Transpiration and the Structure of the Epicuticle in Ticks

Ramsay (1935), 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 epicuticle. Any agents such as abrasive dusts, wax solvents or detergents, which interrupt the continuity of this layer, at the same time greatly increase transpiration. Water loss through the wax layer is also enormously increased if the temperature is raised above a certain critical value. In the present paper the methods devised by Wigglesworth for demonstrating the properties of the waterproofing layers in insects have been applied to a number of species of ticks (Acarina; Ixodoidea). The account also includes observations on the structure and deposition of the epicuticle, and on the functions of the dermal glands. The outermost layer of the tick cuticle visible in ordinary sections has hitherto been referred to as the ‘tectostracum’ (e.g. Ruser, 1933). As this paper will show, the similarity of this layer with the insect epicuticle is so marked that the abandonment of this term seems fully justified.

Ramsay (1935) and Wigglesworth (1945) have shown that if whole insects with their spiracles covered are exposed for short periods to different constant temperatures, transpiration at first increases slowly as the temperature is raised, then much more abruptly when a certain temperature is exceeded. At this ‘critical temperature’, which varies widely in different insects, the waterproofing layer becomes more permeable to water (Beament, 1945).

Similar experiments have been carried out with a series of ticks of different species. The technique employed has already been described (Wigglesworth, 1945). Small batches of ticks, usually from two to six, were killed in ammonia vapour and their spiracles covered. They were then placed in a container of metal gauze which could be suspended over phosphorus pentoxide in a conical flask immersed up to the neck in a large water-bath. The temperature was then raised by intervals of about 10° C. and maintained constant for 30 min. at each temperature. After each exposure the ticks are weighed to determine water loss.

The rate of transpiration is expressed in mg./sq.cm, of surface/hr., and the results are therefore directly comparable with those of Wigglesworth. The surface areas of examples of each species have been estimated by cutting up the cuticle and either simply spreading the pieces on squared paper (engorged ticks) or by making measured drawings of the pieces with the aid of a camera lucida (unfed ticks). Since all the ticks were of approximately similar Shape, two curves, one for unfed and the other for engorged ticks, could be constructed relating weight and surface area. Such a measure of evaporation implies that it is the apparent surface area and not the minute surface irregularities which, under the conditions of the experiment, are important in determining water loss. This assumption can easily be justified. During engorgement ixodids become enormously distended and the deep folds of the epicuticle are flattened out; nevertheless, there is, of course, no increase in the total surface area of the epicuticle, for this layer is completely inextensible. The relationship between apparent surface area and evaporation during engorgement is illustrated by the following example.

Six days after attachment unfed females of Ixodes ricinus, after removal from the host, had a mean surface area of 0-59 sq.cm, and lost water in dry air at 25°C. at the rate of 11·5 mg./sq.cm. during 24 hr. The average weight was 39·5 mg. Immediately after dropping from the host (8th day after attachment) the average weight was 311 mg., the surface area 1·87 sq.cm, and the rate of water loss 11·1mg./sq.cm.

The rates of water loss were first determined at a single temperature (25° C.) and with a series of living ticks representative of the families Argasidae and Ixodidae. The results obtained by exposing unfed and engorged ticks for 24 hr. to dry air are given in Table 1. The species form a graded series ranging from the least resistant member I. ricinus to the most resistant member Ornithodorus delanoei acinus. In the former species water loss is approximately fifty times more rapid than in the latter. Ixodid ticks, it will be noted, are consistently less resistant than argasids, although species such as Hyalomma savignyi approach the degree of impermeability characteristic of the Argasidae.

Table 1 also shows that unfed ticks lose less water per unit of surface area than do the corresponding engorged stages. This difference, usually about fivefold, is too large to be explained by errors in determining surface area and is probably due to the ability of the unfed ticks to oppose evaporation by secretion. As was previously shown (Lees, 1946) the engorged tick usually lacks these powers. In the following experiments dead engorged ticks have been used almost exclusively.

The rates of transpiration in a series of ticks exposed to different temperatures are shown in Fig. 1. The approximate ‘critical temperatures ‘and—as a more accurate standard of comparison—the temperature at which evaporation into dry air equals 5 mg./sq.cm./hr., have been read from the evaporation curves and are set out in Table 2.

All the species investigated fall into two distinct groups; moreover, what is particularly significant, the groupings correspond with the systematic position. In Ixodidae a sudden increase in transpiration takes place at a temperature which ranges from 32 to 45° C. In all Argasidae, on the other hand, the critical temperature is at least 15° C. higher and varies in different species from 62 to 75° C. The break in the evaporation curve is much less abrupt in the Argasidae, particularly in the higher members. Within each major group significant specific differences can be distinguished. In the Ixodidae there is a relatively compact subgroup including the more resistant species of Ixodes, and the genera Amblyomma, Haemaphysalis, Dermacentor, Rhipicephalus and Hyalomma. Ixodes ricinus emerges as an exception among the Ixodidae, having the low critical temperature of 32°C. In the argasid group there is little difference in the evaporation curves of Argos persicus and species of Omithodorus such as O. moubata. O. delanoei, however, has a critical temperature higher by about 5° C., and O. savignyi by over 10° C.

A comparison of the results given in Table 1 where the ticks were desiccated for a longer period at a temperature well below the critical temperature, with the series given in Table 2 based on the evaporation curves, shows that the two arrangements correspond closely. As in the case of insects, therefore (Wigglesworth, 1945), those species most resistant to desiccation also have the higher critical temperatures.

