Clinal variation explains taxonomic discrepancy in the calling songs of shield-back katydids (Orthoptera: Tettigoniidae: Tettigoniinae: Aglaothorax)

Understanding the relationship between mating trait evolution and speciation requires data from a variety of taxa. I analysed the calling songs of the shield-back katydid genus Aglaothorax, which represents a little-known lineage of acoustic insects. Field-collected males of all described species and most subspecies were recorded under controlled laboratory conditions. Discriminant analysis was 90.5% successful in separating song types, but the song types thus delineated did not correspond to currently recognized taxa as defined by morphology. The discrepancy is caused by two species, Aglaothorax morsei and Aglaothorax longipennis, the songs of which vary in a cline across the Transverse Ranges and the Los Angeles Basin of southern California. From west to east, pulse rate and duty cycle decrease and interpulse interval increases. Selection for species recognition at a contact zone, sexual selection and interactions between signals and the environment are considered as causes of the cline.