Nov. 13, 2013
Ian Dworkin is an associate professor in the Department of Zoology and the author of, Genes Gone Wild, a blog about genes, and the crazy things they do to little critters in different environments.
As numerous commenters on the blogs and on Twitter have pointed out this fly is part of the family of true fruit flies that include several that are known to startle jumping spiders (causing them to do a short retreat). This retreat is likely because the flies have evolved to mimic aggressive behaviors of the spiders themselves.
This work was initially described more than 25 years ago in a pair of papers in Science. These papers beautifully demonstrate the adaptive utility of markings on the wing combined with a rowing action of the wings that could achieve this mimicry. Neither the markings on the wings nor the rowing behavior alone were sufficient to induce the aversion behavior in the spiders (the spiders retreat).
Indeed those of us who took biology courses in university in the early to mid-1990s probably remember this example being taught to us. What's more is that it seems to be fairly widespread among species in this family of flies (each research group used a different species of Tephritid fly and spider). Another paper tested about 18 different species of jumping spiders with the medfly, and showed that most spiders responded with the retreat as well. This suggests that this adaptive wing morphology and behavior combination is probably pretty ancient.
A few years ago, when I was hosting a lab barbecue in my backyard, we were lucky enough to get to watch the intricate little behavioral "routine" between a fly and a jumping spider (in this case the bold jumping spider). The spider approached the fly, got into its attack posture, and then the fly did its "wing rowing" display. The spider "retreated" (took a short jump back), and the fly took off, successfully evading getting eaten. Not too shabby,eh? How often do you get to watch this for real?!
Two years ago I got to watch this happen again, this time with a picture-winged fly. I happened to have some collecting vials. So I collected the flies. I then put the fly in a small dish with a jumping spider so I could get some simple video of it. You can watch the video above in all its grainy, low-quality glory.