The effect of temperature on transpiration through the cuticle of the unfed tick has been tested in Dermacentor andersoni. Fifteen unfed female ticks were killed in ammonia vapour, desiccated in the usual manner and weighed together. The resulting evaporation curve was almost identical with that of the engorged ticks of the same species. Clearly, therefore, a waterproofing layer with similar or identical properties is present in the unfed stage.

Different species of ticks occupy a wide variety of ‘ecological niches’, and their powers of resisting desiccation are evidently adapted to meet the conditions normally encountered in their environment. All the species considered here leave the host after each blood meal* and are therefore exposed during most of their life cycle to the microclimatic conditions prevailing in the locality where they are dropped. The Argasidae, mainly nocturnal feeders, usually infest the host’s burrow or nest. Ixodid ticks, which remain attached to the host for several days, are sometimes confined to similar haunts, but when the host is a wandering animal they tend to be dropped more or less at random over a wide range of territory. The type of vegetation and the microclimatic conditions it affords then becomes of considerable importance. There can be little doubt that comparatively small differences in transpiration will be of great significance from the point of view of distribution and ecology. The following notes illustrate the range of habitats-occupied by some of the species under discussion: they may be considered in relation to the results summarized in Tables 1 and 2.

Owing to its susceptibility to desiccation, Ixodes ricinus, a common parasite of hill sheep in Britain, can survive only on rough hill and moorland grazings which provide a permanently moist microhabitat (MacLeod, 1936; Milne, 1944). The character of the vegetation varies, but in northern England the tick infests poorly drained grazings with rough grasses, bracken and rushes which, on dying back, yield a thick ‘mat’ of moist debris overlying the soil. The damp spongy cover afforded by heather associated with mosses also forms an ideal habitat. In Norway, I. ricinus is abundant only along the seaboard, on wet hillsides with deciduous woods and alder bushes or near the coast over swampy ground supporting junipers and heather (Tambs-Lyche, 1943).

Under natural conditions the European species I. canisuga is probably confined to the earths and burrows of mammals such as foxes and badgers and of birds like the sandmartin (Milne, 1947; Nuttall, Warburton, Cooper & Robinson, 1911). However, a particularly favourable, if secondary, host is the sheep dog, and the kennels of these animals are often heavily infested. In northern Britain the ‘kennels’ are often disused stables, and in a typical example in which meteorological records were kept, the humidity conditions were found to be much more adverse than are those of the moist microclimate in which I. ricinus lives (unpublished observations). But I. canisuga has correspondingly greater powers of resistance.

Amblyomma americanum is also a relatively susceptible species. In Georgia and South Carolina where the chief hosts are deer and cattle, the tick occurs principally in the wooded coastal areas and particularly in the wetter parts of these areas.* Infestations are heaviest on small islands off the coast supporting woodland and salt marsh; and on the mainland they are heaviest near streams and in woodland which is lowlying and swampy. Ticks are more numerous in wooded areas with a dense stand of underbrush and a thick mat of decaying vegetation covering the soil than in localities where the brush is reduced and the mat absent.

Haemaphysalis punctata occurs in south-eastern England (Kent) as a parasite of sheep and other domestic stock. The tick is normally found in lush meadowland near the coast, but engorged ticks dropped by animals on temporary leys, which may have only a thin growth of grass affording little protection, can survive and develop for at least one season.

The three species of Dermacentor show somewhat greater powers of resistance. Their environment is similar in many ways. In localities of western England where the European species D. reticulatus is common, the tick is found infesting such places as the grassy verges of cliff tops, overgrown orchards or meadows with long permanent grass. The habitat of the American dog tick D. variabilis in Massachusetts is described by Smith, Cole & Gouck (1946). It occurs abundantly amongst beach grass along the shore, amongst roadside vegetation, and over rough ground with thick tangled grass and blackberry. Thick grass provides excellent cover for the meadow mice which are the hosts of the larval and nymphal stages. Typical situations for D. andersoni in the Bitter Root Valley of Montana are rocky scrub-covered slopes supporting a large rodent population (Cooley, 1932). In any of these localities the ground may become very parched during the summer months. The adults of D. variabilis are carried principally by dogs and are therefore sometimes dropped inside houses. Nevertheless, unlike Rhipicephalus (see below), this species does not survive in buildings unless high humidities are artificially maintained.*

The distribution of R. sanguineus in warm climates is cosmopolitan. Throughout the United States this species occurs principally in houses or other buildings where dogs are kept.* In the northern States the ticks survive in winter only if the buildings are kept heated. The tick can thus maintain itself in spite of the ensuing low humidities. In the southern States Rhipicephalus may infest yards as well as houses. In such cases the yards may be bare of grass or with merely a sparse covering of grass; yet in spite of exposure to the full drying effect of the sun the ticks survive readily.

The genus Hyalomma is predominantly African and Asiatic and is among the most resistant of the ixodid genera, several species extending into regions where very dry or even desert conditions prevail. H. savignyi is abundant in the settled regions of Palestine such as the Esdraelon Valley where cows are raised. In addition to infesting the five-stock enclosures in this district, some unfed ticks which have dropped as the engorged stages from camels and other animals may be found running in the heat of the sun on sand dunes and over the surface of the desert near the Dead Sea.*

The Argasidae are much more richly represented in warm than in temperate climates, and even such species as extend into cooler regions choose dry, dusty habitats. The following are typical examples: the African species Omithodorus moubata infesting the floors and walls of native dwellings; the North American species O. parkeri recovered from the nests of the burrowing owl, Speotylo (Jellison, 1940); Argas persicus, widely distributed in warm countries and a notorious pest of the poultry roost.

