This curious result raises a question which is not alien to the vainglorious theories affecting attenuated viruses, serums and vaccines. A larva subject to complete metamorphosis is stung by the Scorpion; we might readily say that it has been vaccinated, in the sense that it has been inoculated with a virus fatal under the future conditions, but tolerable in its effects in the present stage. The patient does not seem affected by the sting; it begins to eat again and continues its larval work as usual. The virus, however, cannot fail to act, in one way or another, on the animal’s blood or nerves. Might it not lessen the vulnerability which results from the transformation? Can the adult be rendered immune by a habit acquired during the larval stage? Might it be able to resist the virus as Mithridates was able to resist poison? In short, is the insect with a complete metamorphosis whose larva has been stung capable of itself withstanding the sting? That is the question. The confirmatory arguments are so urgent that we are at first tempted to answer: “Yes, the adult will resist.” But we will leave experiment to speak for itself. With this object preparations are made with four sets of subjects. The first consists of twelve Cetonia-larvæ, which, after being stung in October, have been revaccinated, that is to say, stung a second time, in May. The second set is also composed of twelve Cetonia-larvæ, but these have been stung once only, in May. Four chrysalids of the Spurge Hawk-moth form the third. They belong to caterpillars stung once, in June. Lastly, I have some cocoons spun by the Silkworm whose vaccination, attended by a flow of blood, I have described above. The Scorpion will once more play his part with each lot after the hatching has taken place. The Silkworm Moth is the first to respond to my impatience. The Moth is there in two or three weeks’ time, bustling about in readiness for the pairing. The stab received as a caterpillar has not cooled his ardour in the very least. I subject him to the test. The attack is laboured and the blow is not clearly struck. No matter: all those attacked perish after a death-struggle lasting a day or two. The previous vaccination has made no difference to the result: they succumbed before and they succumb after. But these are feeble witnesses, on whom it is not wise to rely. I shall achieve more, I feel convinced, with the Hawk-moths and especially with those sturdy subjects the Cetoniæ. Well, the Hawk-moths whose caterpillars have received the virus which theoretically should render them immune retain their normal vulnerability: when attacked by the sting, they succumb instantly, exactly like the others, who did not at the larval age undergo a preventative inoculation.