The insect wing is formed from an epithelial sheet that folds during development to establish a saclike tissue with an upper and a lower epithelial monolayer. The adult cuticle formed by the upper and lower monolayers has a distinctive pattern of thickened regions called veins. The venation pattern on the lower surface matches that on the upper surface. As demonstrated by transposition of grafts from the upper monolayer, determination of venation pattern occurs prior to pupation in both wing monolayers. However, the pattern is not expressed until later in adult development. Expression of this determined pattern occurs autonomously in most circumstances. One circumstance in which the pattern fails to be expressed is in pieces of the upper monolayer that are isolated from the lower monolayer before adult cuticle deposition and expression of venation pattern. The only evident interaction between the two monolayers of the wing occurs during a 3-day period, 6-8 days after pupation. During this time, the basal laminae segregating upper monolayer from lower monolayer disappear, and the basal ends of cells form desmosomal junctions at the interface between upper and lower monolayer. Transposition as well as isolation of tissue fragments from the upper monolayer suggest that this interaction between the basal surfaces of the two monolayers is a prerequisite for evocation of venation pattern.