Finally, the exceedingly resistant Argasidae, such as Ormthodorus savignyi and O. delanoei acinus, exhibit an even greater degree of xerophily. O. savignyi, a species widely distributed in Africa, Arabia and elsewhere, is commonly encountered partly buried in loose dry sand near the resting places of caravans or in places where camels have been quartered. The very large tick, O. delanoei acinus, has been found in a similarly arid environment (Whittick, 1938).

In insects impermeability is conferred by a film of wax or oil in the epicuticle, and if this layer is interrupted by rubbing the insect with an inert abrasive dust, such as alumina, water loss through the cuticle is enormously increased (Wigglesworth 1945). The same method has been applied to ticks with the object of demonstrating the waterproofing layer.

Table 3 shows the results obtained with three species. In each case the dorsum of the tick was abraded by drawing it several times over a length of filter-paper covered with alumina dust. It can be seen that this treatment greatly increases transpiration, both in unfed and engorged ticks.

After abrasion with the dust the epicuticle appears undamaged, but in the engorged stages of some ticks (particularly in Ixodes canisuga) the cuticle is so smooth that the abraded areas appear as glassy patches from which the waxy bloom has been removed. These areas soon blacken as water evaporates and the gut, which is filled with darkened blood, begins to adhere to the epidermis.

In most insects the presence of alumina in contact with the cuticle does not lead to increased transpiration unless there is actual abrasion; in the cockroach, however, merely sprinkling the dust on the body causes the mobile waterproofing oil to be adsorbed and thus increases water loss. Experiments in which ticks were suspended or otherwise prevented from moving showed that even if the dorsum was completely covered by dust there was no significant increase in evaporation. The following are examples of the results. In dry air at 25° C. an unfed female Dermacentor andersoni lost 1·4% of the original weight per diem before dusting and i’3% after dusting. Comparable values for an engorged female Ixodes ricinus were 7·5 and 9·6%, and for Omithodorus moubata 0·6 and 0·7%. This evidence therefore suggests that even in Ixodes ricinus, a species with a very low critical temperature, the waterproofing agent is a solid wax and not a mobile oil.

A similar increase in evaporation is observed if the dead tick is rubbed with dust. Since, however, water loss from the unfed tick increases after death, it is of interest to show that evaporation can be still further increased by abrasion. The following results have been obtained with unfed females of Hyalomma savignyi. Each value represents the average loss of weight from five unfed female ticks during 24 hr. in dry air at 25° C.

After death evaporation increased from 0·8 to 20·8% of the original weight per diem; but the dead ticks abraded with alumina lost as much as 57% per diem.

Wigglesworth (1945) has shown that the extent of the abrasion of the wax layer in insects can be revealed by immersing the insect in ammoniacal silver nitrate, for reduction of the reagent then occurs only in those areas where the polyphenols in the epicuticle are exposed by the removal of the overlying wax layer. If a normal unfed tick is treated with 5 % ammoniacal silver nitrate for 2 hr. no reduction of the reagent takes place except within the ducts of the dermal glands, which become filled with a conspicuous solid deposit of silver. Owing to the transparency of the larva it was possible to observe the formation of this curious precipitate under the microscope. The following observations were made on the larva of Ixodes ricinus.

About 1 min. after immersion in ammoniacal silver a minute granule appears in the centre of the lumen near the surface of the epidermal cell where it extends into the duct. The granule rapidly grows in diameter until after 15 min. it has formed a round plug entirely filling the duct. Eventually, as its growth continues, it may come to distend the duct and even push the epidermal cells away from the cuticle. Viewed by reflected light the precipitate at first appears as a white mass. Since a similar plug is also formed if the tick is immersed in a solution of silver nitrate in nitric acid, its formation may be due to the precipitation of chloride and not to the reduction of the silver by a polyphenol. Perhaps chloride is present in a thin film of moisture bathing the outer surface of the epidermal cell, and as it is precipitated by the reagent more chloride is withdrawn from the haemolymph.

Before describing the results of treating abraded ticks with ammoniacal silver it is necessary to recall certain features of the cuticle structure. In ixodid ticks the integument shows considerable specialization in different regions of the body. Over the hard inextensible regions, such as the legs and scutum, the epicuticle is flat and the underlying ‘endocuticle’ dark and sclerotized down to the epidermis. Over the remaining parts of the body (the ‘alloscutum’) the epicuticle of the unfed tick is deeply folded to allow for expansion during feeding, while the endocuticle is unpigmented and elastic (Fig. 2 A). The integument of Argasidae shows less regional specialization and there is only one main type of cuticle. In Ormthodorus moubata the cuticle surface is raised into hemispherical tubercles, some of which are surmounted by bristles (Fig. 3 A). Over the surface of the tubercles the epicuticle is smooth, but between adjacent tubercles it is raised into puckered folds which become flattened out after engorgement. The endocuticle is everywhere white and extensible. A further difference concerns the musculature. In Ixodidae the dorsoventral muscles are arranged in rows on the cuticle, but there is no modification in the pattern of the epicuticle overlying the insertions ; externally their presence is marked only by the long furrows running over the integument. In Argasidae, on the other hand, the cuticle is considerably modified over the insertions. The boundary of each muscle bundle is clearly delineated and encloses a number of polygonal units representing the insertions of individual fibres (Fig. 3 A).

