Stained plastic window
In my mother鈥檚 kitchen, one pane of the window was replaced by perspex so that an extractor fan could be installed. Fifteen years later the fan still works but the perspex 鈥� as clear as the surrounding glass when it was cut 鈥� is now an opaque yellowy orange. Why did it change clarity and colour?
鈥� The material that is used to glaze the window was most probably a polycarbonate plastic containing bisphenol-A. Under the constant influence of the ultraviolet component of sunlight this compound forms 2,2鈥�-dihydroxybenzophenone, which absorbs in the visible region of sunlight and frequently causes the yellow coloration that is described by the questioner.
It is interesting to note that this last compound can, by dissipating the energy of further absorbed photons as vibrational energy in the form of heat, act as a stabiliser for the window polycarbonate, preventing further deterioration. Therefore, while the discoloration is annoying, it is unlikely that any further damage to the plastic will occur, and the window should not, after 15 years, craze or fall out.
Advertisement
Stuart Fairgrieve,
SPF Polymer Consultants
Kidlington, Oxfordshire, UK
Lightweight scales
My 5-year-old neighbour Oliver wants to know why butterflies and moths are not able to fly when they lose the dusty surface of scales from their wings 鈥� when they collide with a wet window, for instance.
鈥� The idea that butterflies cannot fly without their scales is something of a myth. Some tropical butterflies are naturally devoid of scales, while bee hawkmoths, whose loose scales fall off the instant they take to the air, are flying aces. The presence of scales may increase lift, but their contribution is probably quite small. Butterflies and moths are generally waterproof enough to withstand light or even moderate rain, but with a wet window it would be the weight of the water and the surface tension making the wings stick together that disables them.
Scales make up the patterns on the wings and are embedded in the wing membrane, where they overlap like tiles on a roof. Indeed, the Latin name of the order of butterflies and moths, Lepidoptera, means tile-winged.
The patterns are important in mate recognition and in providing camouflage or warning coloration, but they also enable flight-muscle function by helping the insects warm up. The 鈥渨hite鈥� butterflies hold their wings half open in the sun to reflect extra heat onto their bodies: species with dark colouring hold them full open to warm the air beneath. Hair-scales on the body help retain this heat.
Aside from water, another hazard is wear and tear, caused mostly by contact with foliage. Yet very tattered specimens typically still fly well. They manage this because much of the wing is superfluous. Lift comes mainly from the robust leading edge, through the creation of vortices as the wings flap and pitch, and because of the light weight and large surface area of butterflies and moths, flying takes very little energy. This leeway in wing design leaves space for eyespot markings, which divert bird strikes away from the head. And the comma butterfly has a naturally ragged outline that disguises it as a dead leaf. The wing surfaces do provide some manoeuvrability, but it is mainly the parts nearer the body. So if tattered individuals do seem to be struggling, this is more likely the result of a wrenching injury or of ageing joints and muscles.
Scale loss is even put to advantage. Males of many species have special scales that release scent as they are lost to entice females in courtship. They may smell of flowers, cigars and even chocolate. The monarch butterfly dusts them from its wings using brushes at the tip of the abdomen. Other adaptations are less glamorous. Female gypsy moths 鈥� a serious forestry pest across much of the northern hemisphere 鈥� use scales to disguise their egg masses under a coat of gritty dust.
Finally, the Australian butterfly Liphyra brassolis major lives in ant colonies and its larvae have armour plating. But the emerging adult has sticky scales that clog the ants鈥� jaws until it has flown the nest.
Leonard Winokur
Southampton, UK
鈥� The coloured patterns on the wings of butterflies and moths are created by overlapping pigmented scales. These are loosely attached to the wing surface and are quite easily dislodged by rubbing.
The wing scales are not essential for flight 鈥� anyone who has set up a light trap to study moths at night will have good experience of trying to identify those that have flown into the trap but have lost most of the identifying colour patterns from the wings.
It is possible that those moths that have lost wing scales as a result of colliding with a solid object such as a window are unable to fly because the wings themselves have been damaged, rather than the scales.
Some moths, such as those in the family Sesiidae, are commonly known as clearwing moths. These have scales on their wings when they emerge from the pupa, but most of these are shed during their first flight.
The wings are then largely devoid of scales, giving them a transparent appearance. These moths have no difficulty in flying, which is further evidence that wing scales are unnecessary for flight.
Andrew Halstead
Principal entomologist
Royal Horticultural Society Garden
Wisley, Surrey, UK
This week鈥檚 question
Take a lettuce break
I have heard that lettuce contains large quantities of caffeine which it uses to poison predators in an attempt to avoid being eaten. Is this true and if it is, how much does it contain?
Alan Veggham
Poole, Dorset, UK