Structural analysis of the haller's organ sensor found in males and females of the hard ticks (Acari: Ixodidae): Ixodes scapularis, Amblyomma americanum, and Dermacentor variabilis

The Haller's organ is a sensory structure that is unique to ixodid ticks and assists in host seeking behaviors. Previous research demonstrates that this organ functions as a hygrometer and is responsible for chemoreception of carbon dioxide, ammonia, and other chemicals such as pheromones.  Presented here are the results of a detailed comparative study of the morphology of the Haller's organ in three important North American tick species: Ixodes scapularis, Amblyomma americanum, and Dermacentor variabilis.  We looked for possible differences in morphology both between and within these species and between males and females for each species.  Our imaging method, Environmental Scanning Electron Microscopy (ESEM), provides a highly detailed image of the organ that yields improved image resolution over methods used in earlier descriptive work (mainly from the 1970s).   Using geometric morphometric techniques, our studies show high levels of intraspecific, within-sex variation in the morphology of Haller’s organ as well as between species and sexes. We hypothesize that there are differences in the Haller’s organ between species because of different host seeking behaviors -  I. scapularis and D. variabilis tend to passively quest while A. americanum tends to actively seek for hosts.  We also hypothesize that the Haller’s organ of males and females of the same species differ in morphology because males also detect the females’ pheromones to find mates and reproduce.  Future studies will determine how this variation in receptor structure allows for physiological and ecological differences both within and between tick species.