After ticks have been rubbed in alumina dust and treated with ammoniacal silver, the epicuticle always shows considerable blackening over the abraded regions. The deepest staining occurs over the alloscutum of lxodidae, even in those species, such as Ixodes canisuga, in which the epicuticle itself is colourless. There is less reduction over the hard dark areas. Clearly the epicuticle must contain polyphenols which in the normal tick are protected from reacting with the silver by the overlying waterproofing layer. In regions where the cuticle is heavily sclerotized there appears to be a corresponding reduction in free polyphenols. After abrasion the polyphenols are chiefly exposed over the elevated regions of the integument, particularly the crests of the epicuticular folds in unfed lxodidae (Fig. 2B, D) and the surface of the tubercles in Omithodorus (Fig. 3B, C). When the tick has fed and the epicuticle is smoothed out, reduction is much more widespread. In lxodidae the whole epicuticle blackens over the abraded part (Fig. 2C, E); in Omithodorus the dust is able to reach the folds between the tubercles. The affected bristles also show irregular patches of stain along their length.

When ticks are kept in damp air after being rubbed with alumina, they are able gradually to restore their impermeability. Some examples are given in Table 4. However, as Wigglesworth (1945) points out in the case of insects, the initial degree of impermeability is never fully recovered.

The process of recovery, as revealed by immersing ticks in ammoniacal silver after different time intervals have been allowed for recovery in damp air, is similar in all species. In Omithodorus the details can be followed on the smooth cuticle of the tubercles. The whole of the abraded region darkens at first, but 20 hr. afterwards the staining area has retreated towards the summit of the tubercle, small outliers of stain remaining nearer the base. After 3 days staining is no longer perceptible, and the waterproofing layer again completely covers the underlying polyphenols.

The waxes of the epicuticle are secreted by the epidermal cells and are discharged by the pore canals. This can clearly be observed in engorged Ixodidae, such as Ixodes ricinus, where, during recovery from abrasion, the areas progressively covered bear no relation to the distribution of the dermal glands. The waterproofing layer cannot then be laid down by these glands, as has sometimes been held (Yalvaç, 1939).

The deposition of wax over the abraded regions is in other respects markedly dissimilar in the two families of ticks. In Ixodidae wax is deposited in an apparently orderly fashion and after complete recovery has taken place in saturated air, deposition ceases. The cuticle then appears normal when viewed under the binocular. In Ormthodorus moubata, on the other hand, wax is laid down very irregularly and deposition apparently continues until moulting commences. In the adult (Fig. 4) enormous thicknesses of wax are secreted, chiefly over the tubercles which, as we have seen, are most liable to abrasion. These deposits of wax dissolve immediately in cold chloroform and are clearly unprotected by a further covering of cement (see below).

The humidity to which the recovering tick is exposed has a considerable influence on the growth habit of the wax deposits in Omithodorus. Fig. 4B shows an example of recovery in saturated air. The wax filaments, as they have grown out from the surface of the epicuticle, have assumed a definite crystalline form, and many tubercles when viewed from the summit are seen to have a rosette of tabular wax crystals radiating from the summit. Often these crystals have striations running through them in the longitudinal and transverse planes. Deposits laid down in drier air (recovery at room humidity) often lack any definite crystalline form (Fig. 4C) and usually appear as thick caps of wax with rounded contours perched on the summit of the tubercle. In 3 weeks the wax may have attained a thickness of nearly loo/u,, that is, greater than the entire thickness of the cuticle. There is no very conspicuous difference in the quantity of wax secreted in moist and dry air, such as there is in Rhodmus (Wigglesworth, 1945).

In a previous paper it was shown that after desiccation unfed ticks readily take up water through the cuticle when exposed to humidities near saturation (Lees, 1946). A covering of impermeable paint can be applied to any part of the body, including the spiracles, without preventing the uptake of water through the remaining uncovered cuticle. Presumably, therefore, the site of uptake is not localized in any way. Most unfed ticks reach a state of equilibrium at a humidity of about 90%. In view of these circumstances one would expect that if the wax layer were interrupted, water uptake would proceed normally from saturated air while at a humidity only slightly above the point of equilibrium, e.g. 95% R.H., uptake through the undamaged cuticle would be offset by the water lost to the unsaturated air through the abraded regions.

Unfed females of Dermacentor andersoni were used in investigating the effects of abrasion on water uptake. Now, if unfed ticks are desiccated repeatedly they take up water after each desiccation at very nearly the same rate when exposed to the same high humidity. Accordingly, the ticks were first desiccated individually in dry air at 25° C. for some 10 days and were then exposed in small containers to humidities of 95 or 100% R.H. The values for water uptake serve as the controls. The same ticks were then re-desiccated and abraded by rubbing the dorsum with alumina. After this treatment the dust was washed off in a stream of water, the ticks were carefully dried and reweighed and exposed again to the appropriate humidities. A sensitive torsion balance was used for weighing the ticks.

The results were unexpected (Fig. 5). After abrasion the desiccated tick fails to take up any water for 24 hr. or so, either from 95 % R.H. or from saturated air. During this period uptake would normally be most rapid. The weight may sometimes fall slightly. After this initial delay the ability to secrete water is rapidly regained and uptake proceeds at the same rate as it did prior to abrasion. It is clear from this observation that the abrasion of a limited area of cuticle temporarily inhibits the activities of all, or nearly all, the epidermal cells, many of which underlie normal, undamaged cuticle. Evidently the epidermal cells are interconnected in some way, and the interruption in the function of one group of cells may influence the normal functioning of an adjacent group.

