March 2017

There is a lot happening in space right now with probes looking at distant planets and asteroids, and many planned space missions that will launch soon to investigate gravitational waves, dark matter and all sorts of other phenomena.  One thing all of these spacecraft have in common is the need for power.  Most experiments are long-term and so battery power is not an option and neither is any sort of refuelling.  So they will get their power from the sun using solar panels.  The problem with that is that the panels need to be really big especially if they are going to be a long way from the sun.  In order to use large panels that need to be fitted inside much smaller launch modules, they will need to be folded.  Not only will they need to be folded, but they will need to unfold reliably because there will be no engineer there to give them a kick.  The designers, however, are looking much closer to home to see how best to fold them.  There are two things in nature that unfold reliably – leaves and beetles wings, and studying these has allowed the designers to improve the designs making them both lightweight and easy to deploy.

It may not be immediately apparent, but if you look at a freshly emerged beech leaf and at the size of the bud it has just emerged from, then there is a big mismatch.  The bud is much shorter and less wide than the leaf, and that is because the leaf is folded up inside the bud.  That initial burst from the bud is relatively rapid as sap is forced into the veins and the leaf flattens out, ready to quickly use the sun’s energy to provide the sugars and proteins the tree will need for growth.  The leaf will carry on growing until it reaches full size, but nowhere near as rapidly as that initial unfolding from the bud as can be seen in time-lapse videos of the process.  (There are plenty of examples on UTube.)

Even more remarkable than the unfolding leaf, is the beetle’s wing.  And it is remarkable because it has to fold as well as unfold.  Most flying insects have four wings – butterflies, bees, dragonflies to name a few, but beetles’ wings have evolved so that the forewings form a tough shell to protect them from predators, leaving the hindwings to do all the flying.  (Evolution didn’t quite get it right in this case though.  The whole point about flight is that it gets you away from predators, but if you’ve got a tough shell that predators can’t penetrate, then why have wings as well?  So, many beetles – appropriately called ground beetles – only have the tough outer shell that is firmly welded shut.)

Violaceous Ground Beetle - note the one piece wing case.

  If you look at a ladybird, for example, you may see its wing-cases open as it prepares to fly, but the unfolding of the hindwings is almost instant and too fast to follow.  You have a much better chance of seeing them fold the hindwings when they land and you will see the much longer hindwings tuck themselves under the wing-cases.

The remarkable thing about this wing folding is that there are no muscles in the wing itself.  The folding and unfolding is all done by subtle flexing of the wing at the joint where it is attached to the insect’s body, as well as having veins that curve and bend in such a way that they pop into a rigid structure when flight is required, and un-pop into folds when compactness is required.

Talking of veins, I didn’t realize until relatively recently that the veins in insect wings aren’t veins.  That is, they don’t carry blood around the wings.  When the adult insect first emerges from its cocoon, the wings are soft and it has to pump fluid into the veins to expand the wings to their full size. (Think butterfly and chrysalis.)  But once expanded, that’s it, the fluid doesn’t circulate. The veins are just there to stiffen the wings and if a piece of wing falls off or is knocked off by a predator, the veins don’t leak because there is no fluid after the initial inflation.  Amazing!

If you want to see leaves unfolding or ladybirds wings unfolding, then you don’t have long to wait, it’ll be happening in a garden near you any day now.

If you want to prove to yourself that a leaf can fit in a smaller bud, try this -

Take a piece of A4 paper and fold it lengthwise like so -

Then mark a line at about 60 degrees from about a third of the way along the fold, like so -

Then fold along the line you have drawn and fold back and forth in half-inch strips to make a shape something like this -

Then open out the paper like this -

Then the tricky bit is to reverse the folds on one half of the paper so that peaks become troughs and vice versa. (Use the edge of a ruler to fold it against.) It should then look like this. -

You can then cut it into a rough leaf shape if you want, like so -

Then you should be able to fold it enough to fit inside its bud, or at least so that it is both shorter and narrower than the original piece of paper - nature is much better at origami than us humans!