The joint of the fore-wing inLymantria is described and compared with the joints of other lepidoptere. 2. The morpho- and histogenesis of the joint elements are investigated. The tegulaanlage can be seen at the end of the last larval period, as a double tissue. 3. After extirpation of parts of the wing-anlage and implantation of this section into the dorsum of the same caterpillar at the 9th day of the last larval stage, the extirpated tissue cannot be replaced. These parts maintain their state of determination when isolated and differentiate the elements lacking in the joint of the butterfly after molting of the imago. 4. The parts of the tegula in the joint and the implanted fragment are measured. The sum of these two measurements is always lower than the 100%-value of the tegula of the opposite, untreated side. 5. By extirpation and implantation the prospective joint elements on the wing disk can be determined. 6. Apart from a short delay resulting from the narcosis and the shock following the operation, the last larval stage is not significantly prolonged.