This effect of abrasion is not due to the fact that the epidermal cells are fully engaged in repairing the damaged waterproofing layer and are unable simultaneously to perform the task of secreting water. The full powers of water uptake are recovered long before the final degree of impermeability is restored, that is, before the deposition of wax over the abraded regions has ceased. The following example will clarify this statement. An unfed female tick lost 1·3 % of the original weight per diem in dry air at 25° C. and gained 8·8 % during subsequent exposure to saturated air for 1 day. This tick was then re-desiccated, rubbed in alumina and exposed to saturated air. There was no increase in weight on the first day, but on the second day after the cuticle had been abraded the tick gained 9·8% in weight. During the third day the tick was exposed to dry air and lost 14·2% in weight; yet 15 days after rubbing with alumina the loss of weight per diem in dry air had fallen to 3·8%.

It is possible to establish an approximate relationship between the area of cuticle abraded and the suppression of water uptake. A single unfed female tick was desiccated and the rate of uptake in saturatéd air determined. After re-desiccation, an area of cuticle on the dorsum was marked out and thoroughly abraded by rubbing it with a tiny ball of filter-paper dipped in alumina. Uptake in saturated air was again determined. After several days had been allowed for recovery this procedure was repeated with a larger area of cuticle, and so on. The results showed that suppression does not become appreciable until a comparatively large area of cuticle has been abraded. In one example the abrasion of 6·1 % of the total surface area only reduced uptake from 6·2 to 3·7% Per diem; whereas when 15% was abraded uptake was reduced to 0·9%. This area was equal to nearly half the entire surface area of the dorsum and was underlain by an estimated 22 % of the total epidermal cells.

Wigglesworth (1945) has shown that if whole insects are extracted with a wax solvent such as chloroform, evaporation through the cuticle is greatly increased. The precise effect in different insects depends on whether the wax layer is covered and protected from solution by an additional ‘cement’ layer, as in Rhodnius; also on the nature of the wax itself. In an insect with a soft, low melting-point wax transpiration is increased by mere exposure to chloroform vapour, but the latter has a much smaller effect in species waterproofed by hard, high melting-point waxes.

Similar experiments have been carried out with several species of ticks representative of the Ixodidae and Argasidae. Engorged females were killed in ammonia vapour and then either extracted with chlorofortn or exposed to chloroform vapour. Their spiracles were subsequently covered. The results are shown in Table 5. In Ixodidae with a low critical temperature, such as Ixodes ricinus, water loss is markedly increased by exposure to chloroform vapour alone. In Dermacentor andersoni and Hyalomma savignyi, both ixodids with much higher critical temperatures, chloroform vapour and even extraction in the cold has a much smaller effect. Argasid ticks, as typified by Ormthodorus moubata, are extremely resistant to chloroform vapour and to cold extraction ; but water loss increases dramatically after extraction in chloroform at 50°C. The differences noted between Ixodes on the one hand and Dermacentor and Hyalomma on the other, are probably associated with the relative solubility of the waxes laid down by these species, for the wax layer does not appear to be covered by a further protective layer. In Omithodorus, on the other hand, an insoluble layer of cement is present overlying the wax (see p. 396), and this protects the wax from solution except in hot chloroform when the cement is broken down.

The action of chloroform in attacking the waterproofing layer has also been studied by immersing ticks in ammoniacal silver after chloroform treatment. Engorged females of Ixodes ricinus show widespread staining over the surface of the epicuticle after chloroform extraction or exposure to chloroform vapour; in Dermacentor the amount of staining after exposure to chloroform for a similar length of time is much less pronounced. If Ormthodorus is exposed to chloroform vapour for 1 hr. there is usually no visible reduction after silver treatment; and after extraction in chloroform at 18° C. for 1 hr. only a few very small black specks appear on the muscle insertions, at the tips of the bristles and on the tubercles (Fig. 6 A). After 15 min. in hot chloroform, however, the muscle insertions blacken completely, the folds between the tubercles stain deeply along their crests and the bristles are often partly filled with a solid black deposit (Fig. 6B). There are usually patches of stain on the crowns of the tubercles, but in most specimens it is clear that by no means all the wax has been removed by this treatment. These observations support the conclusions drawn from the study of water loss after chloroform treatments.

It is known also that in insects transpiration can be increased if detergents are applied to the cuticle (Wigglesworth, 1945). Table 6 shows some results obtained with ticks. The full specifications of the materials used are given in the paper cited above ; they were applied as a thin smear to the backs of living engorged female ticks.

P 31, a refined mineral oil, greatly increases evaporation in Ixodes ricinus but has little or no effect in more impermeable species such as Amblyomma cajennense and Omithodorus moubata. The wax detergents diglycol oleate and the cetyl ethers of polyethylene glycol, R2211 and C 09993, show progressively greater powers of increasing transpiration, as they do in insects. But none of these materials is outstandingly effective in Amblyomma and they are almost without effect in Omithodorus.

