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“In considering the mechanisms involved in the ex. change 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. Never. theless, no direct evidence of such a component was ad. vanced in this paper (i.e., Lees 1946A).

“Ramsay (l935B), and more recently Wigglesworth

(1945) and Beament (1945), have shown that the imper.
meability 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 transpira.
tion. Water loss through the wax layer is also enor.
mously increased if the temperature is raised above a
certain critical value. ...... nothods 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 depo-
sition 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 hither.
to been referred to as the mtectostracum" (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:

ml. Ticks owe their impermability primarily to a superficial layer of wax in the epicuticle. After expo. sure 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. 1;

alomma marginatum Q: savignyi) 7; afid in Argasidae from 635C.

(0rnithodoros moubata) to 75°C. (Q. 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 huidities. 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 suface _ a fact which suggests that the cells are func_ tionally 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 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 temperatues. In An. gasidae the cement layer is very resistant to extraction but is broken down by boiling chloroform.

“S. The cuticulin, polyphenol, and wax layers are all secreted by the epidermal cells. The water. proofing 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

[graphic]

glads shortly after emergence. In Ixodidae the dermal glands udergo 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 0. moubata is capable of surviving in the dry niches in which domestic popu. lations occur. However, we still lack data on the actual rel. ative humidity of these niches in nature. we know only that the tampan can withstand these conditions in laboratory investiga_ tions. Ad it should be stressed that we still know nothing about preferences an critical levels of temperatue and humidity among burrow_haunting populations. The Bahr El Ghazal collections, from warthog burows in the “Nile sponge area", especially excites curiosity in this respect.

Laboratory studies on the optimum temperature ad humidity conditions under which 0. mouba a survives have resulted in wide_ ly differing data and conclusion . The reports in question are those of Cunliffe (1921) and Brett (1939) together with those of Robinson (19426) and others already reviewed in the section on the life cycle of Q. moubata.

[graphic]

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“, mor_ tality is high, but under mdry conditions“, €£% 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 hami, found that (at 25%.) higher rel. ative humidity (up to 8Q%) was more favorable for survival of eggs, larvae and first instar nymphs (the only stages and in. stars tested) though a proportion of all eggs were able to dev.. elop 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 inconsist_ ency between Brett's findings and the known fact that domesticated populations of Q. moubata are chiefly inhabitants of drier areas

[graphic]

‘I

4- =,;:Q\r\Gene's orgor;-;L‘>*s"',()esophogus

L

Central qonglionp

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*4 A , ' ‘Q , .9. Com! organ ‘\ _ e = \§\\ Accessory _ /. I Q|9d_-____ -- 7 ‘ _Sc1liv0ry Filter A Qlflnd chamber, I ' f ' 0 "<1 Cf » 1‘ v: u - » " ' if /1 i Stornoch and 5 ‘ diverticula /’ , Molpighion ‘A fubulé Recmi\ OrnpU"O

Figure 56
Q CRNITHODCROS MOUBATA INTERNAL ORGANS

[After Burgdorfer (19515, with permission of
the editor of ACTA 'l‘ROPICA_7

PLATE XIX

[graphic][subsumed][merged small][merged small][merged small][merged small]

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.

ORNITHOOROS MOUBATA DIACRAMMATIC SACITTAL VIEW

ZrAfter Burgdorfer (1951), with permission of
the GOILOI Of ACTA TROPICAL7

[graphic]

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