by Matthew Cobb
I have a new favourite fly. It is the aptly named Ornidia obesa, a syrphid (aka hoverfly) that is metallic green. As its name indicates, if it played soccerball it would provoke a chorus of the traditional song ‘Who ate all the pies?’.
Here it is, in all its tubby glory (it’s only 1cm long), is a female O. obesa, in a great picture by Scott Nelson from here. Look at those legs – they’re so strong, almost like a bee’s legs.
Males have their eyes much closer together, touching at the top (presumably to find females):
Here is a video by Diego da Cruz Pereira, of O. obesa on a leaf, in Brazil:
And here is a picture of three females laying eggs on a leaf, from Jim Conrad’s Naturalist Newsletter:
The leaf was situated above a compost heap onto which Jim reported he jettisons urine (indeed, he says he noticed the flies while he was peeing on the compost heap). The maggots hatched out, dropped onto the compost heap and then did their maggoty thing.
O. obesa will happily feed on carrion, but it does not like cocaine, it appears. A recent study investigated whether it could be used in forensic entomology for estimating the time of death of a corpse, but noted that while the flies were happy to breed on a pig that had been shot dead (218 flies came out), they were not impressed by the pig that had been killed with an overdose of cocaine (0 flies). I haven’t been able to read more than the abstract, so I’m not sure whether this was because the flies did not lay on the cocaine-killed pig, or if the eggs or maggots or pupae did not survive.
Anyway, it is a very pretty fly. Here are some questions that I would like some answers to:
Why are the flies so stubby? Is this an adaptation, and if so to what?
Why are they metallic green? Is this something to do with feeding on carrion/excrement?
Why are the eyes metallic green too?
I came across O. obesa in a new on-line guide to the Syrphidae of the nearctic region, produced by most excellent Biological Survey of Canada.
Canada has a track record of producing excellent material about flies – ‘Agriculture Canada’ produced the stupendous 3-volume ‘Manual of Nearctic Diptera’, from whence I took the picture of the male, above. It is long out of print (a shame as it is a lovely object), but you can read it (and search it!) in these three massive PDF files (be patient) put on-line by the Entomological Society of Canada: Volume 1, Volume 2 and Volume 3.
I have some very similar flies here in Ecuador in my yard. They hover in one spot for a while, and when they do, they look very much like Euglossid bees, which are often metallic green, and are stubby, and also have green eyes. So my guess is that all three of your questions have the same answer—mimicry of stinging Euglossid bees.
Which begs the question: why do the bees look like that?
I don’t know. Most of the smaller euglossine bee species are metallic (purple, blue, green, reddish-copper).
I seem to remember that as this fly hovers, it moves its legs conspicuously, the way many bees move their legs while hovering to transport pollen or waxes from one part of their body to another for storage. I can’t imagine this serves any function for these solitary flies; it may be mimicking the leg movements of euglossid bees.
Incidentally the author of the video also has a video of a potential model, the green Euglossine bee, Eufriesea violacea.
Nice images
Ah the joys of peeing on a compost heap – I managed to scare a black racer snake out of my compost heap with a misaimed stream. The black racer is harmless but one can still feel vulnerable when a snake comes thrashing towards you when you have your hands otherwise engaged.
Reblogged this on Mark Solock Blog.
I love their eyes! Look at that compound eye glory and they are pretty to boot!
It makes me wonder how they got the pig to overdose on cocaine. Did they encourage the pig to party too hard? Got him snuffling lines with his little pig snout?
Please note that no pig has ever died by overdosing on cannabis.
Many thanks Matthew, I was not aware of these pdfs online and indeed was looking for one the other day! They also produced the two fabulous volumes: Manual of Central American Diptera, which are much more recent.