The nymphal exuvium of ticks consists almost entirely of the old epicuticle which, apart from the hard cuticle of the legs and scutum in lxodidae, alone remains undigested by the moulting fluid. The exuvia of Ixodes ricinus and Omithodorus moubata show many of the chemical characteristics of typical insect epicuticles—for example, insolubility in cold mineral acids and solubility in hot caustic potash. If nymphal skins of either species are extracted for 1 hr. in boiling chloroform and are then warmed gently in nitric acid and potassium chlorate, there is at first some evolution of gas from the inner side of the cuticle. On further warming the membrane expands and begins to disintegrate, liberating oily droplets which are themselves readily oxidized. Evidently a fatty material which cannot be extracted with lipoid solvents is incorporated in the epicuticle. The results of this test are similar to those described by Wigglesworth (1947) in Rhodnius, and it is likely that the rigid basal layer of the epicuticle in ticks is similar to, if not identical with, the ‘cuticulin’ layer of insects. This substance is provisionally regarded by Wigglesworth as a polymerized lipo-protein subsequently tanned by quinones.

Treatment with ammoniacal silver after abrasion, or chloroform treatment, shows that in all ticks the cuticulin layer is covered by a film of polyphenols which in turn is overlain by the waterproofing layer. In Ormthodorus moubata the wax layer is covered by a further layer of cement which, as we have seen, resists extraction in cold chloroform. This layer can be demonstrated readily in paraffin sections which have been cleared in xylol. Fig. 7 shows the cuticle in a moulting tick; the wax layer has been dissolved from the old cuticle and the thin covering of cement (c) has frayed away from the surface. It will be noticed from this figure that a similar layer is absent from the new cuticle although moulting is far advanced. The cement layer itself forms a perfect mould of the contours of the epicuticle apart from its seeming absence over the bristles. Seen in section the cement appears to be of very uniform thickness, but the fact that hot chloroform exposes the polyphenols with much greater ease over the muscle insertions than over the puckered folds and the tubercles, suggests that this layer is actually thinnest over the muscle insertions.

No evidence of the presence of a cement layer was obtained in any of the ixodid ticks examined. Sections of the cuticle of Ixodes ricinus, I. canisuga and Hyalomma savignyi were prepared after clearing in xylol, but no detached superficial layer could be detected. The conclusion that the wax layer in Ixodidae is unprotected by a further cement covering, is, as we have seen, supported by the results of the chloroform extractions.

With the object of determining the time relations and mode of deposition of the waterproofing layer, moulting nymphal ticks were dissected out of their old cuticles and immersed in ammoniacal silver. In Rhodnius a picture of the extent of wax deposition can be derived from this technique, as the polyphenols are secreted before the overlying waterproofing layer (Wigglesworth, 1947). Three species of ticks were examined, namely, Ixodes ricinus, Dermacentor andersoni and Omithodorus moubata. Since the duration of the moult at constant temperatures often showed considerable variations, it has been convenient to distinguish four stages as a broad guide to the general course of development. These are as follows:

• From the beginning of engorgement to the time of separation of the epidermis from the old cuticle.

• From the end of A to the time of onset of the secretion of moulting fluid. During this period the cuticulin layer is laid down.

• From the end of B to the time of withdrawal of the moulting fluid.

• From the end of C to emergence.

During stage B, that is, before the appearance of moulting fluid, it is almost impossible to carry out the dissection without damaging the new cuticle. If pieces of cuticle at the end of stage B are exposed to ammoniacal silver without previous fixation some ill-defined staining may develop in the epicuticle; but in many preparations no blackening occurs and polyphenols may be absent. Shortly afterwards, as moulting fluid begins to appear, the old cuticle can be removed successfully and the new cuticle exposed to silver from the outside. In the three species investigated there is comparatively little staining even at this early stage. But if the cuticle is first rubbed with alumina there is widespread reduction over the elevated regions. This shows that not only is the polyphenol layer fully established, but a large part of it is already covered by the waterproofing layer. Thus in ticks the waterproofing layer is laid down comparatively early in development, probably immediately after the secretion of the moulting fluid.

The waterproofing layer is never quite complete, however, at the beginning of stage C. In Ormthodorus the muscle insertions and some of the stellate folds blacken completely, and there is some reduction over the summits of the tubercles and at the bristle tips (the silver stain often appears in the form of spiky crystals) (Fig. 8 A). After the withdrawal of the moulting fluid the extent of the silver staining gradually diminishes, the last region to be covered by the waterproofing layer being the muscle insertions. This takes place just before emergence (Fig. 8B). In Ixodes and Dermacentor the deposition of wax is even further advanced at the stage when moulting fluid is abundant. Exposure to silver results in some irregularly distributed patches of stain appearing over the scutum and alloscutum ; but their total area is extremely small when compared with the area of cuticle over which the polyphenols are protected from staining (Fig. 9). After the moulting fluid has disappeared these areas are soon covered over.

The waterproofing layer is deposited at such an early stage that some doubt arises as to how the moulting fluid is withdrawn. Possible sites of absorption are the external openings, including the mouth and spiracles, the whole cuticle, or the small patches of cuticle over which the waterproofing layer is still incomplete. An attempt to determine the site of withdrawal in D. andersoni by injecting neutral red into the ecdysial fluid of moulting nymphs was only partly successful ; for although the dye soon appeared in the Malpighian tubes as the fluid was withdrawn, no concentrations of dye were visible in the cuticle or elsewhere. On the whole it seems more likely that the fluid is withdrawn through those parts of the cuticle which happen to remain unwaterproofed.

