Stinging nettle hair

Other living things besides animals also use acid to sting. The stinging nettle bush has sharp, hollow hairs that contain various chemicals, including formic acid, that irritate the skin of any animal that is unfortunate enough to rub up against it. 

The elemental distribution of Ca, Si, and Mn in the hair of the common stinging nettle Urtica dioica) obtained using the Oxford University PIXE microprobe. The color code for Ca is yellow, >3.4 M orange, 2.0-3.4 M red, 1.5-2.0 M dark blue, 1-1.5 M blue, 0.5-1.0 M light blue, 0.1-0.5 M white, <0.1 M. The color codes of Si and Mn are similar. The PIXE data show that the tip mainly is made up of Si (presumably amorphous silica), but the region behind is largely made up of Ca (calcium oxalate crystals). The base of the hair contains substantial amounts of Mn. The pictures were kindly provided by R. J. P. Williams. 

Formic acid—HCO,OH—46—occurs in the acid secretion of red ants, in the stinging hairs of certain insects, in the blood, urine, bile, perspiration, and muscular fluid of man, in the stinging-nettle, and in the leaves of trees of the pine family. It is produced in a number of reactions by the oxidation of many organic substances sugar, starch, flbrin, gelatin, albumin, etc. by the action of potash upon chloroform and kindred bodies by the action of mineral acids in hydrocyanic acid during the fermentation of diabetic urine by the direct union of carbon mon-... 

Serotonin acts as neurotransmitter in the brain of animals (E 3.2). It is, however, also of importance as a hormone for the maintenance of normal gut mobility in the gastro-intestinal tract (E 3.1). Serotonin accumulates in the trichomal fluid of the stinging nettle (Urtica dioica) which is injected into the body of animals breaking the hairs during predation of even touching. In addition to the other constituents of the nettle toxin, e.g., acetylcholine and histamine, it provides protection against predation (E 5.5.3). 5-Hydroxy-L-tryptophan and serotonin accumulate in the seeds and fruits of several plant species, e.g., bananas, pineapples, tomatoes and walnuts. At least in walnut seeds serotonin serves in the detoxication of ammonia during maturation and abscission of the fruits (E 2.2). 

A relationship similar to that between tyrosine and dopamine exists between the amino-acid tryptophan and the brain substance serotonin or 5-hydroxytryptamine (Figure 8.9). Tryptamine is known in the venom of scorpions, while serotonin is found widely in the venom of honeybees, centipedes, and at least two spiders. Serotonin is not found in the venom of ants or solitary wasps but social wasps have it in quantity, as much as 1 pg per insect. It is present in the barbs of the larvae of the Tiger moth Arctia caja (Plate 11) and a satumid butterfly larva. Their origins in insect venoms are presumably from phenylalanine but this has not been proven. The same substance is in the hairs of the common stinging nettle Urtica dioica. 

An electron micrograph image of the hairs on the leaf of a stinging nettle. 

Formic acid also occurs in the hairs of certain caterpillars, and in the sting of nettles. 

Fu, H.Y., S.J. Chen, R.F. Chen, et al. 2006. Identification of oxalic acid and tartaric acid as major persistent pain-inducing toxins in the stinging hairs of the nettle, Urtica thunbergiana. Ann. Bot. 98(l) 57-65. 

Figure 7.5. Photomicrograph of the stinging hair of nettle. [Her (1) by permission of Cornell University Press.]. -i .

Without silica, nettles do not develop the ability to sting. Barber and Shone (112b) described experiments in which the nettle Urticaria dioica was grown in a culture solution nominally free of silica. The leaves showed little stinging ability. Then a solution of silica was added to the culture medium and in 2 weeks the stinging hairs... 

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