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Tick species having marked host predilections may occupy the same geographic area as the favorite host or only a certain segment of the host range; for example, H. houyi appears to be present in most areas where its favorite host, the ground squirrel Euxerus erythropus, occurs. On the other hand, the elephant para site A. tholloni is not known from the northern and southern periphery of its host's range, the Varanus lizard parasite A. exorna tum does not follow its host into arid areas, and such hyrax para sites as H. bequaerti and its related species occur in only a restricted area of the host's range.
Distributional data for most argasid species of the world are extremely sparse due to the specialized and laborious techniques necessary for collecting them. Even such a commonplace species as that presently considered to be 0. moubata, long believed to be well understood geographically, is now confused by a mounting body of biological evidence that tends to discredit or at least to modify critically earlier impressions but is yet too limited to be definitive.
The number and variety of hosts an individual species utilizes throughout its life cycle profoundly effects its distribution and density. A single host tick such as B. decoloratus that feeds on easily available herds or groups of antelopes and cattle is spared many of the dangers resulting from being forsaken as a newly hatched larva or a newly molted nymph or adult in an inhospitable environment where it must seek an entirely different type of host for survival. It may thus become locally numerous and also be carried af ar by both wild and domestic hosts under conditions frequently favorable for survival and reproduction. Similarly, a species that utilizes the same type of host for all developmental stages, even though it releases and reattaches two or three times, has advantages over one that cannot survive unless a certain variety of hosts for nourishing the different stages are present.
The frequency with which immature stages of several African amblyommas attack birds probably accounts in part for their relatively wide distribution and population density. In semi arid climes of North Africa and the Near East, where small mammals are rare or localized, the great predilection of immature hyalommas
for birds is most important for the survival and distribution of these species. Bird migrations may be of extreme importance in spreading certain ticks; in Egypt a considerable body of yet un studied data is being amassed on this subject.
The apparent adaptability of some byalommas and rhipicephalids in altering the number and kinds of hosts they require when and if the situation demands is a most important factor where the supply of a variety of hosts is a critical factor of survival, which in turn determines distribution.
The only serious studies on African tick distribution are those of Theiler (1948,1949A,B,1950A,B,C,1956), Theiler and Robin son (1953A), and Theiler and Salisbury (1956), for South Africa. In these, a combination of valuable criteria, i.e., vegetation plus range, mean, and seasonal distribution of temperature and rainfall, are carefully correlated with localities from which ticks have been collected. Vegetation types are shown to have a close kinship to outlines of tick distribution and the same appears to be largely true in the Sudan. These reports should be studied by anyone interested in the geographical distribution of ticks. Theiler frequently mentions uneven distribution, an aspect of the overall picture often most difficult to evaluate because of the complex factors mentioned in the present brief discussion.
For epidemiological and economic purposes tick species to be considered zoogeographically are more numerous than those that may be readily considered academically strictly as indicators of zoogeographic Districts. Domestic animal parasitizing ticks, so important from the stand point of human and animal diseases and numerically so common in observations and collections, are often more widely distributed than they might be in the absence of domestic hosts. Similarly, host specific ticks of wild animals, which are frequently those rapidly disappearing with the advance of civilization and hunters, are becoming more restricted in their contemporary range. In the following discussion an at tempt has been made to strike a functional medium from both the practical and the academic standpoints.
The known Sudanese tick fauna comprises 62 identifiable spe_ cies plus two additional subspecies. It is conservatively esti. mated that twelve to fifteen additional species remain to be dis covered here and that when a future count is made the presently unknown components of the fauna will include approximately equal numbers of northern, eastern, and western species. Of the 64 known forms, ten are either Palaearctic in origin or are tenta tively referred to this Region, one is an introduced American (Neotropical) species, and 53 are Ethiopian.
