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(Ornithodoros moubate) to 75°C. (C. savignyi). Species having higher critical temperatures are more resistant to desiccation at temperatures within the biological range. A broad correlation is possible between these powers of resistance and the natural choice of habitat. Argasidae infest dry, dusty situations whereas Ixodidae occupy a much wider variety of ecological niches.

"2. If the tick cuticle is rubbed with abrasive dust, evaporation is enormously increased. Living ticks partially restore their impermeability in moist air by secreting wax from the pore canals on to the surface of the damaged cuticle.

"3. Unfed ticks are able to take up water rapidly through the wax layer when exposed to high humidities. Water uptake, which is dependent on the secretory activities of the epidermal cells, is completely inhibited by the abrasion of only part of the total cuticle surface - a fact which suggests that the cells are functionally interconnected. Resistance to desiccation at low humidities is achieved by a dual mechanism: active secretion and the physical retention of water by the wax layer.

"4. In Argasidae the epicuticle consists of four layers: the cuticulin, polyphenol, wax, and outer cement layers. Only the three inner layers are present in Ixodidae. Since the wax layer is freely exposed in the latter group, chloroform and detergents have a marked action in increasing transpiration, particularly in those species with low critical temperatures. In Argasidae the cement layer is very resistant to extraction but is broken down by boiling chloroform.

"5. The cuticulin, polyphenol, and wax layers are all secreted by the epidermal cells. The waterproofing layer, which is deposited on the completed polyphenol layer, is secreted by the molting tick relatively early in development and may be nearly complete by the time molting fluid is abundant. In Q. moubata the cement is poured out by the dermal

glands shortly after emergence. In Ixodidae the dermal glands undergo a complex cycle of growth and degeneration, but their products appear to add nothing of functional significance to the substance of the cuticle."

Lees' important contributions indicate why O. moubata is capable of surviving in the dry niches in which domestic populations occur. However, we still lack data on the actual relative humidity of these niches in nature. We know only that the tampan can withstand these conditions in laboratory investigations. And it should be stressed that we still know nothing about preferences and critical levels of temperature and humidity among burrow-haunting populations. The Bahr El Ghazal collections, from warthog burrows in the "Nile sponge area", especially excites curiosity in this respect.

Laboratory studies on the optimum temperature and humidity conditions under which 0. moubata survives have resulted in widely differing data and conclusions. The reports in question are those of Cunliffe (1921) and Brett (1939) together with those of Robinson (1942C) and others already reviewed in the section on the life cycle of Q. moubata.

Cunliffe found that a saturated atmosphere has no inhibitory influence on molting but is decidedly unfavorable for vitality (only one specimen passed the third nymphal stage under these conditions). Even under "medium conditions of humidity", mortality is high, but under "dry conditions", 66% of the nymphs complete metamorphosis and the rate of development is increased. High temperature increases the number of eggs laid but decreases fertility, longevity, and time required for metamorphosis.

Brett, on the other hand, found that (at 25°C.) higher relative humidity (up to 80%) was more favorable for survival of eggs, larvae, and first instar nymphs (the only stages and instars tested) though a proportion of all eggs were able to develop at any "low humidity normally met with in nature". He also found that the first nymphal instar is much more resistant to desiccation than larval and egg stages. The apparent inconsistency between Brett's findings and the known fact that domesticated populations of O. moubata are chiefly inhabitants of drier areas

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Figure 56
Q CRNITHODCROS MOUBATA INTERNAL ORGANS
ATAfter Burgdorfer (1951), with permission of
the editor of ACTA TROPICA 7

PLATE XIX

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Spirochetes of African tick-borne relapsing fever, Borrelia duttonii, are illustrated, as short wavy lines, in the positions they occupy in the tick's body. Note their escape routes from the tick's body and into the host's body while the tick is feeding.

ORNITHODOROS MOUBATA DIAGRAMMATIC SAGITTAL VIEW

ATAfter Burgdorfer (1951), with permission of
the editor of ACTA TROPICA 7

PLATE XX

- 158

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is explained on the basis of Williams' (1923, 1924A,B) and Buxton's (1932, 1933) exposition of the comparatively high humidity in sand, cracks of walls, and soil in areas that are otherwise dry. Brett's discussion and the comparison of his findings with those of Cunliffe and of other workers, especially those of Robinson (1942C) discussed on p. 137, which corroborate those of Brett, should be studied for their practical importance by anyone concerned with O. moubata. Since only careful and thorough research in the field as well as in the laboratory can conclusively settle the matter, a more complete discussion of this question is hardly in order here.

Structure and Function
Introduction

No thorough studies of the internal anatomy and histology of O. moubata have been undertaken. What has been done on certain aspects of these subjects is reviewed in the following paragraphs. On the whole, workers have been content to accept Christophers" (1906) careful though still somewhat general description of the internal anatomy of O. savignyi as also applicable to O. moubata. Recently, Burgdorfer" (1951) # provided a short account of the internal anatomy of O. moubata and some of his excellent illustrations are reproduced (Figures 56 to 58). However, O. moubata deserves more specialized attention than it has thus far been accorded. These two species differ in habits, habitats, distribution, and receptivity to pathogenic organisms. It may be expected, therefore, that under their leathery shells, which also differ, significant anatomical and physiological differences remain to be demonstrated.

Internal Anatomy

The general features of the internal anatomy of these two species are similar and Christophers' (loc. cit.) description of

a dissection of 0. savi as present low, applies equally well to Q. moubate # #: differences noted):

"Over the whole dorsum lies a fine membranous expansion of tracheae and trabeculae of the fat body. Lying in this, in the median line, is the delicate tubular heart. Posteriorly, at about the junction of

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