A concurrent study of the nesting biology of the solitary bee Osmia (Helicosmia) chalybea Smith (Hymenoptera: Megachilidae: Osmiini) and its cleptoparasite Stelis (Stelis) ater Mitchell (Hymenoptera: Megachilidae: Anthidiini) proposes that their larval cocoons allow gas exchange between ambient air and internal cocoon air and provide a water-vapor barrier, thus enabling the immature bees to avoid desiccation and simultaneously protecting them from invading parasites and predators. The anterior pole of the cocoon of each species has a nipple-like feature that, although structurally different in each case, seems to allow gas exchange while filtering out predators and parasites. The purpose of the current study was to test whether the anterior ends of the two cocoons allow gas exchange whereas the posterior ends are indeed essentially air- and water-vaportight. Results clearly support the assumption that cocoons of both species function as predicted and raise the interesting speculation that cocoons of other bee taxa may serve the same functions.