The colors and patterns found on eggs can affect the survival of offspring. A well-camouflaged egg keeps it hidden from would-be thieves, for example, while pigments help to protect the developing embryo from UV radiation. According to a new Current Biology study, a female stink bug “chooses” the color of her eggs based on the amount of light that’s reflected off the surface: Darker eggs for the tops of leaves, lighter eggs for leaf undersides. She selectively controls egg pigmentation based on light perception.
The spined soldier bug, Podisus maculiventris, is pretty common in fields, backyards, and even (unfortunately) houses across North America. University of Montreal’s Paul Abram uses them as hosts for parasitic wasps. This is how he noticed that darker-colored stink bug eggs tended to appear on black squares in a crossword puzzle on the newspaper used to line the bottom of their cage. Lighter eggs showed up on the light squares. When he tried replicating this observation using petri dishes painted black or white, he got the same surface brightness results.
"We did a whole suite of experiments to determine whether females control egg color or whether eggs themselves are responding to the light," Abram explains in a statement. "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."
He and his colleagues also conducted experiments using soybean plants to see where eggs of different colors are laid. Darker eggs, which are better protected from UV radiation, were laid on the tops of leaves. Since leafy material is very good at filtering UV light, eggs laid on the tops need more sunscreen. To the right you can see a female spined soldier bug and the range of egg colors that she’s capable of laying, from a pale yellow to a dark brown or black.
"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 adds. "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." The variety of egg colors laid by birds and other insects are typically the result of age or diet changes – not a sensory cue from the environment.
Furthermore, the team found that the eggs aren’t darkened by melanin – the pigment that’s responsible for our differences in skin and hair color – but by a previously unknown pigment. The stink bug egg's pigment may be an example of convergent evolution: It absorbs different wavelengths of light in a similar way as the melanin that’s found throughout the animal kingdom.
Images: Andrea Brauner (top) & Leslie Abram (middle).