Written by Joe Ballenger

Wasp picture, via AaE FB inbox

Some of the more interesting questions we get aren’t even about living insects. People have lots of questions about dead bugs, too.

Hey there, I just had a quick question, I was in my backyard when I came across this little guy in the dirt. I’m not sure if the coloration is because of it being dead, or it was like that to begin with. I was hoping you could shed some light for me!

This question is neat, because it combines two of our previous posts. Nancy has written before on how insect eyes work, as well as structural coloration.

So why did the eyes of this bug turn white after it died?

Eye Color

So let’s start with flies.

Drosophila Wild Type (Left) and Sepia (Right) mutant eye types. Image credit: Carolina Biological Supply, via Flickr. Image Credit: CC BY NC-ND-2.0

There are very few biological systems we understand better than that of the common fruit fly, Drosophila melanogaster.  In fact, much of what we understand about how genetics work in animals comes from this animal.

In the early 1900s, Gregor Mendel’s work on heredity was rediscovered and a biologist by the name of Thomas Hunt Morgan wanted to demonstrate that heredity worked in animals in a similar manner. His system was a small fly, known as Drosophila melanogaster, and the first mutation he discovered was a male-linked mutation which made fly eyes white. Throughout the years, scientists have bred out a whole host of color mutants in flies…these are all broken steps in pigment biosynthesis.

Insects have pigments which absorb excess light, and in life, this pigment moves up and down in the eye depending on how it’s needed.

Generally day flying insects have one rhabdom associated with one ommatidium. In night flying insects, one rhabdom can receive information from several ommatidia.

To quote Nancy:

Insects that are completely nocturnal modified the structure of the compound eye. Normally, the inside of the ommatidium is lined with pigment cells. This prevents light seeping into the adjacent ommatida. Usually this is good because the more light that floods in makes your resolution shoot down the tubes.

The stuff you see in wasp eyes is generally the same stuff that makes Drosophila eyes different color. In life, the wasp eyes are black like the ones in the picture below.

Asian Chalybion sp. Image Credit: Arlan Suresh, via Flickr. License info: CC BY 2.0

Sun Bleaching

The pigments in insect eyes are called pterins. The types of pterins which are found in the eyes are what dictate the colors. They also have a really complicated construction process.

Insect eyes have some really complicated mixtures of Pterins, but ultimately they all do the same job. Their job is to absorb light in as much of the spectrum as they can. In some cases, the different pigments allow the insect to absorb specific wavelengths to filter out specific colors.

Since pterins absorb light, that light transfers to the molecule and causes chemical changes. In this post, we’re not interested in the absorption of the light…we’re interested in the decay of the molecules.

When UV hits the pterins, it generates highly charged molecules called free radicals. These free radicals react with portions of the molecules, causing them to break. Once these pigments break, they can’t absorb light anymore. The result is that the eyes turn white after awhile.

So why doesn’t the bug’s body turn white?

The bug’s body isn’t colored using pigments. It’s color is structural, and results from how light moves through the insect’s coloration. There’s photodegredation that’s happening, but because the color isn’t a result of pigment, the bug’s body doesn’t fade as quick.

If you go to a display of butterflies, this is easy to see. Butterflies colored by pigment, like Luna moths, will be faded. Those colored by structural coloration, like Morpho, will still have their brilliant colors.

The Bottom Line

Joe’s bleached work shirt.

Bleaching is a phenomenon that occurs in a lot of places. When you spill bleach on your clothes, highly reactive chemicals break down the eyes which give them color. The shirt to the left is one of my work shirts that’s been damaged by bleach.

When insects are left out in the sun, UV light creates highly reactive species which eventually break down pigments. The bugs can become more brittle, and eventually fade, but not all parts of the bug are equally damaged. Many of the structural components, including those that make them brightly colored and shiny, won’t break down as fast.

The ultimate result is bug eyes which stay white, but bodies that remain shiny and new looking.

Works Cited

Kim, Heuijong, Kiyoung Kim, and Jeongbin Yim. “Biosynthesis of drosopterins, the red eye pigments of Drosophila melanogaster.” IUBMB life 65.4 (2013): 334-340.