Sunday, 31 March 2013

Why do stinging nettles sting?

Why do stinging nettles sting?

There are actually over 30 species of stinging nettle, with the most common being Urtica dioica, existing throughout the northern hemisphere.

How do stinging nettles sting?

If you look closely, you can see tiny hairs covering each nettle. The long hairs found on stinging nettles are called trichomes. These trichomes are the parts of a stinging nettle that stings you. (1) The stinging structure of the nettle is similar to the hypodermic needle. (2) Each hair is just a single cell that is elongated into a fine point. The walls of this point contain a material that glass is made of, called silica. Therefore, the sting is essentially a tiny glass needle. At the tip of the hair, there is a tiny glass bobble. Below the tiny glass bobble, there is a point of weakness in the cell wall, meaning that when the tip of the hair is touched, the glass bobble breaks off, leaving a sharp edge which can then penetrate skin. The sting is not just due to the fact that the skin is being pierced; it is also due to toxic substances which are present in a swollen sac at the bottom of the hair. It is the injection of these toxins that makes the stinging nettle sting.   The main toxins in this liquid seem to be histamine, serotonin, acetyl choline, and formic acid. This liquid full of toxic substances is under pressure. Therefore, when the top of the hair is knocked off, the toxins are released into the skin. The needle is pushed down as it penetrates the skin, which squeezes the sac at the base of the hair, increasing the pressure of the liquid. This helps to push the liquid up the hair and into the skin. This is similar to pushing down onto the plunger of a syringe (the increase in pressure forces liquid out). (1)


Little is known regarding the cellular and molecular mechanisms that occur when an organism is stung. It is thought that histamine causes the initial reaction when an organism is stung. However, it is believed that further reactions occur between an organism and the stinging nettles chemicals. It is thought that stinging nettles may contain additional toxic substances that are toxic to the nerves, causing a secondary release of other toxins. (7)

Being able to sting is an effective way for stinging nettles to avoid being eaten. (3)

Information about the toxins in stinging nettles

  •  Histamine causes inflammation, which is why we tend to take antihistamines against allergies.
  • Serotonin and acetyl choline are neurotransmitters so they fire off our nervous system. (1)
  • Serotonin and acetyl choline combine to make histamine stronger, causing an allergic reaction in most people who get stung by stinging nettles. (4)
  • Ants produce formic acid, which is the substance that makes their bites hurt.
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The toxins trigger the pain receptors in skin, which causes inflammation and irritation. If you are badly stung, the effects can last to up to twelve hours. (1)

So, why can we sometimes pick stinging nettles up without being stung?

The hairs on the stinging nettles grow in a certain direction. On the stem, they tend to grow upwards. On the leaves, they tend to grow outwards. (6) If you stroke the plant in the direction of these tiny hairs, the stinging nettle tends to not sting you. (5)

By Lauren Watmough 

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  1. So are nettle stings an adaptation to deter grazing herbivorous mammals? Does the adaptation do its job (to prevent the leaves getting eaten, thus affecting vigour/reproduction of the plant)? How can we get evidence???!

  2. The stinging hairs on a nettle have developed as a defense against grazing animals. Few animals, minus goats and hungry sheep, will touch these stinging nettles when the sting is active. This means many insects can live in the area and move between the stinging nettles without activating the sting (1). The adaption stops animals eating the plant as they do not want to be stung. In fact,other plants, such as the white dead nettle, mimic the image of a stinging nettle in order to deter grazing animals. However, they do not actually have stinging hairs (3). As stinging nettles are not eaten by grazing animals, they can spread and reproduce more than other plants as there are more, living available plants to reproduce. Research has shown that in heavily grazed areas, nettles have more stinging hairs in order to increase their defenses (2).



  3. This is really interesting, I didnt realise there is a toxin that made stinging nettles hurt when touched, I thought it was the hairs getting into the skin.