The egg, larva, and pupa of Caurinus dectes are described and illustrated. Eggs are ellipsoidal, coated with black cement, and are attached to bryophytes. The larva is orthosomatic, with much-reduced thoracic legs, and further differs from other larval Boreidae in: amphipneustic spiracles, number of stemmata (7), placoid sensilla present in membrane of first antennal segment, and egg burster on frons in first instar. Three larval instars are recognized from analysis of head measurements and partial rearing. Larvae feed in stemmines or galleries in leafy liverworts (Jungermanniales). Pupation occurs in a silk-lined cell or cocoon, otherwise unknown in the Mecoptera; the pupa is exarate, with decticous mandibles. The seasonal occurrence of egg eclosion (February-March), pupation (July-August), and principal adult activity (October-April) is typical for the Boreidae, but only C. dectes is univoltine in this family. Successive annual generations appear to overlap through extended egg diapause and adult perenniation.