New Scientist - USA (2019-11-23)

54 | New Scientist | 23 November 2019

Nettle effect

How is it that stinging nettles don’t

seem to affect my dog’s sensitive

nose and ears or his relatively

fur-free belly while I respond rapidly

with a very painful skin rash?

Fiona Hussey

Aylesbury, Buckinghamshire, UK

Nettles are covered in tiny,

hair-like structures called

trichomes with fragile ends that

break off when touched. The ends

act like needles, injecting chemicals

that cause irritation and pain.

The outer layer of skin on a dog’s

nose is very thick, so I suspect that

the trichomes are too delicate to

penetrate it. The rest of the dog’s

body has hair, and even though

the ears and belly have much less

hair than elsewhere, it probably

still provides protection.

Pamela Manfield

Monmouth, UK

A friend recently told me that he

hasn’t felt nettle stings since a

boyhood experience. At the age

of 6, riding his new bicycle while

wearing shorts and a T-shirt,

he overbalanced into a patch

of nettles and was badly stung

all over his body. Since then, he

hasn’t had a problem with them.

Perhaps it is the same for dogs.

Dogs probably are shielded from

stings for the most part, but can feel

them. A recent article in Veterinary

Record Case Reports (DOI: 10.1136/

vetreccr-2018-000672) describes

two gun dogs that were treated

for suspected toxic effects from

sustained exposure to nettles – Ed

Hourglass figure

Why do wasps have such

a narrow waist? What could be

the advantage of having such

a narrow join between thorax

and abdomen, which seems

like a very weak point?

David Muir

Edinburgh, UK

Key to the evolution of certain

insects was a change from feeding

on plants to laying eggs on or

in the bodies of other arthropods.

The eggs hatch and the larvae

eat their host from the inside,

eventually killing them.

Around 250 million years ago,

at the start of the Triassic period,

a species of insect evolved to have

a narrow waist, called a petiole.

This adaptation allowed greater

flexibility and manoeuvrability

of the ovipositor, the tubular

structure on the female’s rear

used to deposit eggs.

This insect was the last common

ancestor of bees, wasps and ants.

In some families of Apocrita,

the wasp-waisted insects, the

ovipositor has morphed over

time into a stinger – as lots of us

have discovered to our cost.

Hazel Russman

London, UK

Wasps and bees have a narrow

section joining their thorax to

their abdomen, so they can curl

their abdomens under to sting

you while resting on your skin.

Dan O’Donovan,

Solihull, West Midlands, UK

The waist is narrow for flexibility.

This area needs to act like a hinge,

but unlike with a finger joint in

humans, it isn’t surrounded by

flexible skin and flesh. Instead,

it has a hard outer casing and a

thin membrane that can flex only

a little. If this hinge were thicker,

the membrane would have to

flex further and more quickly.

The joint is only as wide as

it needs to be for itself, nerves

and other plumbing.

Getting used to it

I grew up in Perth, Western

Australia, where summer days are

often in the high 30°Cs. I found this

warm, but not too hot. But after a

year living in the UK, I was wearing

shorts when it was just 21°C.

Obviously, I had acclimatised

to the local weather. Is this a

psychological or physiological

process? Do any changes occur

in the body?

Neil Holmes

Bromsgrove, Worcestershire, UK

Acclimatisation will be a factor,

but differences in relative

humidity of the air are probably

more important. In warm

weather, the cooling effect

of the evaporation of sweat from

the skin is vitally important.

When the air temperature in

Perth is in the high 30°Cs, the

relative humidity is usually very

low, often about 40 per cent or

lower. This allows sweat to readily

evaporate from the skin, keeping

the body comfortable.

The air in the UK in summer

is usually more humid. At 21°C

and a relative humidity of 70 per

cent, there will be less evaporation

of sweat from the skin and so

less heat loss. So the body will

feel comfortable at this lower

temperature.

Eddy Richards

Allanton, Scottish Borders, UK

When I moved to Kenya, I

experienced the same adaptation,

but in reverse. After a few months,

I started to put on a jumper when

the temperature dropped below

about 24°C. Returning to the

UK was a bit of a shock, because

it was snowing at the time,

but I rapidly reacclimatised.

However, even 30 years later,

I still find high temperatures

perfectly pleasant. This makes

me think that the effect is

psychological and is a reaction

to changes in the typical

temperature locally.  ❚

This week’s new questions

Time to think Are longer-lived animals such as turtles or

tortoises typically more intelligent than those with shorter

lifespans? Deborah Hartstein, Sydney, Australia

Mind fuel How many calories go on running the brain?

Could I burn more calories by trying to do lots of difficult

mental arithmetic or solving the New Scientist cryptic

crossword? Could I actually think myself thinner?

Thomas Reid, Dundee, UK

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