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addition to such extrinsic environmental factors as physical barriers, temperature, and humidity. For ectoparasites that commonly attack both wild and domestic animals a double set of values needs be employed.
The distribution of parasites that utilize widely differing hosts in each developmental stage is effected not only by the physiological range tolerable to the ectoparasite but also by that tolerated by the several types of hosts required to complete the life cycle. In families of insects consisting of many species certain basic distributional patterns are more easily discerned than in the family Ixodidae composed of relatively few species variously adapted to a wide range of environments and hosts.
In Africa and in the Sudan in particular, where both domestic animals and many wild animals make long migrations in search of water and grazing, particular caution should be exercised in geographic evaluation of collections consisting only of male ticks, which remain attached to the host for many months. Males may be unaccompanied by females, which generally feed for four to ten days, either because the reproductive season is not yet at hand or because they have been transported far beyond their normal range by hosts wandering in search of food and water. When outside their normal geographic range, these unassociated males are not only false zoogeographic indicators but of reduced epidemiological significance since they do not normally leave the host and attach to another. Inasmuch as domestic animal parasitizing ticks are frequently those also directly or indirectly associated with human diseases these considerations assume additional importance.
The wide disparity in size and beauty between different species of ticks as well as in degree of exposure or concealment of feeding sites on the host may give a false distributional picture when collections are gathered haphazardly and by nonspecialists.
It goes without saying that in great areas of the world and especially in the Sudan, remoteness of considerable areas and lack of general and specialized interest by collectors may often result in skewed data or samplings that are far from homogenous.
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, #. ground squirrel Euxerus erythropus, occurs. On the other hand, the elephant parasite A. thoIIoni is not known from the northern and southern periphery of its host's range, the Varanus lizard parasite A. exornatum does not follow its host into arid areas, and such hyrax parasites as H. bequaerti and its related species occur in only a restricted area o e 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 afar 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 semiarid 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 unstudied data is being amassed on this subject.
The apparent adaptability of some hyalommas 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 Robinson (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
be readily considered academically strictly as indicators of zoogeographic Districts. Domestic animal parasitizing ticks, so important from the standpoint 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 attempt has been made to strike a functional medium from both the practical and the academic standpoints.
The known Sudanese tick fauna comprises 62 identifiable species plus two additional subspecies. It is conservatively estimated that twelve to fifteen additional species remain to be discovered 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 tentatively referred to this Region, one is an introduced American (Neotropical) species, and 53 are Ethiopian.
H. dromedarii A. rsicus
#: excavatum # reflexus
H. # 7.I. s. simplex
H. marginiatum I. Vespertilionis
The hyalommas are Palaearctic in origin and distribution although 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 species 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 outside of the Palaearctic Region in Africa are vague.
I. vespertilionis and A. persicus probably should be treated as Palaearctic species that have been able to establish themselves as far south as the Cape through bat and human agencies.
Consideration of 0. 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. O. savignyi in the Sudan is confined strictly to arid areas. The Bat Parasite I. s. simplex is known only from a few, scattered Oriental, Palaearctic, and Ethiopian records that defy zoogeographical evaluation. The pigeon parasite A. reflexus, probably Palaearctic 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 9. savignyi might be considered as normal inhabitants of the semideserts or short grass savannahs of the Sudan, although extensively distributed by domestic animals. The presence of H. detritum and H. # 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.T.A. 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 elsewhere except for the now cosmopolitan R. s. sanguineus, A. brumpti that reaches a comparatively few miles # Egypt, and # variegatum that has become established adventitiously in the West #.
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. # have been discussed under Palaearctic species
The single specimen of A. pomposum 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