1. The dermatitis caused by contact with the larva of Megalopyge opercularis is produced by a poison introduced by the hollow, specialized setæ of its cuticular tubercles. It is not produced by the ornamental hairs, or by the tissue juices of the animal.
2. The poison appears to be of the nature of a venom, combined with protein vehicles, and may be itself a protein.
3. It is rendered inert by boiling, or by heating to 55°C. for a considerable period of time.
4. It is apparently stored in sacs at the base of the setæ, but whether secreted there, or by hypodermal glands, remains to be determined.
5. It diminishes in virulence after the larva has spun its cocoon, and is no longer active after the caterpillar is dead.
6. The poisonous spines cause localized necrosis of the human epidermis, followed by the formation of small vesicles. The cellular reaction to the poison is chiefly lymphocytic.