This research seeks to discover what changes occur in the sensory periphery and the CNS during the evolutionary transition between earless and eared (hearing) animals. What are the underlying mechanoreceptive peripheral and CNS substrates for hearing? Do new processing circuits arise specifically for hearing and acoustic behaviors? Or do the afferents from the new sensory system link to existing circuits? Insect systems provide an excellent model for studying these questions since the fundamental structures of auditory receptors and ears are remarkably similar in vertebrates and invertebrates. Further, hearing has evolved many times in insects (greater than 14 compared with less than 3 in vertebrates) so there are many parallel cases to compare and contrast in search of fundamental rules for the evolution of a new auditory system. Praying mantises, the focal animal of the project, are unique in possessing a single, ultrasound-sensitive ear in the ventral midline of the body. Previous research has identified the homologs of auditory system components in the mantis closest sister group, cockroaches (without tympanate hearing) and has detailed the nymphal development of the mantis ear. The current project will define the transition in behaviors-from non-acoustic (cockroach; mantis nymphs before hearing develops) to acoustic (adult mantis)-that must accompany the acquisition of hearing. This will be accompanied by an examination of the physiology of auditory internuerons and their homologs as a first step in understanding the CNS circuitry changes underlying the behavioral changes. The project will study the evolution of ontogenetic paths that have led to different auditory capabilities among mantises. It will focus on the developmental contrast between hearing males and deaf females in species with auditory sexual dimorphism, emphasizing the role of juvenile hormone and ecdysteroids. The project seeks to substantiate the hypothesis that many seemingly disparate insect ears have, in fact, evolved independently from exactly the same structures on the insect body. Thus, there are a very limited number of evolutionary hot spots on the body suitable for building an ear. Immunostaining will be used to map chordotonal organ distribution in the abdomen and thorax of a number of tympanate insects to determine exactly which chordotonal organs have become tympanal organs-the expectation is that three or four chordotonal organs out of greater than 90 account for most insect ears. This line of research will lead to a clearer understanding of the neural and non-neural characteristics that define a hot spot for auditory system evolution.