The quote above is, of course, from Thomas Henry Huxley; it was his reaction after learning about Darwin’s simple but powerful idea of natural selection. But I had exactly the same reaction after reading a new paper in The Proceedings of the National Academy of Sciences by Shevtsova et al. (reference below; free online access). The paper shows that insect wings, viewed under certain conditions of light, display stable patterns of iridescence that might be of profound behavioral (and evolutionary) significance.
I’ve spent 42 years looking at flies—ever since I was a sophomore in college—and I must have seen millions of them under the microscope. Usually we look at them against a white background, which we use for inspecting the beasts for mutations or sorting them into males and females for mating. Very occasionally, as when I dissected out female ovaries to count the number of ovarioles, I’d put them on a black background, which makes the white eggs more visible.
On those occasions, I could see an iridescent pattern in the wings, like what you see on the surface of a soap bubble or an oil slick on wet pavement. I never paid that pattern any attention, and that was my mistake.
Unlike every other fly person who’s seen the patterns, the authors of the PNAS paper—Ekaterina Shevstova, two Swedish colleagues (C. Hansson and J. Kjaerandsen), and Dan Janzen, the well-known American naturalist and entomologist—didn’t ignore the colorful patterns. They found that these patterns (see examples below) are produced by interference between light reflected off the two surfaces of the chitinous wing membrane, that they are stable within an individual but often variable among sexes and among species, and that they’re found in most Hymenoptera (wasps and bees) and Diptera (flies). They call the patterns “wing interference patterns” (WIPs), and discovered that they’re best seen against dark backgrounds, and when the light strikes at certain angles.
The paper is well explained by Ed Yong at Not Exactly Rocket Science, so I won’t go over the findings in extenso (plus, the paper is pretty easy to read). First, look at a few of the WIPs. Here’s Figure 1 of their paper (click to enlarge):
G, at the top right, is a hymenopteran (Closterocerus coffeellae), showing how the difference in background can suddenly reveal a hidden iridescent pattern. H and I, below that, show the common fruit fly, Drosophila melanogaster (the one I’ve seen millions of), showing the same background-dependent pattern. J and K are the wings of another species of Drosophila (D. guttifera), which has patches of pigment on the wings under “normal” illuniation but bright iridescence before a black background. The authors suggest that maybe the pigment spots are there not to be seen on their own, but to control the patterns of iridescence, which is what the other flies are really looking for.
A and B show two different but related species of hymenopterans, demonstrating that the WIPs can be highly visible when displayed in the right circumstances, and invisible otherwise. C shows the wasp Neorileya: like many insects, it has a dark abdomen, and so the WIPs are visible when the wings are folded against the body. D shows a sepsid fly displaying a bright pattern of color during a wing display (this may be a mating display; in many flies the males vibrate and waggle their wings when courting females).
Finally, E and F show the WIPs appearing in two fly species against different backgrounds, including the green of a leaf.
What are these patterns for? The authors suggest that they may be of profound significance in insect communication. (See Ed Yong’s article for a precis). As I suggested above, for example, they could be used in mating displays, with females preferring the patterns of males from their own species, since related species may differ in pattern. The paper provides intriguing evidence that within a species of flies and wasps, the WIPs of males differ from those of females, making these patterns (like the colors or plumage of many birds) sexually dimorphic traits.
Whenever males and females differ in an ornamental trait without obvious adaptive significance, one hypothesis is that the trait evolved by sexual selection (see WEIT for an explanation). This is particularly true if, in a group of related species, the males differ in pattern but the females don’t. This invariance of a female trait coupled with strong variation in a male trait that could be related to mating is a strong signal of sexual selection. And that’s the pattern we seem to see, at least in part of Fig. 3 from their paper:
In this figure, each column represents one of three related species of parasitoid wasp in the genus Achrysocharoides. Two “replicate” males of the species are above the line, and the female is below the line. So, for example, B and C are males of one species, and D is the female of that species. F and G are males of a related species, H the female, and so on.
What you see is that while the WIPs of males vary among the species (but don’t vary much within a species), the females of different species (D, H, and L) are all pretty much the same. To me, this implies sexual selection: the males diverge by sexual selection driven (perhaps) by female choice, while there’s no selection on the females to diverge.
This is precisely the pattern seen in insect genitalia: very often closely related species show big differences in male genital shape, while the females of those species are all the same. As William Eberhart has pointed out often, this suggests sexual selection involving female choice. (See his wonderful and underappreciated book, Sexual Selection and Animal Genitalia.)
WIPs have other evolutionary significance. They might be used not to recognize individuals of the same species for mating, but to discriminate against individuals of different species that have different WIPs. That is, they could constitute a reproductive isolating barrier that contributes to speciation.
The authors (probably Dan Janzen, who studies fig wasps) point out another possible “use” of WIPs. Female fig wasps pollinate the figs (and lay their eggs) by squeezing through the tiny hole at the bottom of the fig (the ostiole). To get through, they break off their wings and leave them on the outside of the fig, at the same time secreting a fluid from their abdomen that glues the wings in a “protruding and visible position” on the fig. This might serve as a signal to other females to leave that fig alone, since it’s already occupied with the reproductive output of a previous female.
Here’s a female about to enter the ostiole of a fig. After she enters, and lays her eggs, she dies inside. Every time you eat a fig, you’re eating at least one dead wasp.
The WIPs are useful taxonomically, too: the authors have used them to diagnose “sibling species” of wasps: species that are almost morphologically identical. It turns out that despite their similarity, the species have diagnostic wing patterns. Once you separate species based on those patterns, you can begin to see subtler differences in morphology that you might have missed.
This paper doesn’t cure cancer or anything, but it’s a really nice presentation of a ubiquitous pattern in nature that may have important evolutionary explanations. And how stupid of all of us drosophilists (and other entomologists) not to have paid attention to it!
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Shevtsova, E., C. Hansson, D. H. Janzen, and J. Kjaerandsen. 2010. Stable structural color patterns displayed on transparent insect wings. Proc. Nat. Acad. Sci. USA Early edition. doi/10.1073/pnas.1017393108.
