Many aquatic insects have no visible respiratory organs. Respiration occurs by simple diffusion of dissolved oxygen across the cuticle and into a closed tracheal system. In many groups, there may be terminal abdominal processes such as a medial filament, cerci, urogomphi, paraprocts, epiproct, etc., but these are primarily sensory in function, rather than respiratory. Dragonfly nymphs (Odonata: Anisoptera) have no external respiratory organs, but have a unique system of internal rectal tracheal gills; externally the abdomen bears short, stout cerci, paraprocts, and epiproct. Larvae of some flies and beetles have no external respiratory processes, but have a pair of prominent terminal abdominal spiracles for obtaining atmospheric oxygen.