Automeris io is the best known of the nettling caterpillars. It is the larva of the Io moth, one of the Saturniidæ. The mature caterpillar, ([fig. 33]), which reaches a length of two and one-half inches, is of a beautiful pale green with sublateral stripes of cream and red color and a few black spines among the green ones. The green radiating spines give the body a mossy appearance. They are tipped with a slender chitinous hair whose tip is readily broken off in the skin and whose poisonous content causes great irritation. Some individuals are very susceptible to the poison, while others are able to handle the larvæ freely without any discomfort. The larvæ feed on a wide range of food plants. They are most commonly encountered on corn and on willow, because of the opportunities for coming in contact with them.
The larvæ of the brown-tail moth (Euproctis chrysorrhœa) (fig. [35] and [36]), where they occur in this country, are, on account of their great numbers, the most serious of all poisonous caterpillars. It is not necessary here, to go into details regarding the introduction of this species from Europe into the New England States. This is all available in the literature from the United States Bureau of Entomology and from that of the various states which are fighting the species. Suffice to say, there is every prospect that the pest will continue to spread throughout the Eastern United States and Canada and that wherever it goes it will prove a direct pest to man as well as to his plants.
Very soon after the introduction of the species there occurred in the region where it had gained a foothold, a mysterious dermatitis of man. The breaking out which usually occurred on the neck or other exposed part of the body was always accompanied by an intense itching. It was soon found that this dermatitis was caused by certain short, barbed hairs of the brown-tail caterpillars and that not only the caterpillars but their cocoons and even the adult female moths might harbor these nettling hairs and thus give rise to the irritation. In many cases the hairs were wafted to clothing on the line and when this was worn it might cause the same trouble. Still worse, it was found that very serious internal injury was often caused by breathing or swallowing the poisonous hairs.