杏吧原创

Colour coding

Question: Why is house dust always grey?

Answer: Here in the dry, dusty Middle East, our house dust is closer in
colour to the pale sand outside.

Chana Lajcher

Jerusalem College of Technology,

Israel

Answer: Our dust is red, as we live in north Texas, close to the Red River.
Our land is composed mainly of red clay.

S. Tinney

Texas

Answer: Under normal conditions, house dust consists mostly of sloughed-off
skin cells. Amazingly, most of the dust on top of the wardrobe is dried-out
human skin.

Dry skin is a translucent grey colour, and consequently, so is the dust.
There鈥檚 no other colour in it because the blood vessels are much deeper down and
they are not lost as the skin grows.

Other forms of dust, such as windblown soil, can be different colours, and
the colour gives clues to their origin.

John Morton, School of Applied

Sciences

University of Glamorgan,

Pontypridd

And if we have any readers that question whether all that house dust is
really dead skin, here鈥檚 a proposal for an experimental test鈥擡d

Answer: Grey household dust is largely human skin. Although humans come in a
selection of colours, the pigments are found below the layer that generates our
outer layer of dead skin. To test this, perhaps a reader would like to bath in
woad and report if the dust turns blue.

Roger Wilkins

Felixstowe, Suffolk

White out

Question: How do mountain-dwelling birds and mammals protect themselves from
snow-blindness?

Answer: Any animals whose sight is important to them, and which must deal
with bright light, have several mechanisms for protecting their vision.

Some have pupils that can contract to almost nothing, while others have 鈥渄ark
glasses鈥� in the form of a third translucent eyelid, or nictitating membrane. In
mammals, this is the more or less functionless little fold in the inner corner
of your eyes, but in birds it can cover the exposed part of the eyeball and
permits them to look at very bright light.

Yet another approach is to add external shading or filtering structures. Our
own eyelashes are attenuated examples, and some sportspeople who play under
floodlights find that artificial eyelashes are marvellous for reducing
glare.

Furry animals鈥� eyelids probably have a similar effect, and you will notice
that many birds have beautiful eyelashes. Ostriches and the African ground
hornbill have the most gorgeous examples.

Then there is the question of coloration. Many animals and birds have masks
or streaks through or round their eyes. Apart from making the eyes less
conspicuous targets in an attack, they tend to be placed to reduce glare.

Jon Richfield

Dennesig, South Africa

Pixies at home

Question: A small toadstool that glows brightly at night is growing on a
rotting log in my garden in south-east Queensland. What is the evolutionary
advantage of such a characteristic? Does it attract things which it eats, or
does it repel things which would eat it?

Answer: Fungi glow not to attract things which they eat but to attract things
which eat them. Many fungi benefit from animal dispersion. Truffles, for
example, emit a strong aroma that attracts animals, which then eat the
subterranean fungus. The spores pass through the gut and are not only dispersed
but also provided with nutrients.

Virtually all members of the fungal family Phallales (stinkhorn
fungi) give off an unpleasant odour redolent of rotting meat or sewage, which
attracts beetles or flies. These disperse the spores, either when they pass
through the gut or when they become attached to the insects鈥� head or thorax.
Similar dispersal mechanisms are common among flowering plants.

Phosphorescent fungi attract night-flying insects, such as moths, which
perform the same role. It is quite common to find the fruiting bodies of such
fungi moth-eaten and full of larvae, by which time the glow is reduced to that
of the palest moonlight.

Michael McBain

Australian Fungal Mapping Project

Monash University

Answer: Luminescent species of fungi attract small flies and other insects,
which help to disperse the spores of the fungi and, because of the faeces they
produce, contribute to the growth of the fungi. The problem is that bad guys are
also attracted, such as fungus gnats, some of which eat the spores and,
therefore, are harmful to the fungal propagation.

But nothing is simple in biology: the light emitted by the fungus also
attracts tiny parasitic wasps that attack fungus gnats. They lay their eggs in
the adults or in the gnats鈥� eggs, thereby killing off some of the spore
stealers.

Why aren鈥檛 all fungi luminescent? Leaving aside 鈥渦ltra weak photon
emissions鈥�, which occur in all species, few species produce light bright enough
to be seen by the human eye. What鈥檚 more, even luminescent fungi can be grown
under conditions in which they do not glow, and non-luminescent mutants show no
growth defects whatsoever, so the light is clearly not of great importance.

As far back as 1962, McElroy and Seliger suggested that fungal luminescence
was a vestigial phenomenon, preserved in a few species. It originated after
photosynthetic organisms evolved, when oxygen was toxic to most species and the
best way to get rid of it was to chemically reduce it to water. The production
of light accompanied this reaction. All the successful oxygen-removing organisms
at that time, according to McElroy and Seliger, might have been potentially
luminescent.

Victor Meyer-Rochow

Department of Biology

Oulu University, Finland

This week鈥檚 question

Boom boom: Why, when the space shuttle is returning to Earth, do you hear two
separate sonic booms a few seconds apart?

Andy Horanic

Orlando, Florida