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approximately equal length; segment 2 arising from pedicle, with straight outer margin, convex inner margin, and subparallel ante rior and posterior margins; segment 3 with lateral margins slightly converging to bluntly rounded anterior margin, posterior margin straight though forming a slight, more or less downcurved projec tion at juncture with inner margin; segment 3 ventrally with short, broad spur reaching basal margin; segment 1 narrow, pediclelike, not observed to bear a ventral knob as in male; palpi with hairs as illustrated. Hypostome larger but otherwise similar to that of male.
Scutum three-fourths as wide as long; outline slightly convex anterior or eyes, slightly converging from eyes to juncture of cervical grooves, abruptly converging posteriorly; anterior emar. gination deep. Cervical grooves reaching lateral margins as in male and delimiting an anterolateral area that is hirsute and somewhat punctate as in male. Surface otherwise smooth except for a transverse row of hairs bordering anterior margin between cervical grooves and a few scattered hairs arising from primctations in cen tral field. Eyes on lateral margins at scutal midlength, slightly convex.
Spiracular plate of similar construction to that of male but differing slightly in outline. Genital apron a wide oval at level of coxae II. Genital grooves subparallel to level of spiracular plates, thence divergent to level of posterior margin of anus.
Integument with numerous fairly regular rows of short hairs on both dorsal and ventral surfaces; hairs more dense on posterior margin between spiracular plates.
Legs similar to those of male except that the free segments are narrower and tarsi more elongate; tarsus IV lacking ventral subapical spur; anterior projection of coxa I variable, as dis tinctly produced as that of male or more truncate.
NYMPH (Figures 352 to 358): Capitulum. Basis capituli three times as wide as long, with straight basal margin, rounded junctures, and divergent lateral margins. Palpi four times as long as wide; segment i forming a slight pedicle; segments 2 and 3 of approximately equal length and subrectangular; apex
more or less bluntly rounded; segment 3 ventrally with a short, wide spur not reaching basal margin of segment. Hypostome similar to that of adults but with smaller corona and 3/3 dentition in files of eight denticles.
Scutum with length_width ratio approximately equal; outline gradually diverging from scapulae to midlength, thence abruptly converging to narrow, bluntly rounded posterior margin. Surface lightly shagreened and with a few scattered hairs; cervical grooves as in adults (not visible in all specimens); eyes small, indistinct or invisible.
Spiracular plate subcircular, with six large goblets in a circle. Integument with long hairs.
Legs with lengthwidth ratio of free segments intermediate between those of male and females sexes; coxa I not so deeply cleft and trimcate anteriorly; tarsi short, robust, not tapered downwards as in adults; dorsal margin gradually tapering; claws and pads as in adults.
It appears well established that continental Africa is the place of origin and center of distribution of Rhipicephalus ticks. of the 46 rhipicephalid species and subspecies that Zumpt (1950A) recognized in his preliminary generic revision, 39 (including the now cosmopolitan R. s. sanguineus) are endemic in the Ethiopian Famal Region; two are Oriental; and five range from southern Europe and northern Africa into Russia.
The genus Rhipicephalus comprises almost a third of the known Sudan tick fauna and contains more than twice as many endemic African species as any other ixodid genus in the Ethiopian Faunal Region.
King (1926) listed five forms of Rhipicephalus from the Sun dan. In addition, R. capensis has been apparently erroneously referred to the Sudan Tzumpt 1942B). During the present study, seventeen species and two additional subspecies, or nineteen different forms, have been discovered in the Sudan.
Zumpt's (1950A) major contribution in bringing together the basic taxonomic data for this difficult genus will facilitate greatly the final revision of rhipicephalid species. Earlier, Theiler (1947,1949B, 1950A,B) had commenced study of individual species, an effort that is still underway (Theiler and Robinson 1953B, Theiler, Walker, and Wiley 1956). Very careful and thorough studies are also in progress by Walker (1956), who has several further reports completed or in an advanced stage of preparation, Since Zumpt's classic preliminary work, Wilson (1954) described a new species, R. hurti, from Kenya and Santos Dias has provided a number of new names, the validity of which are uncertain. Obviously, now criteria must be sought to sep arate many rhipicephalid species and subspecies. In an attempt to apply characters surrounding the female genital aperture, Feldman Muhsam (1952A) has distinguished another species, R. secundus, among populations appearing to be R. s. sanguineus. However, problems of morphological and biological criteria for this genus are not likely to be solved until more exhaustive field research and patient laboratory investigation have been devoted to them.
Study of comparative morphology in the genus Rhipicephalus and of chaetotaxy in the family Ixodidae led Pomerantzev (1936) to reconstruct radially generic concepts of this family. This approach merits further investigation; however, the pitfalls of hasty conclusions based on worn or poorly-preserved field col lected specimens, in which the chaetotaxic picture is imperfect, may result in additional confusion of species concepts (Hoogstraal 19550).
Rhipicephalids tend to considerable variation in appearance and morphological details owing to crowding on the host, welfare of immature stages, and availability of suitable hosts, factors that play a part in the determination of size, robustness, and even certain physical characteristics. Distinguishing characters in many specimens tend to become so generalized that diagnosis is difficult. This is especially true for females. The question of biological races remains to be explored; many data suggest this phenomenon to be operable in certain groups of rhipicephalids. The genus is divided into clearly defined species and species variable enough to cause confusion. It contains extremely common as well as rare species.
Host predilections within this genus are fairly wide among several groups of available animals, although the lack of interest in other animals easily available in the same area is conspicuous by rarity of records of their infestation. A few species, such as R. pravus, have an exceedingly wide host range, being common ly taken on man and all domestic and many feral animals, such as carnivores, antelopes, hares, birds, elephant shrews, elephants, buffalos, and others. Other species, such as R. distinctus from hyraxes, are known only from a single kind of host. It is significant that immatures and adults of most rhipicephalid species do not attack birds and reptiles.
The life cycle is either the two host or the three host type and hosts of immature stages may be either the same as those parasitized by adults or smaller and different animals.
In some species, records of larvae from both cattle and rodents are so common as to confuse the picture of the preferred hosts of this stage. R. appendiculatus is an interesting example in point. In R. S. simus there is so much data indicating immature stage predilection for burrowing rodents that it is disconcerting to find that in certain areas where this tick is common larvae are
frequently found on other animals. Reasons for these differences are beyond our present ability to explain.
Immature stages of many rhipicephalid species remain un described and distinguishing criteria for a number of those that are known are insufficient for identification of field collected material.
Ecological stratification is quite restricted, various spe cies being confined to forests, highlands, semidesert areas, or certain rainfall conditions. The degree and distribution of relative humidity appear to be the most critical of limiting factors. Vegetation types associated with this factor and in fluenced by the length of the rainy season or proximity to moisture laden air beside the seas can often be associated with rhipicephalid distribution.
Economically, many species are of considerable importance as reservoirs and vectors of a variety of animal and some human pathogens. The kennel tick, R. s. sanguineus, has been shown to have a particularly wide spectrum of actual or potential relation ships as a vector of diseases.