The operation of the larval labium in dragonflies is reviewed and compared with other fast actions in other arthropods. The dragonfly labium is similar to most of these in that a locking mechanism allows energy storage prior to the action, but differs in that its successful operation requires coordinated and rapid operation of two joints, not just one. The locking mechanism resides in the primary flexor muscle of the labium, which alone appears to allow energy for the operation of both joints to be stored and prevents the labium from being protracted during jet propulsion.