The last word–

FLIGHTS OF FANCYWhy did I find the peak of Mount

Etna covered in ladybirds but devoid

of other visible flora or fauna?

(Continued)

My own experience indicates that one of the explanations you gave previously (1 December 2007) is correct. At university, my summer job was as a forest lookout, sitting from June to August in a tower perched on a 2400-metre mountain top in south-east British Columbia, looking for fires. I had lots of time for observation.

During two of the summers I found that my outdoor relaxation was disturbed by biting ladybirds. On further investigation I discovered on both occasions a veritable sea of the creatures under a nearby flat rock. An area of around 30 centimetres square was seething with ladybirds, maybe three insects deep. They showed no inclination to fly away, and because there was no chance of migration or hilltopping [insects being transported on warm updrafts] having brought

them there they must have been preparing to hibernate, though it was quite early in the year for this.

Interestingly, each summer there were predictable swarming times for different insects. For example, over a three-day period, a flying ant infestation would build up and then die down. I assume ants have to keep

assignations simple – such as “let’s meet at the highest place”. Although the lookout windows were covered with climbing ants, the radio antennas above were covered even more densely. This too was not a hilltopping or updraft event, because different species would arrive on different days and there was a build-up and a die-down over the three days. David Shelton British Columbia, Canada

SAND ISLANDSWhen walking along a beach in

Mallorca in late April, I noticed some

unusual patterns being cast on the

seabed by small floating patches of

sand. What causes the patches to

form and why do their shadows

on the seabed have bright fringes

around the edges and around the

gaps in the middle?

The effect is very specific and perhaps more likely to be seen on the west coast of the British Isles than in Mallorca. The sand will be fine, dry and windblown – the sort that forms sand dunes. Blown by the wind, the sand will fly a few tens of centimetres above the beach, and some will be trapped on the surface of any water in its path, including a calm sea.

Dry sand takes a few minutes to wet through, and initially there is a “contact angle” between the air/water interface and the surface of the sand grain. The result is that the grain will be kept on the surface rather as if it were on a trampoline, with the water surface curving down around each grain. The surface tension

associated with this also pulls in adjacent floating grains to form rafts of grains. Around these “sand islands” the meniscus of the water effectively forms a convex cylindrical lens.

If the depth of the pool below is close to the focal length of this lens then the sun’s image will be projected as a line around the shadow below, as seen by your questioner. Eventually the sand grains become thoroughly wet and sink. David StevensonNewbury, Berkshire, UK

The effect in question is a result of surface tension. When the sand settles on the surface of the water it presses it down, so that the level of the water under the sand island is slightly below that of the surrounding water, making the surface curve between the two levels. This has the same effect as a convex lens, focusing sunlight on

the bottom to create the bright fringe around the island’s shadow.

Another example of the same effect is seen around the surface-dwelling insects commonly called pond skaters , which cast disproportionately large and blotchy shadows with a “silver lining” around them. This is because the light that one would normally expect to land directly below the edge of the skater is refracted away from the skater by the curved water surface, effectively enlarging the shadow.A. AndersonRamsgate, Kent, UK

THIS WEEK’S QUESTIONTwisted logic

Can anyone explain why a tree would twist itself almost to destruction in the way this one has (see Photo). It is located at 900 metres above sea level at the north entrance to Yosemite National Park in California. Obviously this isn’t an area where there is 24-hour sunlight, which might induce it to chase itself all the way around, yet it appears twisted through three complete rotations.Stewart BrownBristol, UK

“My outdoor relaxation was

disturbed by biting ladybirds.

I discovered a veritable sea of the

creatures under a nearby rock”

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“Sand grains will be kept on

the surface rather as if they were

on a trampoline, with the water

surface curving around them”

080202_R_LastWord 149080202_R_LastWord 149 24/1/08 5:34:07 pm24/1/08 5:34:07 pm