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The xeric environment in which 0. moubata is capable of sur. vival is best explained by two physiological studies by Lees (1946A, 1947). In his research on water balance in ticks, Lees (1946A) found that among the species studied, 0. moubata shows the greatest ability in limiting evaporation from its own body. In this species, the critical temperature at which water loss increases through the superficial waxy epicuticular layer is also high (Lees 1947). This resistance to desiccation at temperatures within its biological range may be correlated broadly with the argasids' choice of dry, dusty ecological niches.
Lees summarized his 1946A studies, in which Ixodes ricinus was the principal species for research and 0. moubata was one of eight other species used for comparative purposes, as follows:
"The unfed tick gains water from humid air or from water in contact with the cuticle, and loses water by evaporation. Whilst attached to the host the tick is gaining water from the ingested blood and losing water in the excrement. The engorged tick usually lacks the ability to take up water from humid air.
leThe exchange of water takes place mainly through the cuticle. Regulation of the water balance is there fore brought about by the activity of the epidermal cells.
"The cuticle comprises two principal layers, the
water loss from the unfed tick is not closely related to saturation deficiency, particularly at high
humidities. This departure is due to a physiological cause, namely, to the ability to secrete water. The effects of this activity are such that a state of equilibrium is attained at a relative humidity of about 92%; at lower relative humidities it takes up water. The retention of water at humidities below the point of equilibrium is due not only to the physical properties of the epicuticle but also to this secretory activity, for water loss increases when the tick is temporarily asphyxiated, poisoned with cyanide, or injured through excessive desicca tion. Near the point of equilibrium the loss or gain of water over a wide range of temperature is determined by the relative humidity.
iuThe uptake of water from humid air occurs when the tick is in a desiccated condition but ceases as the normal water content is restored. After previous exposure to saturated air the adapted tick at first loses water at relative humidities above the point of equilibrium, but later comes to retain water com pletely.
14Both unfed and engorged ticks possess the abil ity to prevent or to limit temporarily the entry of water in contact with the cuticle.
"The engorging female, originally weighing about 2 mg., ingests about 600 mg. of blood. About 300 mg. or two-thirds of the contained water are usually eliminated before the end of engorgement. Evapore tion from the cuticle may account for a considerable fraction of this, for the temperature to which the attached tick is exposed (about 3700.) is, in Ixodes ricinus, above that temperature at which a marked increase in the permeability of the epicuticular lipoid takes place.
The nine species of ticks examined differ con siderably in their powers of limiting evaporation. This may reflect specific differences in the nature
of the epicuticular lipoid. The order of their resistance
As already stated, Lees has shown that 0. moubata is more re sistant to desiccation than most ixodid ticks. Nymphs exposed to dry (0% R.H.) ar at 25°C. survived for 35 days and lost only from one to three percent of their original weight daily. This survival period is strikingly longer than that of several ixodid tick spe_ cies used in the experiments. After a period of desiccation (five days at % R.H.), o. moubata regains most of its original body weight when placed in 95% R.H. for five days. Water is taken up through the spiracles, for no increase occurred when these open ings were blocked. Loss of water occurs through the cuticle and spiracles (see Spiracular Morphology and Function below).
In order to carry Lees' work one step further, Browning (1954B) conducted a study on the exchanges of water between the atmosphere and 0. moubata. Unfed nymphs were able to abstract water from moist air 795% R.H.) and to restrict their rate of water loss in dry air. This ability was lost (a) in atmospheres containing 30% to 45% CO2; (b) in atmospheres containing more than 90% N2; (c) immediately after the tick fed; and (a) gradually after the tick has been starved for some five months. It was shown that the action of high (30% to 45%) concentrations of CO2 is mainly upon the activity of the epi. dermal cells, possibly mediated through the central nervous system. The concentration required to cause opening of the spiracles is only about five percent. These findings are of considerable interest in relation to Lees(1947) basic work.
"In considering the mechanisms involved in the exchange of water through the cuticle the assumption was made that, in addition to active secretion, the passage of water, and particularly its retention, is also in fluenced by the presence of lipoid material in the cuticle. Ticks show great diversity in their powers of resisting desiccation, and this was thought to be accounted for by the specific nature of the waterproofing lipoid. Nevertheless, no direct evidence of such a component was ad vanced in this paper (i.e., Lees 1946A ).
"Ramsay (1935B), and more recently Wigglesworth (1945) and Beament (1945), have shown that the impermeability of insects is entirely due to a thin, discrete layer of wax or oil in the outermost part of the epi. cuticle. Any agents such as abrasive dusts, wax sol vents, or detergents, which interrupt the continuity of this layer, at the same time greatly increase transpire tion, Water loss through the wax layer is also enormously increased if the temperature is raised above a certain critical value. ... methods devised by Wiggles. worth for demonstrating the properties of the waterproofing layers in insects have been applied to a number of species of ticks. ...... observations on the structure and deposition of the epicuticle, and on the functions of the dermal glands (are provided). The outermost layer of the tick cuticle visible in ordinary sections has hitherto been referred to as the "tectostracum" (Ruser 1933)
(but) the similarity of this layer with the insect epicuticle is so marked that the abandonment of this term seems fully justified.'
The results and conclusions of this work, lees summarized as follows:
"1. Ticks owe their impermeability primarily to a superficial layer of wax in the epicuticle. After exposure to increasing temperatures, water loss increases abruptly at a certain critical temperature. The critical temperature varies widely in different species, in Ixodidae ranging from 32°c. (Ixodes ricinus) to 45°c. Hyalomma marginatum (= savignyi]; and in Argasidae from 6300.
(Ornithodoros moubata) to 7500. O. 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.
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.
113. 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 in hibited 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.
In Argasidae the epicuticle consists of four layers: the cuticulin, polyphenol, wax, and outer cement layers. Only the three inner layers are present in Ixo. didae. 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 0. moubata the cement is poured out by the dermal