Development in Marmara arbutiella Busck is hypermetamorphic, with 3 behaviorally and morphologically distinct larval forms. There are 6–8 sap-feeding and 2 nonfeeding, structurally differentiated instars. The early instars are legless, dorsoventrally compressed, prognathous sap feeders. The 1st of the nonfeeding instars, which we call the transition instar, never issues from the cuticle of the previous (feeding) instar; instead there is apolysis without ecdysis. It is characterized by reduction—the cuticle is transparent, the feeding structures are nonfunctional, and the legs are vestigial. Moreover, the stadium is ephemeral, lasting no ≤24 h. The 2nd nonfeeding stage is a fully legged and ambulatory instar, with rudimentary feeding structures and a functional spinneret. Upon issuing from the mine, this instar spins a cocoon that is elaborately decorated with clusters of 31–114 pearly bubbles that are extruded from its anus and then individually attached to the exterior of the cocoon. We contrast the unique development of Marmara with other gracillariids and interpret the transition instar as an ontogenetic bridge, a stage of risk and metabolic cost without function.