other scientific sources, and offered to researchers.

One of those photos, of course, is the subject of this blog post. In June of 2015, Nicolas ventured out in search of a rare butterfly. While he never did see the butterfly, he did spot drone flies behaving in a curious way. “I was looking closely at all the wildflowers along the trails, and I noticed those hoverflies on a daisy. I had never seen this interaction of 3 flies, and took a series of images to try to have the 3 in focus.”  

I reached out @matthewvosper, the top iNat identifier of orange-spined drone flies, for some information about the taxon and this behavior. He told me that the genus Eristalis, which has been split reapeatedly since its original description in 1804, contains about 100 species and has about 50k more observations than any other hoverfly genus on iNat.

The larvae of Eristalis and similar genera live in muddy stagnant pools. They use their long breathing-tube tails like a snorkel, providing them with the rather delightful moniker “rat-tailed maggots”. They can be attracted to your garden by making simple “hoverfly lagoons”.

E. nemorum is one of the smaller species of Eristalis. It is characterised (though not uniquely) by its glassy clear wings with a very small wing stigma (the stigma is often important for identifying Eristalis). It's a very typical-looking Eristalis with clearly triangular orange spots near the front of the abdomen (which can be faded in females), and narrow white bands at the segment margins. It has a bit of a ginger-haired scutum, normally well dusted with a characteristic pattern. It has a holarctic distribution, and it is most similar to E. hirta and E. rossica (which also have short stigmas).

One of the most distinctive things about E. nemorum however, is the unique behaviour exemplified in this Observation of the Week. A female enjoying a flower will be “guarded” by a potential mate hovering directly above her. When the male finds her he touches her with his legs before assuming this position (apparently to confirm the identity - they have been observed touching small dark objects in this way and flying off). They can hover like this for many minutes. When the female leaves, the male follows her. This behaviour doesn't guarantee a mating opportunity however, as other males can get involved. Additional males hover in a towering stack of suitors, maintaining a constant distance from each other. It's not entirely clear how this situation gets resolved when the female flies off! This behaviour is a diagnostic character of E. nemorum.

Nicolas (above) continues to post his photos to iNaturalist and use it as a way to identify his subjects. “I haven't spent a lot of time exploring images from over the world,” he says, “but I'm planning a trip to Greece in April, and iNaturalist will be a nice tool to prepare for it.”

(Photo of Nicolas was taken by Elisabeth Gaillard. Some quotes have been lightly edited for clarity.)

- you can find Nicolas’s website here!

- check out our Identifier Profile on a group of fly identifiers who’ve made ID resources for hoverflies!

- here’s some footage of Orange-spined drone flies courting in Ireland.

- this is a nice paper describing courtship behavior for orange-spined drone flies. [PDF]

Posted on January 30, 2024 07:25 PM by tiwane