Stink Bug Egg Color Change

Sometimes scientific questions come from the most unusual places. In Paul Abram’s case it came from a crossword puzzle in a newspaper lining a cage full of stink bugs. Abram, a PhD student at the University of Montreal, doesn’t actually study stink bugs: he studies the parasitic wasps that infest them. But in the cage, with the newspaper and crossword puzzle, he noticed something remarkable about the stink bug eggs. He observed that darker-colored eggs tended to appear on the black squares of the puzzle and the lighter-colored eggs on the light squares.

Out of curiosity (I mean, come on, he’s a scientist!) he then replicated this observation using Petri dishes that were painted black or white. What the heck? Sure, egg color variation occurs in nature within a species and, in fact, certain birds and insects will lay subtly differently colored eggs typically in response to changes in age or diet, but Abrams suspected something else was going on here.

To understand why these spined soldier stink bugs (Podisus maculiventris) do this, Abram conducted experiments on soybean plants to identify where the stink bugs lay different colored eggs. He found that darker eggs are laid on the tops of leaves and that lighter eggs are laid on the bottom of leaves. Since leaves are very good ultraviolet light filters, Abram suspected that the egg color adaptation is likely a form of sunscreen for eggs laid on tops of leaves. A follow-up experiment showed that, indeed, the dark pigment protected developing eggs from ultraviolet radiation.

“We did a whole suite of experiments to determine whether females control egg color or whether eggs themselves are responding to the light,” he says. “What we show is that color is likely influenced by how a female stink bug perceives the ratio of amount of light reflecting off of a surface to the amount of light coming down from above her head… We suspect that these bugs possess some kind of physiological system that receives visual input from the environment and then modulates the application of a pigment in real time.”

Abram expected this pigment to be melanin, which is responsible for skin and hair color differences in humans and is found in many other families of animals. Instead, a biochemical analysis revealed a potentially new, unnamed pigment. The stink bug’s use of this pigment may be an example of convergent evolution, as it absorbs different wavelengths of light in a similar way to melanin.

“This is the first animal found that can selectively control egg color in response to environmental conditions, but we really doubt that it's the only one,” he says. “It highlights the fact that adaptations similar to this are probably hiding in plain sight, waiting to be discovered.”

This amazing, plain sight discovery was published last month in Current Biology.

Images: muscogeegirl/Flickr