Summary.1A comparative account is given of the respiratory adaptations of the larvae and pupae of the family Psephenidae.2The structure of the spiracles of the larvae of the Eulichidae is described.3The structure and function of a simple but previously unknown respiratory adaptation, the spiracular brush, are described.4An account is given of the evolution of the cribriform type of spiracle from the biforous.5The methods of attachment of Psephenid pupae to the larval cuticle and to the substrate are noted.6Abdominal gin‐traps in the pupa of one of the Eubriinae and in the larva of the same species are described. The protective devices called gin‐traps have not previously been recorded in insect larvae.7Instances in the Psephenidae of the mimicry of the larva by the pupa of the same species are noted. These appear to be the first recorded instances of mimicry amongst aquatic insects.8A key to the immature stages of the subfamilies of the Psephenidae is given.9Afropsephenium terricolaPaulian is shown not to belong to the Psephenidae or the superfamily Dryopoidea.10A new definition of the family Dryopidae is given.11It is shown that the larvae of the aquatic Dryopidae are terrestrial and not aquatic, and that this group and the Hydraenidae are apparently the only exceptions among insects to the rule that if the adults are aquatic the larvae are also aquatic but terrestrial adults may have aquatic lar