Readers’ wildlife photos

August 7, 2023 • 8:15 am

Well, folks, this is the last substantive batch of photos I have in the tank. If you want more this week, you’ll have to provide them. These come from our most regular regular, Mark Sturtevant; his captions are indented and you can enlarge the photos by clicking on them.

This post has been on my mind for several years, beginning with an encounter that I had with a weird little fly on a bridge. Here is that fly (I think Pseudotephritina sp.), and I had probably shared it here once upon a time. It was marching up and down on the bridge rail while continually waving its wings. I did my best to photograph the little insect, which was no bigger than a fruit fly, with my little 50mm lens on extension tubes. But I wasn’t the only one interested in the fly. There was also a jumping spider, and it definitely was intent on having the fly for a meal! As the spider stalked closer, the fly would suddenly turn to it, waving its wings, and the spider would flee! This was repeated several times until the spider gave up. Did the stripes on the wings look like spider legs to the jumping spider? Jumping spiders do signal to each other by waving their legs. This is how they avoid conflict.

Now one must not make too much of this impression from a one-time encounter like that. But many flies in several different families have boldly patterned wings which they wave around. While this is known to act as intraspecific communication, it is thought that in at least some species flies also use this kind of display to scare off free-roaming spiders like keen-eyed jumping spiders. There is, for example, a classic paper concluding that another fly, Rhagoletis zephyria, would frequently display its patterned wings when stalked by jumping spiders, and spiders would tend to stop their approach in response. Here is a picture from that study, and one can definitely see that the fly does look like a jumping spider:

R. zephyria is part of a large species complex of flies that all strongly resemble each other. From the BugGuide web site, I count 18 species in North America. One of these is the apple maggot fly (R. pomonella), and I do have two apple trees and I see what I presume is that species of fly in the yard from time to time. It should be mentioned that the apple maggot fly is also a classic example of sympatric speciation, since the flies originally relied on hawthorn trees as their host. [JAC: the idea that the two host races of this fly formed sympatrically is probably not correct; see Coyne and Orr 2009). But with the introduction of apples into the country, some of them jumped to apple trees and there is now significant reproductive isolation between the two populations. Anyway, one of the flies appeared on my back porch last summer, and because I was able to catch it I could at last act on what has been on my mind for many years. Would this fly use its wings to deter a jumping spider? Mind you, this is a different species from the one described above, but … maybe? The following pictures record the results of this admittedly informal attempt to test that hypothesis.

Here is the fly, feeding on slices of sour green apples. It was quite content to just sit there and feed since I had starved it for a day.

Now when this fly turns away from the camera, one can certainly see that its wing markings are very much like spider legs. Both males and females display their wings when encountering one another, but what would happen if I introduced a jumping spider?

So out to a local field I went, and soon returned with a test subject—a handsome male Phidippus clarus [see citation below]. What would happen if the two met? Would the fly react to the spider? Would the spider react to the fly (other than making a meal of it)?

In my arena I had the fly, feeding away, and the spider was kept several inches away under a clear plastic cup. When the spider was facing the fly, I would then lift the cup and make ready with the camera. After about 10 tries (I should have counted, but I didn’t), I could definitely say: I am not sure! Most times the spider did look at the fly, and sometimes it paused to look at it, as it was doing here for some seconds. But then it would turn and walk away. At no time did it stalk the fly, nor did it hustle off like it was fleeing. So I can’t “read” what the spider saw of the fly other than that it wasn’t prey.

Meanwhile, the fly just kept feeding, and it did not seem to react to the spider at all. But on one occasion – just one! – the fly certainly did seem to react to the spider by suddenly spinning around (it was facing away before), and it held out its wings. Here is that moment, with the fly out of focus in the background.

And here is a second picture, now focused on the fly. That is not a relaxed posture. The spider for its part just paused briefly, and then moved away.

I don’t know what to say about this informal experiment, other than that the one response from the fly encourages me to try it again. I am currently keeping an eye out for more of the flies.

As a kind of postscript, there is this lovely paper which proposes that many species of insects from several different orders may be mimicking jumping spiders to ward off predation. There are lots of cool and enticing pictures, and the readers here will certainly enjoy having a look.

Thank you for looking!

10 thoughts on “Readers’ wildlife photos

  1. Fascinating story and great pictures! I wonder why the flies-in-peril didn’t, well, just fly away. Perhaps they were making a strategic calculation: it’s better to try to make the spider go away so that the fly can continue to partake of its current food interest. Flying away, while precluding death, would mean giving up the current food item and having to find another. On balance, it may be more advantageous to stay and fight than to leave and risk starvation.

    1. That is a good question. If we may freely speculate, perhaps it relates to a couple factors, one being what you bring up about staying near a food resource. My impression has been that many flies with patterned wings walk around conspicuously, waving their wings, and they are often seen on or near their food sources. This would be where they defend territories and mate. So one can see why they might be reluctant to fly off since those that do might not find their way back or they might lose their territory.
      Meanwhile, the patterns on some of them might also resemble a spider, and those could experience selection to further modify that pattern and for behaviors that thwart spiders that approach them. Those that do would not need to fly off.

  2. Thank you for this captivating spider and fly soap opera – and also for the fascinating article you reference.

  3. Wonderful. The mimicking of jumping spiders by many orders of insects is one of my favourite topics and I love photographing a new example. Great to see the miniexperiment. Thank you.

  4. Well, we had wild Wild WILDlife last night. About 10:30 last night, Flo yelled that there “seemed” to be a bird in the house — she couldn’t see it but the cats had gone crazy!

    It turned out to be a bat, orbitting the house faster than Flo could turn her head, and leaving both cats in its wake. I opened all doors with expectatioin that the bat would exit, but it kept orbitting for at least 15 minutes. Finally turned off all indoor lights off and exterior lights on… Out it went — it was clearly making its living off insects drawn to lights..

    Sorry no pix, and I ust couldn’t keep this in til next Caturday.

    PS: And I loved the spider-mimicking fruit flies. Score one for science posts.

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