A study of the embryonic development of the Rocky Mountain spotted fever tick Dermacentor andersoni (Stiles) (Acarina, Ixodidae)
The following work describes some phases of the embryonic development of the Rocky Mountain spotted fever tick, Dermacentor andersoni (Stiles), from the time of oviposition until the embryonic development is completed and the larval tick emerges from the chorion. Research into the embryonic development of Dermacentor andersoni was undertaken for several reasons. The problem was of interest to the writer, since as research into the broad field of arthropod embryology, it was felt to be closely related to the writer's previous study and research in the field of insect embryology. It was thought that elucidation of the various phases of tick embryology might later enable comparison with the comparatively well established concepts concerning embryonic development in insects. An investigation into the literature dealing with the embryology of arthropods shows that considerable information has accumulated concerning the embryology of the Acarina. Most of this, however, deals with the embryonic development of the mites, and very little describes embryology as found in the ticks, specifically. In addition to the scarcity of information on this subject, almost all the investigations were done in the latter part of the nineteenth century, and are not readily available. No descriptions of the embryonic development of any member of the genus Dermacentor has been found. The limited information as well as the fact that it is not available without difficulty seems to warrant the present effort. Many members of the order Acarina are of medical and economic importance. Most ticks are vectors of disease and this is true of Dermacentor andersoni. This species of tick is a vector of Rocky Mountain spotted fever, tularemia, Colorado tick fever, and "Q" fever, and is the cause, in human beings and some domestic animals of the little understood condition, tick paralysis. It is known that, in three of the diseases named, the causative organism is transmitted hereditarily from the adult to the larva through the egg. The causative organisms thus transmitted are Dermacentroxenus rickettsi, causing Rocky Mountain spotted fever; Pasteurella tularensis, causing tularemia; and Rickettsia burneti, causing "Q" fever. In order to determine the relationship of the causative organism to the embryonic tissues, knowledge of the embryonic development is required. The Rocky Mountain spotted fever tick is of particular importance in Saskatchewan since the southwestern part of the province lies within its area of distribution. The Tick and Bubonic Plague Survey of the Saskatchewan Department of Public Health reports finding Dermacentor andersoni infected with a highly virulent strain of tularemia in a number of localities. This was found at Govenlock, Maple Creek, Eastend, Shaunavon, and Val Marie in 1950, and at Eastend in 1951 (1). Medical reports also indicate the presence of some Rocky Mountain spotted fever infection in these areas. Since Dermacentor andersoni plays such an important role in the transmission of several serious diseases, and since the causative organisms in several instances is passed on from the adult to the larva through the egg stage, it is of importance that the complete development of the tick Dermacentor andersoni be understood. Dermacentor andersoni requires two full years to complete its life cycle. It passes through three morphological stages--the hexapod larva, the eight-legged nymph, and the sexually mature adult. Each stage requires a mammalian blood meal before moulting. The rather lengthy period required for complete development in the postembryonic life of this tick is in accord with the long embryonic period of six weeks.