Time to learn about apple maggot flies

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MarkSturtevant
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Time to learn about apple maggot flies

Post by MarkSturtevant »

The name is not appealing, but apple maggot flies (Rhagoletis pomonella) are interesting for different reasons. Here is an apple maggot fly that I found hanging around our apple trees last summer. Although rather pretty, they are considered a serous pest to apples. I found that I could photograph this one very easily by simply giving it slices of apple. So while the fly is slurping up sour apple juice, allow me to tell you about apple maggot flies.
ImageApple maggot fly by Mark Sturtevant, on Flickr

One interesting thing about them is that they are a classic example of sympatric speciation. This form of speciation is where one species becomes two species while still living in the same area, rather than after they are separated to live in different areas. What happened to cause this in this species is the introduction of apple trees to North America. You see, their native host plant are hawthorn trees, which are related to apples. However, when European settlers brought over apple trees, some of the flies started to prefer the new trees as their host plants. This was first noted to happen by 1850. So now there are two populations of the flies, one that continues to live and breed on hawthorns (these are sometimes called hawthorn maggot flies), while the apple maggot flies prefer apple trees. Although there is considerable reproductive isolation in the wild between these populations, they are still considered one species although I am not sure why.
This sort of thing has apparently happened several times, by the way, as there are several identical-looking species/races/populations of these flies that live on native snowberries, honeysuckle, blueberries, and other fruits.
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Now when this fly turns away from the camera, we see another interesting thing about them. Those bold stripes on the wings look a lot like spider legs, don’t they? A look-alike species (this one is given its own species name), was found to use this wing pattern to deliberately mimic jumping spiders, and this helps prevent attacks by those visual predators.
ImageApple maggot flies are spider mimics by Mark Sturtevant, on Flickr

Experiments showed that when a jumping spider approaches one of these flies, the fly is seen to turn to face its predator and spread out its wings – suddenly making them look a lot more spider-like. The spider would then frequently move away rather than attempt to have a meal. This is described in this classic paper: https://www.researchgate.net/publicatio ... _Predators

So I set out to see if I could make this remarkable interaction happen with what I presume is an apple maggot fly.
I soon managed to find a suitable jumping spider (a male Phidippus clarus). If they were to cross paths, would the fly use its wings to mimic a jumping spider? Would the spider react as if it came upon one of its own kind? In truth, it can be hard to get animals to behave like you’d want them, but I do believe I managed to get a partial positive result.

Over several trials, with camera in hand, I arranged for the two to encounter one another. Pretty much every time … nothing happened. The fly slurped up apple juice, and the spider, while at times glancing at the fly, showed little obvious reaction to it.
But on one occasion – and I don’t know what was special about this occasion -- the fly definitely reacted to the nearby spider by suddenly snapping around to face the spider while holding out its wings. Its reaction was quick. You can see the fly in the background here, although very out of focus. The spider meanwhile seemed to not react much other than to move away.
ImageApple maggot fly versus jumping spider by Mark Sturtevant, on Flickr

Here the spider had moved on, and the fly is still in its spider-mimicking pose. That was pretty cool! But I am not sure what the spider thought of it.
ImageApply maggot fly versus jumping spider by Mark Sturtevant, on Flickr

I did several more trials over a few days. But I got nothing more out of either of them. But that one result was still interesting enough that I definitely would like to try this again!
ImageApply maggot fly versus jumping spider by Mark Sturtevant, on Flickr

It has been proposed that this particular kind of mimicry, where insects mimic jumping spiders in order to avoid predation by them -- is rather widely seen in insects. There is this interesting paper that makes the case for this: https://peckhamia.com/peckhamia/PECKHAMIA_179.1.pdf You should scroll through and look at the fascinating pictures!

Thank you for looking!
Mark Sturtevant
Dept. of Still Waters

rjlittlefield
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Re: Time to learn about apple maggot flies

Post by rjlittlefield »

Lovely! And that paper is a great link. Thanks!

--Rik

AlP
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Re: Time to learn about apple maggot flies

Post by AlP »

Excellent.

colohank
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Re: Time to learn about apple maggot flies

Post by colohank »

If the fly sensed the presence of a jumping spider and felt threatened, why would it opt to display the pattern on its wings rather than simply flying away? The wing ruse might work, but why take the chance that it wouldn't?

Scarodactyl
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Re: Time to learn about apple maggot flies

Post by Scarodactyl »

The moment it tries to flee the spider knows it's prey, and flying may not help given how fast and high it can jump.
I envision it a bit like this, the fly represented by the basketball
https://i.imgur.com/bBwtjoD.gif

MarkSturtevant
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Re: Time to learn about apple maggot flies

Post by MarkSturtevant »

colohank wrote:
Sun Mar 12, 2023 7:48 am
If the fly sensed the presence of a jumping spider and felt threatened, why would it opt to display the pattern on its wings rather than simply flying away? The wing ruse might work, but why take the chance that it wouldn't?
It's a good question, and of course there are flies that don't have the markings and/or the behavior that goes with it and those should fly away (and some will escape). But I think Scarodactyl could be on the right track. Jumping spiders are fast when they commit to an attack. I've seen one bring down a fly in flight. Here is my hand-wavy scenario for how this spider mimicking strategy might evolve.
1. The flies come from a group that already use patterned wings to display to each other. That is a wide characteristic of flies in this family (Tephritidae). Tephritids and many species in related families, like Drosophilidae, often signal to each other with patterned wings. It's all about sex. There are Drosophilids that seem to also mimic spiders with their wings.
2. Some of these flies happen to cause jumping spiders to at least pause in their approach since they look vaguely like another jumping spider. These flies would have a survival advantage. This sort of thing, where a trait "accidentally" is beneficial, is called an excaptation. Excaptations can result in further selection upon that trait to be better at doing that new thing.
3. Thru excaptation, there is subsequent selection for flies to have a more spider-ey look, and along with that, selection for behaviors that increase survivorship when a spider approaches. They turn to the spider and hold out the wings. Some will wave their wings and move from side to side (jumping spiders move jerkily, and also signal to each other by waving their legs to signal they are a fellow spider. That is to avoid conflict).

One should wonder: Is any of this behavior from the flies a case where the fly thinks the spider is a female fly? Do male flies ever try to court a spider? Would a male jumping spider ever attempt to court one of these flies? Male flies and spiders are very randy, and so one might wonder! I have seen male jumping spiders court the wrong species of female.
Mark Sturtevant
Dept. of Still Waters

Lou Jost
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Re: Time to learn about apple maggot flies

Post by Lou Jost »

Very nice story, and great work trying to make it happen!

Pau
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Re: Time to learn about apple maggot flies

Post by Pau »

Mark, great article and very well illustrated! =D> =D> =D>
I've been delighted recalling well the first evolutionary phenomenon example: I have used it many times when teaching Biology.

I didn't know the spider mimicking case, very very interesting.
Pau

Marcepstein
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Re: Time to learn about apple maggot flies

Post by Marcepstein »

Really nice and very interesting :!:

Bob-O-Rama
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Re: Time to learn about apple maggot flies

Post by Bob-O-Rama »

Its a nice and informative series. I have no patience with live subjects, so this always seem miraculous to me. The mimicry is really well done, especially how the head coloration matches a rotting apple, kinda "selling" the pattern of the wings / body.

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