Q: Do woodlice have any natural predators? I have watched spiders leap onto
them and then recoil as if in disgust. I have a variety of spider species in the
house to control flies to an acceptable level, but woodlice carry on multiplying
regardless. What is the environmentally friendly solution?
A: Most spiders are not keen on eating woodlice. This is partly because
woodlice secrete chemicals which many spiders find unpalatable, and partly
because of their tough armour. Common wolf spiders, for example, do not normally
eat woodlice. However, members of the spider genus Dysdera are woodlice
specialists. These slow-moving, short-sighted, nocturnal spiders have huge
fangs, which they use to stab woodlice simultaneously from above and below,
before injecting a powerful toxin which paralyses their victim in seconds. Two
species of Dysdera are common in southern England.
Woodlice are eaten by a wide range of vertebrates, including shrews, but this
is little use in controlling domestic populations. The key to woodlice control
is that they prefer cool, dark and damp conditions. So buy a humidifier, turn up
the central heating, switch on all the lights, and your woodlice problems should
Department of Animal and Plant Sciences, Sheffield University
A: There seems to be a widespread belief that woodlice are vermin to be
rooted out with the same vigour as Colorado beetles from a potato patch, but
there is no basis for this prejudice. Woodlice are the most inoffensive of
creatures. They make their living by eating dead and partially rotten plant
material, and by grazing on the algae that grow on damp surfaces.
With their weak mouthparts they are incapable of damaging sound timber or
attacking living plants, they have no interest in pieces of meat or jars of jam
and do not carry disease. They are also very special creatures, as the only
truly terrestrial crustacean, they are more closely related to crabs and
lobsters than to spiders, silverfish or the other common household fauna.
The environmentally friendly solution is, therefore, to do nothing. Just
enjoy sharing the habitat of these thoroughly endearing little creatures.
Q: I have seen a device called a storm glass which consists of a glass tube
filled with a liquid containing crystals. The shape of the crystals is supposed
to change according to the pressure. How does this device work? Apparently,
Darwin used one on board the Beagle.
A: I send this letter to you with some urgency. It concerns the answer to the
question about the storm glass. There is the possibility that somebody may be
seriously hurt trying to make this device without having first thought about
what they are going to do.
Camphor is a pungent organic chemical, which is combustible. It has a history
of use as a plasticiser in explosives and is readily oxidisable; ammonium
chloride decomposes when heated, releasing toxic fumes and potassium nitrate is
an oxidising agent. Mixing these three chemicals together in a dry state could
very probably result in the potassium nitrate oxidising the camphor, causing it
to combust and the ammonium chloride to decompose. The consequences may be
fatal. If this device really must be made, probably the safest way to do this is
to dissolve the potassium nitrate and ammonium chloride in the water, then add
the ethanol and finally the camphor.
Q: Why are most eggs egg-shaped?
A: Eggs are egg-shaped for several reasons. First, it enables them to fit
more snugly together in the nest, with smaller air spaces between them. This
reduces heat loss and allows best use of the nest space. Second, if the egg
rolls, it will roll in a circular path around the pointed end. This means that
on a flat (or flattish surface) there should be no danger of the egg rolling
off, or out of the nest. Third, an egg shape is more comfortable for the bird
while it is laying (assuming that the rounded end emerges first), rather than a
sphere or a cylinder.
Finally, the most important reason is that hens’ eggs are the ideal shape for
fitting into egg cups and the egg holders on the fridge door. No other shape
A: Most eggs are egg-shaped (ovoid) because an egg with corners or edges
would be structurally weaker, besides being distinctly uncomfortable to lay. The
strongest shape would be a sphere, but spherical eggs will roll away and this
would be unfortunate, especially for birds that nest on cliffs. Most eggs will
roll in a curved path, coming to rest with the sharper end pointing uphill.
There is in fact a noticeable tendency for the eggs of cliff-nesting birds to
deviate more from the spherical, and thus roll in a tighter arc.
This week’s questions
Cold feet: Why do Antarctic penguins’ feet not freeze in winter when they are
in constant contact with the ice and snow? Years ago I heard on the radio that
scientists had discovered that penguins had co-lateral circulation in their feet
that prevented them from freezing but I have seen no further information or
explanation of this. Despite asking scientists studying penguins about this,
none could give an answer.
Carbonised: I have just bought a second hand soda siphon and have been trying
to make it work. The theory is that you fill it with water, close it and then
screw a bulb of gas into the socket on the side. The bulb is pierced and
supposedly makes the water fizzy as well as providing the pressure to pump the
water out of the siphon. However, something is amiss. After I have added the
gas, the pump action begins to work but the water is not carbonated. What is
Blowfish: Fish don’t fart, why is this?
Brentwood, California, USA
Megaquestion: ISO metric measures employ a fixed prefix system: deci-,
centi-, kilo-, mega-, and so on.
Why don’t we talk about megametres or gigametres? For instance, New York is
about 5 megametres from London and the Earth is about 12 megametres in
And why don’t scientists talk in terms of multiples of seconds for long
periods of time such as gigaseconds or teraseconds, instead of years. It seems
that one second (like the metre or kilogram) is either too small or too large
for practical scientific use.
Come to think of it, why is the kilogram the basic unit of mass instead of
the gram? Who had the idea of making a thousand of something a base unit?
I could go on for centiseconds, but my dinner is getting cold…