The hydrophobic properties of the cuticle in Ixodes and Dermacentor are such as would be expected from the foregoing remarks. If moulting nymphs are dissected out of the nymphal exuvia and the moulting fluid blotted from the surface of the imaginai cuticle, droplets of water introduced on to the surface by means of a fine pipette fail to wet it. And after moulting the cuticle remains strongly hydrophobic. In Ormthodorus the properties of the cuticle are more complex. In the presence of moulting fluid the cuticle is easily wetted by water, and after its withdrawal and replacement by air it remains hydrophilic for a time and then becomes highly hydrophobic. After emergence the cuticle remains hydrophobic unless the tick is immersed in water, a treatment which again renders the cuticle hydrophilic. This may normally occur when the tick feeds and becomes bathed in its own coxal fluid. The lessening affinity of the cuticle for water after the withdrawal of the moulting fluid is not associated with the increasing extent of wax deposition, for treatment with silver often showed that deposition was further advanced in cuticles that were easily wetted than in others which had already become hydrophobic. Presumably the waterproofing agent in this species contains mobile polar as well as apolar elements. In the presence of the moulting fluid the hydrophilic endings in the surface will be oriented outwards, but as air replaces moulting fluid the outwardly directed endings become predominantly apolar. The increased affinity of the cuticle for water following immersion is more difficult to account for, however, as by this time the wax layer has acquired a further covering of cement.

The cement layer in Omithodorus is laid down shortly after moulting. This can be shown by extracting ticks in different stages of development with cold chloroform and by estimating water loss through the cuticle. These results have been compared with the water loss from unextracted ticks in the same stage of development (Table 7). Transpiration through the new cuticle is always rapid during stage C when moulting fluid is present. After its withdrawal and the completion of the waterproofing layer impermeability becomes fully established; but until the time of emergence transpiration is greatly increased by cold chloroform extraction. Within a few hours of moulting, however, the effect of extraction begins to diminish, indicating that the cement is being laid down. In ticks which have moulted 1 hr. previously, extraction with chloroform exposes the polyphenols very easily over the muscle insertions and to a varying extent along the folds and summits of the tubercles. 15 hr. after moulting the waterproofing layer is usually completely protected from extraction. The time of deposition of the cement layer is therefore much the same as in Rhodnius (Wigglesworth, 1947).

At certain stages in the developmental cycle of ixodid ticks the dermal glands discharge a visible product on to the surface of the cuticle. The material in question is a thick yellow ‘grease ‘which slowly spreads over the cuticle or collects round the gland openings (Fig. 10). It is never discharged by the unfed tick and only begins to appear a few days after full engorgement. But even in the same species some stages may fail to discharge the grease. None of the material is visible in recently engorged larvae and nymphs of Ixodes ricinus, for example, while there is a copious discharge from the dermal glands of the engorged, egg-laying female. In Dermacentor andersoni and other species, on the other hand, there is an abundant greasy exudate from the glands of the engorged nymph (Fig. 10). In Argasidae no products visible to the unaided eye are ever secreted by the dermal glands.*

It has been suggested that the function of the greasy material in Ixodes ricinus is to waterproof the cuticle (Falte, 1931) and, similarly, in Hyalomma anatolicum to protect the cuticle and ‘sense organs’ from desiccation (Yalvaç, 1939).*

In Ixodes ricinus the yellow grease is easily removed from the cuticle of the egglaying female by wiping the surface with extracted cotton-wool. The material proved to be completely insoluble in cold chloroform and xylol, and only sparingly soluble in hot chloroform. Since it is known that all insect waterproofing agents are readily soluble in chloroform (Beament, 1945), and that in Ixodes also chloroform extraction removes the waterproofing layer from the cuticle (p. 393), it is most unlikely that the dermal gland exudate has a waterproofing function. Indeed, as has already been indicated, the true waterproofing layer is laid down by the epidermal cells and is completed before moulting. The histological development of the dermal glands in Ixodes and Dermacentor has been followed with the object of throwing light on the significance of the greasy exudate. In Ormthodorus the histology of the glands has been examined in relation to their probable function in secreting the cement layer. The results are described below.

The openings of the dermal glands lie round the periphery of the tubercles, and the ducts, numbering from one to eight according to the size of the tubercle, run inwards to the epidermis which forms a deep pocket extending into the interior of the tubercle (Fig. 3 C). The dermal glands can best be examined in whole preparations of the cuticle by focusing downwards through the thickness of the tubercle. At most stages during the development of the last nymphal instar the dermal glands remain invisible. Signs of activity become apparent, however, at two stages in the moulting cycle : just before the secretion of the moulting fluid (Fig. 11 A) and a few hours after moulting, as the cement layer is being deposited (Fig. 11C). The histological appearance of the glands is very similar during both phases of activity, the lumen gradually filling with a colourless secretion. During the latter phase of activity the glands appear never to attain the distended and conspicuous condition of the dermal glands in the fourth stage nymph of Rhodnius shortly before the deposition of the cement layer (Wigglesworth, 1947).

The dermal glands in this species undergo a complicated cycle of activity, the significance of which is by no means clear. There are two types of gland each provided with an elaborate cuticular duct of characteristic form. From the description by Yalvaç (1939) of the dermal glands (‘drüsensinnesorgane’) in Hyalomma anatolicum it is clear that the larger type with a wide neck and a frilled opening corresponds with his ‘sensillum sagittiforme’, and the smaller, which has a narrow neck and a slit-like opening, with his ‘sensillum hastiforme’. In common with other species of Ixodidae the gland cells in Dermacentor attain a gigantic size at certain stages in the developmental cycle. The general fate of the glands has been followed through from the nymphal to the adult stages.

