Some excellent studies have been made by various workers on the morphology of the sclerites of the thorax of insects. Few of these, however, have dealt with the thoracic sclerites of Lepidoptera. Many of the figures of the thorax of Lepidoptera that may be found in our present literature are either very unsatisfactory drawings or are merely included in a general study of all orders of insects with respect to the morphology of this body-region. No American investigator up to now has devoted much study to the different types of development in the lepidopterous thorax.