By Paul Abram
PhD Student, Université de Montréal
When Pink Floyd recorded their epic, psychedelic instrumental “Any Colour You Like” for the classic album Dark Side of the Moon, were they inspired by a predatory stink bug?
Well … probably not.
The spined soldier bug (Podisus maculiventris), can’t actually lay any colour of egg it likes – but the real range of possibilities is pretty impressive.
Almost three years ago, when I started working with stink bugs and their parasitoid wasps, I noticed this astounding variation in the colour of the eggs of the spined soldier bug. I was surprised to find that nobody had looked into the cause of this variation or its potential functions. In fact, the function of insect egg colouration seems to have been a bit neglected in general. While I was initially hesitant to start on the dangerous path towards a PhD “side-project” (code for “I would like to take much longer to finish my degree, please”), I eventually caved.
In 2013, I was visiting a colleague’s lab where newspapers are used as a laying substrate for these bugs, and I noticed that there seemed to be a loose correspondence between the colour of the egg masses and the darkness of the paper, especially in high-contrast places like crossword puzzles. I wondered – could stink bugs actually adjust the coloration of their eggs to match the darkness of the laying surface? If so, this would be the first case of an animal able to selectively control the colouration of its eggs.
Back in Montreal a few months later, I started working on this question with an undergraduate summer student, Marie-Lyne Desprès-Einspenner. We did the simple experiment of putting individual females in Petri dishes painted white, black, or black on one side and white on the other.To our surprise and excitement, we got some nice results. First of all, it was clear that individual stink bugs could lay eggs across the whole spectrum of egg colours, and that the egg colour variation wasn’t just a result of advancing egg development. Additionally, stink bugs tended to lay darker eggs in the black petri dishes than the white ones; and, in the bi-coloured dishes, overall darker eggs on the black side than the white side. These effects were subtle, though, compared to the most important and unexpected factor: where the eggs were laid. Eggs tended to be lighter when laid on the underside of the lid (which was lit up from above) than when laid on the side or the bottom of dishes.
So, individual stink bugs can lay eggs of a range of colours, depending on where they are laying. Our next question was: how does this capability express itself on natural laying surfaces? We did some experiments using soybean plants, and figured out what seems to be the key to this whole thing: the stink bugs have a very strong tendency to lay darker-coloured egg masses on the tops of leaves (which have a relatively low surface brightness, like our black dishes), and lighter-coloured masses on leaf undersides (which have a high surface brightness due to light passing through from above, similar to the lids of our white dishes).
Because leaves are excellent filters of ultraviolet (UV) radiation from the sun (protecting most insect eggs, which are usually laid on leaf undersides), and dark pigmentation often acts as a ‘sunscreen’ in nature, we wondered if dark colouration would protect developing stink bug eggs from a lethal sunburn when they are laid on the tops of leaves. Eric Guerra-Grenier (another undergraduate researcher in the lab) and I tested this in the lab by exposing differently coloured eggs to different doses of sun-mimicking UV radiation.
The results were crystal clear – darker eggs are better-protected from UV radiation than light eggs, with a strong dose-dependency with respect to UV radiation intensity and egg colouration.
This was an exciting find, but begged the question: what is the pigment that makes eggs dark, anyway? The clear answer was that it must be melanin, which is responsible for most dark animal pigmentation, including in us humans, and is also really good at protecting against UV radiation damage.
Eric and I did the obvious thing, sending hundreds of (freezer-killed) stink bug eggs to two melanin biochemists in Japan. Our collaborators ran a suite of tests to confirm that the egg pigment was melanin. But…it turned out that the egg pigment wasn’t melanin! Right now, we simply don’t know what this “mystery pigment” is (maybe something totally new to science?).
As is common in research, we are left with more questions than answers. What is the physiological mechanism that allows stink bugs to selectively apply pigment to eggs? In evolutionary terms, why lay eggs on UV-exposed leaf tops in the first place? And why still lay some light eggs on leaf undersides? Could the pigment also have a role in camouflage, thermoregulation, or water retention? Do other, closely related (or why not distantly-related) insect species also have this capacity? We’re currently working on some of these questions, and I hope that we get to try to answer all of them eventually.
If you’d like, you can find a lot more details about our findings, including the answer to “does UV radiation affect the control of egg colour?”, in a newly published paper (remember to listen to the accompanying song while reading) – and stay tuned for more results in the coming months.
In the meantime, fellow entomologists and naturalists, look closely at insect eggs – is there anything interesting about how they’re coloured/patterned?
I would like to suggest additional Pink Floyd song/entomology paper pairings (feel free to suggest your own!):