And, first of all, does the Pine Processionary possess a special glandular structure which elaborates the virus, as do, for instance, the poison-glands of the Wasps and Bees? By no means. Anatomy shows that the internal structure of the stinging caterpillar is similar to that of the harmless one. There is nothing more and nothing less. The poisonous product, of unlocalized origin, results, therefore, from a general process in which the entire organism is brought into play. It should, in consequence, be found in the blood, after the manner of urea in higher animals. This is a suggestion of grave import, but after all quite valueless without the conclusive verdict of actual experiment. Five or six Processionaries, pricked with the point of a needle, furnish me with a few drops of blood. I allow these to soak into a small square of blotting-paper, which I then apply to my fore-arm with a waterproof bandage. It is not without a certain anxiety that I await the outcome of the experiment. The result will show whether the conclusions already forming in my mind will receive a solid basis or vanish into thin air. At a late hour of the night, the pain wakes me, a pain which this time is an intellectual joy. My anticipations were correct. The blood does indeed contain the venomous substance. It causes itching, swelling, a burning sensation, an exudation of serum and, lastly, a shedding of the skin. I learn more than I had hoped to learn. The test is more valuable than that of mere contact with the caterpillar could have been. Instead of treating myself with the small quantity of poison with which the hairs are smeared, I have gone to the source of the irritant substance and I thereby gain an increase of discomfort. Very happy in my suffering, which sets me on a safe path, I continue my enquiry by arguing thus: the virus in the blood cannot be a living substance, one that takes part in the working of the organism; it is rather, like urea, a form of decay, an offthrow of the vital process, a waste product which is expelled as and when it forms. If this be the case, I ought to find it in the caterpillar’s droppings, which are made up of both the digestive and the urinary residues. Let us describe the new experiment, which is no less positive than the last. I leave a few pinches of very dry droppings, such as are found in abundance In the old nests, to soak for two days In sulphuric ether. The liquid, coloured as it is with the chlorophyll of the caterpillar’s food, turns a dirty green. Then I repeat precisely the process which I mentioned when I wanted to prove the innocuousness of the hairs deprived of their poisonous varnish. I refer to it a second time in order thoroughly to explain the method pursued and to save repetition in the various experiments undertaken.