The hyalommas are Palae arctic in origin and distribution al though they invade a narrow vegetated northern fringe of the Ethiopian Region. H. impeltatum has also gained fairly extensive foothold in the savannahs of West Africa. The first three spe cies listed above conceivably could have reached the far flung areas where they do exist even without the assistance of domestic animals although there is little doubt that these vehicles have greatly facilitated their spread. Details of the distribution of H. detritum and H. marginatum in the Sudan and elsewhere out side of the Palaearctic Region in Africa are vague.
I. vespertilionis and A. persicus probably should be treated as Palae arctic species that have been able to establish themselves as far south as the Cape through bat and human agencies.
Consideration of O. savignyi, A. reflexus, and I. s. simplex as Palaearctic species is entirely tentative. The xerophilic tampan ranges from India to Southwest Africa and its distribution has probably been influenced by common association with camels, except in southern Africa where other factors must be considered. 0. savignyi in the Sudan is confined strictly to arid areas.
The Tat parasite I. s. simplex is known only from a few, scattered Oriental, Palae arctic, and Ethiopian records that defy zoogeograph ical evaluation. The pigeon parasite A. reflexus, probably Palae
arctic in origin though possibly Oriental, is seldom established in tropical Africa; the single population of this species known from the Sudan may already have died out (page 75).
In summary, H. dromedarii, H. excavatum, H. impeltatum, and 0. savignyi might be considered as normal inhabitants of the semideserts or short grass savannahs of the Sudan, although extensive ly distributed by domestic animals. The presence of H. detritum and H. marginatum in the Sudan is inexplicable on the basis of our present information. The two bat parasites, I. s. simplex and I. vespertilionis are not unexpected due to their hosts flight range. 1. persicus and A. reflexus are here as a result of human activities.
Fifty-three of the Sudan's 64 tick species appear to have originated in the Ethiopian Faunal Region and few exist else where except for the now cosmopolitan R. s. sanguineus, A. brumpti that reaches a comparatively few miles into Egypt, and A. variegatum that has become established adventitiously in the West Indies.
The Ethiopian tick fauna of the Sudan is largely East and South African in origin. It appears best to dispose briefly of the uncertain elements and of the small and relatively unimportant West African and montane components of the fauna before considering the bulk of the species.
Several species in the present collection are represented by so few specimens and data that inferences as to manner of occurrence in the Sudan or in certain local areas should be postponed pending further facts. A. reflexus, H. detritum, and H. marginatum have been discussed under Palae arctic species Tpage 848).
The single specimen of A. pompo sum appears to be far from its favored montane habitat and movements of cattle between montane areas known to be infested and Yei are not believed recently to have occurred. This specimen may, however, point
to a limited, undiscovered focus in poorly-explored mountains near Yei.
R. longicoxatus appears to be so patently a rare Somali low land tick that the single specimen from Bahr El Ghazal and others reported from French West Africa are difficult to reconcile with previous information, and their significance cannot be evaluated properly until further specimens and data become available.
The Imatong Area mountain masses (page 859) harbor outliers of the flora and fauna of both the West and of the Bast and South African Subregions that occur almost nowhere else in the Sudan. H. parmata, R. bequaerti, and R. kochi are typical West African Subregion forms found in the Imatong and Didinga Mountains. The few specimens of H. parmata from the plains level around Torit are assumed to have been acquired by their hosts in the vicinity of the single stream reaching this area from the mountains.
The bat parasite I. s. simplex has already been discussed under Palaearctic species (page 848). Two other species, I. schillingsi and I. alluaudi, occur on monkeys and on shrews, respectively, in the Imatong forests and reach here from eastern and southern Africa.
R. compositus, a species of eastern Africa that also extends into limited areas of western and southern Africa, is generally associated with highland or montane areas but attacks a wide variety of game animals not necessarily confined to these zones. The several specimens from the Sudan have been found near moun tains but not actually in them. This species is tentatively categorized as montane on the basis of its known distribution elsewhere.
West African Components
West African savannah species in the Sudan fauna are B. annulatus*, D. c. circumguttatus, R. cuspidatus, R. longus and
*B. annulatus is an introduced American species that in the Ethiopian Faunal Region is known chiefly from the West African Subregion.