Smokescreen
Question: During a countryside fire on a still winter evening, the smoke
forms a horizontal layer after rising vertically to a height of a few metres.
Why?
Answer: Normally, the air gets colder the higher you go because of the fall
in air pressure. However, an inversion can occur in winter. Warmer air will lie
above colder air because the lower layer is cooled as the Earth鈥檚 surface loses
heat by radiating it into space. The two layers will be separated by a sharp
boundary.
Smoke rises from stacks because it is warmer than the surrounding air and
thus less dense. But when it hits the inversion layer it stops rising. Though it
is warmer and thinner than the air below it, it is not warm and thin enough to
make it into the layer above. All it can do is spread along the interface and
make the layer visible for interested humans.
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Manfred Mahnig
Meckenheim, Germany
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Question: Pine forests in Europe are frequented by a caterpillar known in
Spanish as processionales because of its habit of migrating by
attaching itself to other individuals to form a snake-like procession. I have
observed as many as 50 in a procession a metre or more in length. This makes
them quite conspicuous. What is the advantage of this behaviour?
Answer: Processionary behaviour evolved independently in the closely related
families of several moth species.
The caterpillars of these species tend to have warning colours and hairs that
irritate the mouths of predators. They also taste bad. The processions are part
of a special, well-established strategy: be nasty, look nasty and stay in groups
so that no one is likely to confuse you with anything tasty. Larvae of many
moths, beetles, hoppers and sawflies use this strategy.
Jon Richfield
Dennesig, South Africa
Answer: The processionary moths belong to the family Thaumetopoeidae. The two
best-known European species are the pine processionary (Thaumetopoea
pityocampa), a serious pest in pine forests, and the oak processionary
(T. processionea), which feeds on oak and occasionally walnut. Their
relative, the bunny-tailed moth Ochrogaster lunifer, feeds on several
species of acacia in Australia.
The caterpillars normally live in large communal nests in tree branches.
However, when they move around they can form a procession, following each other
nose to tail like a miniature train. The caterpillar鈥檚 spinneret, located on its
mouth, produces a trail of silk as it walks. The next caterpillar hangs on to
the silken trail, adding its own silky thread, and so on. Oak processionary
caterpillars may march in several columns, unlike the pine processionary.
These processions can form for two reasons: first, to find a new food source
when a tree becomes defoliated, and second, to search for a suitable pupation
site on the ground.
Processionary behaviour benefits caterpillars in several ways. Because of
their sheer number and the greater quantity of irritant hairs on show, there is
less chance of individual caterpillars falling prey to predators. It also
ensures that individuals don鈥檛 get lost while they are migrating to a new host.
And, when they have reached their destination, communal pupation makes finding a
mate easier for emerging moths.
Maria Bennett
Aberdeen
Answer: Forming processions and communal living make caterpillars very
conspicuous, but the irritant hairs in which they are clothed deter birds and
other predators. All, that is, apart from cuckoos, which have a unique ability
to shed their gut lining when confronted with high irritant loading.
Chris Gibson
Dovercourt, Essex
Answer: I too have seen caterpillars joined together in tandem. With this
type of mimicry, resembling a slender snake, the caterpillars avoid being
recognised by birds and gain protection from predators.
Anton Br酶gger
Oslo, Norway
Answer: Some of the natural history of the pine processionary caterpillar was
described by J. Henri Fabre in the last century. The first six chapters of his
book, translated into English as The Life of the Caterpillar, carry
this information. It is available online at
http://eldred.ne.mediaone.net/jhf/cater.html.
Eric Eldred
Derry, New Hampshire
This week鈥檚 question
Burning rope trick: When I was working at a fire research centre some years
ago, a co-worker and I noticed that burning droplets of polyethylene emitted a
buzzing sound as they fell. We never worked out why.
Recently, I duplicated the phenomenon and photographed the test. I took a
clear polyethylene bag of the kind used by dry cleaners and laundries to protect
clean clothes, and formed it into a rope with knots along its length. I then
hung the rope outside on a windless night and set it alight at the bottom.
I had set up my camera about 30 centimetres from the falling droplets. Each
time a droplet looked ready to fall, I opened the shutter, and kept it open
until the droplet had passed.
I suspect that the frequency with which the burning droplets brighten and dim
as they fall matches that of the buzzing sound. But can anyone
explain why the droplets burn this way?
Vincent Homer
Ras Tanura, Saudi Arabia
Burning polyethylene is dangerous. Children must not try this experiment
unsupervised, and adults should take proper precautions鈥擡d