August 2015

‘How did you spot that?’ is a question I’m often asked when I see something interesting in the undergrowth.  Indeed, it is a question I’ve often asked others when they have found something I didn’t see.  The simple answer ‘Well, I was looking for it.’ - is rarely true.  Most of the time when you spot something, it is because you saw some movement that attracted your attention.  Walking round Rye Harbour reserve, out of the corner of my eye I caught sight of a bird flying into a tree, and heard a familiar call. I scanned the tree for a while before I saw the barred breast of a cuckoo.  The cuckoo rarely sings on an open perch and often tucks itself in amongst leafy branches, which explains why many more people have heard a cuckoo than have ever seen one.  So when I pointed it out to my companion, they said – ‘How did you spot that?’

When it comes to the insect world, you may well spot some movement – a butterfly opening its wings, for example.  But insects are very wary of predators which may also spot movement, so they tend to keep still as far as possible.  If you are interested in insects then there is a tendency to walk slowly and scan bushes or the ground, and quite often you may not be aware of how you saw one.  It may have been a shadow on a leaf, or a tiny splash of a colour other than green.  (Or it may be green like the tortoise beetle that looks just like a flat blister on the surface of the leaf!)

Tortoise Beetle

Just recently I saw my first woundwort shieldbug -

 Eysarcoris venustissimus.  Now, before you congratulate me, you’ll probably want to know what a woundwort shieldbug is.  It is a bit like the common green shieldbug that you’ll find on your runner beans sucking the juices from them.  It is smaller though, in fact it is only about 6 millimetres long. (That’s quarter of an inch in old money.)  It also has exquisite purpley bronze colouring on its back and shoulders, and a black and white connexivum.  (Entomologists like to give posh names to various bits of their chosen group of insects and connexivum is shorter than saying the pie-crust edging round the back and sides of the abdomen of a shieldbug, and I wish I’d never mentioned it now!)

Woundwort Shieldbug

It’s a beautiful insect though, and I certainly can’t say I was looking for it, because before I saw it I didn’t even know that they existed, and it was just the magnification provided by the camera’s viewfinder that allowed me to identify it as a shieldbug.  It was only when I got the photos back home, and looked it up in the field guides that I knew exactly what it was.  It is called the woundwort shieldbug because it lays its eggs on hedge woundwort and the nymphs feed on the leaves.    Hedge woundwort is an attractive plant with spikes of purple flowers and horrible smelling leaves.  Plants that have …wort in their name were once used medicinally, so woundwort was probably once used to heal wounds.   The woundwort shieldbug also lays its eggs on white dead-nettle which is closely related to woundwort.

The eggs hatch out into nymphs, which is a shieldbug’s way of getting round all that dangerous caterpillar and chrysalis stuff that many other insects go through.  The nymphs are mobile and more or less similar to the adult insect, but smaller and they can’t fly or reproduce.  To grow they have to shed their old exoskeleton and puff themselves up to expand the soft skeleton underneath, which then hardens again.  They will go through several of these instars, as each stage is called, before they emerge from the last stage as an adult with wings and reproductive bits.

So how did I spot it? – I haven’t a clue.  I was looking for bumblebees at the time.  Maybe I just knew that there was something beautiful and interesting down there, because I find that if you keep your eyes open, there usually is.

July 2015

On Springwatch the other night, Chris Packham, referring to the blue plumage of something like a swallow, came out with the statement ‘There are no blues in nature’.   A considered response to that statement may be ‘OK Blue-eyes, come out from behind those delphiniums and bluebells and explain yourself!’   What he was referring to, of course, is that there are no natural blue pigments in nature – or are there?  In fact, blue is a relatively uncommon hue in nature, with less than 10% of all flowering plants showing any shade of blue.  Nature is very good at reds and yellows, greens and browns, but nature has almost never evolved a true blue pigment.  Blue pigment, chemically speaking, is actually a fairly recent discovery.  When John Constable painted ‘The Hay Wain’ in 1821, he painted a cloudy sky – not because he was good at clouds, which he clearly was – but because blue pigment was fiercely expensive in those days.  It was made by grinding up the semi-precious stone lapis lazuli.

Salvia - 'Deep Blue'

Chris Packham was wrong regarding plants though, because there is at least one blue pigment in nature – indigo.  Though it is now produced synthetically, indigo dye was originally produced by extracting it from the leaves of the tropical American plant añil (Indigofera suffruticosa).  Indigo dye is most famously used to dye the cotton woven into blue jeans.  Most plants, like bluebells, etc. make their colour using anthocyanins, which though normally red will go blue in a suitably alkaline environment.

But when it comes to the blues in animals, Chris Packham is right.  That doesn’t mean that the bluetit, the swallow, the peacock’s tail, and the common blue butterfly are all suddenly going monochrome, because they have a different trick up their sleeves.  They use what is called structural colour.  Normally when you see blue colour, the blue object has reflected blue light back at you and absorbed light of other colours, and that’s all done at the atomic level with photons of light being absorbed by electrons or not as the case may be. 

Australian Fairy Wren

But a bird’s feather is made mostly of keratin which is mainly a pale brown colour at the atomic scale.  At nanoscale, however, things are different.  (Nanoscale just means things measured in nanometres which are very small – one millionth of a millimetre in fact.)  Nanoscale is way bigger than atomic scale, but still way smaller than you can see with the best optical microscope.  The reason that things are different at nanoscale is that the wavelength of light is also measured in nanometres.  With structural colour there are surface features on the feather that correspond to the wavelength of blue light so that only blue light reflects back.  In the peacocks tail and in the scales on some butterfly’s wings there is a slightly different thing going on, and the surface features are transparent and only half the wavelength of light thick so that when blue light is reflected from both the front and back surface, it effectively doubles the intensity of the light and produces iridescence.  The surface features are normally parallel ridges and this means that the iridescence can only be seen at certain angles.   This is best seen in the blue morph butterfly which is a tropical species not normally seen in Pett, but if you take a close look at the ‘eyes’ on the wings of a peacock butterfly, or the ‘eyes’ on an eyed hawkmoth, you can see a flash of that iridescence.

Eyed Hawkmoth

Peacock Butterfly

So when the baby bluetits have all hatched and they’ve grown their adult plumage, just think how much physics they needed to know to produce it.  Or better still, just marvel at how lucky we are to have such beautiful creatures so commonplace.