Morphometric analysis of the Haller's organ, a chemoreception 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 unique to ixodid ticks and assists in host seeking behaviors. These behaviors vary among ticks - Ixodes spp. and Dermacentor spp. tend to passively quest while Amblyomma spp. tend to actively seek for hosts. Regardless of host seeking behavior, the Haller's organ function is homologous across tick species and responsible for chemoreception of carbon dioxide 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.  Using Environmental Scanning Electron Microscopy (ESEM), we produced highly detailed images of the organ that yields improved image resolution over methods used in earlier descriptive work (mostly from the 1970s).  Using geometric morphometric techniques and running a procrustes ANOVA, our studies showed 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.  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 mate and reproduce.  Future studies will determine how this variation in receptor structure allows for physiological and ecological differences both within and between tick species.