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Do Polar BearsGet Lonely?

THE LAST WORD

Sticky problem

What caused this tree resin,

emerging where a branch was

sawn off, to flow out at an angle,

then vertically downwards,

before flowing in an upward

curve (see photo)?

■ The sap in the photograph is a viscous liquid which readily sets to form a gummy solid.

As with many paints and glues, the setting process begins when volatile solvents in the liquid start to evaporate. Liquid sap emerging from the mass through a horizontal hole is exposed to sunlight and wind, which dry the top surface of the spreading liquid faster than they dry the lower surface. A skin therefore forms on the top surface first, tending to make the emerging resin curve upwards initially.

The shape of the emerging resin is also affected by gravity. The photograph shows several long strips that started out curving upwards but then became

straighter or began to flow downwards as their weight increased. At least one of them broke off under its own weight during formation.

As a side note, there seem to be beads of darker resin in the formations; these could indicate regular periods when the sap flowed more slowly, such as during the night, or when the solvent evaporated more rapidly, as would have happened during the day.Nathan Scott

School of Mechanical Engineering

University of Western Australia,

Crawley

Negative attitude

I have five solar-powered lights

in my garden. Something has spun

silken masses inside their battery

compartments, and in all five lights

they are located at the negative

terminal of the battery. Is there

a reason for this, or is it just

coincidence?

■ It seems unlikely that a web built in such a location would be used for capturing prey; it is far more likely that the tangled silk is formed as a home by a spinning spider. There are 11 types of spider which live in the questioner’s home town of Johnson City, New York state, but only one species builds these kinds of nests: Cheiracanthium

mildei . Originally from the Mediterranean, it arrived in the north-eastern US in the 1940s and lives in low-lying areas, making it

a prime candidate for the source of these constructions.

However, the primary question was why these nests were formed on the negative terminal of the battery. If you look closely at the battery enclosure you’ll see a small spring that keeps the electrical circuit pressed against the battery’s negative terminal. The spiral shape of the compressed spring provides perfect structural support for C. mildei’s tubular nest.

There is one more possibility for the appearance of these structures. Female C. mildei are known to lay up to 200 eggs in five separate egg sacs. The questioner mentions that there are five solar light fixtures, so it may just be that a female C. mildei saw the secluded battery compartments of your solar lights as an ideal spot to lay and defend her eggs.Danny Ben-David

New York City, US

This week’s question

THINGIES

While swimming one morning across Coogee Bay in New South Wales, we came across thousands of these strange creatures (see photo) floating at depths down to about 2 metres.

They were hard but also flexible, with water inside and a small hole at one end. Their length varied from about 3 to 30 centimetres and their walls were between 2 and 5 millimetres thick. Their skin was marked with many small protrusions, the size of which varied from one creature to another. Unlike jellyfish they appeared to be completely harmless.

No one I’ve spoken to from the area has ever come across anything like this. So what were they and why were they there?Philippe Wilmotte

Maroubra, New South Wales,

Australia

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