Several readers have called my attention to yet another amazing case of mimicry, this time in a tephritid fly (the “true” fruit flies). Most people became alerted to this by a semi-viral tw**t by Ziya Tong, which notes that “Goniurellia tridens is a 3-in-1 insect,” and that the photo below was taken by Peter Roosenschoon in Dubai. Roosenschoon is a conservation officer at the Dubai Desert Conservation Reserve.
Those aren’t ants bedecking the fly’s wings; they’re the normal wing markings of this species. But why would a fly have antlike markings on its wing? [UPDATE: Note comments at bottom where an ant expert and two others (including Matthew) think that these are spiderlike markings. I'm coming around to that point of view.]
The issue is discussed in a New York Times‘s “Dot Earth” column by Andrew Revkin, which refers back to the original article in the original article by Anna Zacharias in The National, a United Arab Emirates newspaper. Zacharias describes it:
The image on the wing is absolutely perfect,” says Dr Brigitte Howarth, the fly specialist at Zayed University who first discovered G tridens in the UAE. [JAC: the species has been known since 1910, and is found in the Near and Middle East, Asia, and Asia.]
. . .In the UAE alone, 27 picture wing species are known. Some have wings bearing simple shapes but others, like G tridens, are far more complex.
Dr Howarth first saw G tridens on an oleander shrub in northern Oman. “I was looking at the stem of the leaves and I noticed that there were some insects crawling around. When I sort of honed in I started to notice what I thought was a couple of ants moving around.”
At first she suspected an infestation on the fly’s wings. “But it was so symmetrical that I thought, ‘oh this is not possible’. When I got it under the microscope I realised that these were insects painted onto the wings.”
In contrast to its wings and brilliant green eyes, the fly’s body is a dull greenish grey – “almost cryptically coloured,” says Dr Howarth – that blends into the leaves where it is found
But why the ant markings? Howarth, interviewed by Zacharias, explains:
When threatened, the fly flashes its wings to give the appearance of ants walking back and forth. The predator gets confused and the fly zips off.
That doesn’t make a lot of sense to me. Why doesn’t the fly just “zip off” by flying away when it sees a predator? Confusing a predator by waving your wings just wastes time. Now some tephritids have spider-like markings on them, and that makes more sense. Apparently the predator is a jumping spider, and when it sneaks up on a fly, it sees the spider markings, mistakes them for another spider of its species, and displays to it. That display gives away the spider’s presence, allowing the fly to get away. But I can’t see this happening with ants.
Howarth also suggests a sexual function:
This defence mechanism may also make the fly attractive to potential mates – something that is less of a concern for the average housefly.
“A lot of flies, if a male sees a female that is suitable it just flies up and tries to latch on,” said Dr Howarth. But G tridens has an altogether more amorous courtship, showing off its wings in a colourful dance. And Dr Howarth believes it is no exception.
“If you look at the behaviour, it tells you a lot about the functionality,” said Dr Howarth. “Not everybody gets to mate. The ones that do have something about them that make them more attractive.
“Is it the same in other invertebrates, who knows? It’s very possible that those are in fact for courtship behaviour.”
That makes even less sense. Yes, marks on the wings can help courtship, but why would ant-shaped markings on the wings be necessary for that? It’s not like females are attracted to ant shapes because the flies eat ants, because they don’t. No, there’s something here about the resemblance to the ants that facilitates the fly’s survival or reproduction. Howarth’s final statement doesn’t make a lot of sense to me, either:
This elaborate behaviour may be a response to the fly’s restrictive environment. “Something that can survive anywhere doesn’t need to have as many protection factors,” said Dr Howarth.
The more realistic the picture on the wing, the better its chance of survival and reproduction.
The truth is that we don’t really know why this fly has antlike markings, but we can predict that studying how the fly uses its wings, and knowing more about its ecology, will suggest some testable hypotheses. I’m too harried to think about alternatives now, but I bet some readers, particularly our entomologists and field biologists, can suggest an evolutionary explanation for ant markings.
Finally, here’s a flickr picture by Drew Gardner (I cropped it a bit), who notes, “Another pictured wing fly. This is a small fly about 3 mm long which came to the mercury vapour light. Each wing has an almost perfect picture of an insect, complete with head, thorax and abdomen and six legs.”
h/t: Diana MacPherson, Matthew Cobb, and several others I’ve forgotten (sorry!)
EDIT from Matthew Cobb: Not everyone agrees with the ‘ant’ diagnosis. Starting with ant supremo Alex Wild (a.k.a. @Myrmecos). The following discussion took place on Tw*tter.