There are several different forms of irritation or poisoning from the sting of insects. With most of them, the local pain, swelling and tenderness is due to the injection of formic acid.
A secondary form of poisoning really is due to foreign protein shock. This happens with bees and wasps only when there are a great many stings. The same is true of red ant bites.
In the bite of poisonous spiders, the formic acid is not usually present, and frequently the bite is no more painful than a pin-prick, either at the time or afterwards. The poison of the black widow spider, in a human being, produces grave constitutional symptoms but no local irritation.
Tarantulas do not secrete any definite poison, and the result of a tarantula bite is entirely local, so it is probably due to formic acid. It produces a small white, raised, swollen area, about an inch in diameter, with a smarting and stinging sensation. During the years I served in the army in Texas I saw many tarantula bites, and never saw a person constitutionally sick, nor did I ever see a person who did not recover.
The same may be said of scorpions. The bite results entirely in a local raised, white spot.
All of these formic acid irritations are treated best with equal parts of spirits of ammonia and water, applied as a wet dressing.
Still another form of irritation from insects comes from the mere touch of part of its body on the skin. There are some caterpillars with a type of hair which, on being brushed against the skin, liberates a poison which gives rise to a skin eruption. The larva of the brown-tail moth is the most prominent offender of this kind in this country.
Some insects excrete a poisonous principle through the pores of their body. In this category comes the so-called blister beetle or Spanish fly. The excretion from the Spanish fly is called “cantharidin,” and formerly was used as a substance to produce blisters for counter-irritation in medical treatment.
There is a sort of beetle in the West Indies which excretes an oily substance from glands situated between the second and third pairs of legs. When this insect gets underneath the clothes and walks across the skin, this oily substance is deposited and causes a burning sensation with quite a violent dermatitis. It is possible that there are a great many other insects which do the same thing, and in sensitive individuals produce even more violent symptoms.