Gradients in the Insect Segment: The Orientation of Hairs in the Milkweed Bug Oncopeltus Fasciatus

1. During metamorphosis in Oncopeltus the hair-forming cells orient to some unseen influence in the epidermis. The hairs can thus be used as indicators of the polarity of this influence. 2. Excision of the intersegmental membrane can cause a total inversion of the direction of hair outgrowth in the nearby region. 3. Individual insects, each with a large gap in the intersegmental membrane and an associated pattern of hairs which are unusually oriented, occur occasionally in the culture. 4. A study of these defects, and of Locke's observations on the segmental gradient of Rhodnius, leads to the development of a sand model which can be applied not only to the experiments of Locke on the ripple pattern of Rhodnius, but also to those of Piepho on the orientation of scales in Gadleria. 5. The sand model helps in a description of the hypothetical chemical basis of the gradient in the insect segment. 6. The relationship between the gradient and pattern is briefly discussed.