The large dermal glands (type A) may be considered first (Fig. 12A–D). In the unfed nymph two gland cell nuclei, somewhat larger than the other epidermal nuclei, are associated with each duct; but the cytoplasm is either very sparse or fails to stain with haematoxylin (Fig. 12 A). As soon as the tick begins to feed, however, the gland cell nuclei hypertrophy and secrete abundant deeply staining cytoplasm (Fig. 12 B). By the end of engorgement the glands have attained their maximum size. There are usually two gigantic cells associated with each duct, but sometimes there are three such cells, sometimes only one (Fig. 12C). The cytoplasm of the cells is often somewhat fibrous, and the nucleus, which may be flattened, often rests against a large central vacuole containing fluid. The fate of the cells depends on the general course of development. If the tick fails to moult, the gland cells may persist unchanged for a long period; but if the moult is impending the cells soon begin to show degenerative changes. The cytoplasm becomes yellowish and reticulated and yellow greasy droplets appear (Fig. 12D). The gland then undergoes involution, the yellow residue passing up the duct to the surface of the cuticle. This is the greasy material already referred to and as such it clearly represents nothing more than the end-product of the degenerating gland cells. Sometimes the glandcell nuclei become pycnotic and are thrown out with the cytoplasm; more frequently they remain resting against the epidermis and only the cytoplasm is lost. After the epidermis has separated from the old cuticle a group of small cells near the neck of the old gland are associated in secreting a new duct. A few of the old dermal gland nuclei may still persist beneath the epidermis when the tick moults.

The cells of the small dermal glands (type B) undergo a somewhat different cycle of activity (Fig. 12E–I). In the unfed nymph their component nuclei are indistinguishable from the other epidermal nuclei and only the cuticular duct can be recognized at this time (Fig. 12E). During engorgement, nuclei near each duct hypertrophy and begin to accumulate cytoplasm, just as in the case of type A cells. Again also, maximum size of the paired cells is attained at the end of engorgement (Fig. 12 F). After the tick has dropped from the host the type B cells undergo very rapid involution and a few days afterwards, when many type A cells are still persisting unchanged, all the type B cells have disappeared, the products of the degenerating glands again passing up the ducts to the surface of the cuticle. There remains near the old duct a group of about eight nuclei which stain more deeply with haematoxylin than the other epidermal nuclei (Fig. 12G). Soon afterwards the moulting tick frees the epidermis from the old cuticle. The nuclei remain grouped round the new cuticular duct as it is forming and begin subsequently to secrete cytoplasm. Each group of nuclei thus participates in the formation of one new type of gland cell (type C). These are round or pear-shaped and their cytoplasm is acidophil and without inclusions. The multinucleate type C cells attain a maximum diameter of about 30μ shortly before the moulting fluid is secreted (Fig. 12I). Afterwards they dwindle in size but the groups of nuclei are still visible when the tick moults.

Only one type of dermal gland is present and this resembles the small type B gland of Dermacentor. The general course of development is also similar in many ways (Fig. 13). As the nymph becomes engorged the gland-cell nuclei, hitherto indistinguishable from the other epidermal cells, secrete a large quantity of basophilic cytoplasm. The paired cells again reach their maximum size at the end of engorgement when large vacuoles begin to appear near the nuclei. The neck cell nuclei are clearly seen at this time grouped round the duct (Fig. 13 B). As in Dermacentor the appearance of vacuoles seems to be a sign of impending degeneration, but the cells persist entire until moulting has begun and the epidermis separated from the old cuticle. The cells then undergo involution (Fig. 13 C). Since, however, the glands are now unprovided with ducts the products of degeneration cannot escape to the surface of the old cuticle, but are absorbed instead. Because of this difference in timing the degenerative products of the nymphal gland cells never become visible externally.

The cytoplasm of the type B cells disappears before the nuclei, which may persist until the cuticulin layer has been secreted (Fig. 13 D). The neck cell nuclei remain near, and possibly secrete, the new cuticular duct. They appear to accumulate a little clear cytoplasm but undoubted multinucleate gland cells, like the type C cells of Dermacentor, have not been distinguished with certainty. After the secretion of the moulting fluid the epidermal nuclei become smaller, and when the tick emerges the epidermis again consists of a uniform sheet of cells ; all the nuclei are of the same size and no cell boundaries are visible.

When the female tick becomes engorged the cycle of hypertrophy and involution is once more repeated (Fig. 13E–H). Since the adult does not moult, however, the products of the degenerating gland cells are not absorbed but are thrown out on to the surface of the cuticle, appearing as the characteristic yellow greasy material.

The development of the type A gland cells in Dermacentor is probably very similar to that of the similar cells in Hyalomma (Yalvaç, 1939); this author describes type A cells in engorged ticks discharging their ‘secretion ‘on to the surface of the cuticle, the separation of the epidermis from the old cuticle and the subsequent ‘regeneration’ of the gland cells. The regenerated glands figured (Yalvaç, 1939, figs. 30-32) are typical type A glands each consisting of a pair of gigantic vacuolated cells. Type C gland cells were not described. Yalvaç holds the view that the dermal glands have two main functions, namely, the secretion of a waterproofing material and the elaboration of moulting fluid by the ‘regenerated’ gland. Now their appearance is so similar in Dermacentor that one may question whether the glands described by Yalvaç in Hyalomma were not all type A cells in different stages of degeneration. As mentioned above, a few of these glands may persist beneath the epidermis after it has separated from the old cuticle. Schulze (1942) regards the cuticular ducts of the dermal glands as sense organs. It should be mentioned that his ‘sense cells’, as figured for Ixodes ricinus (Schulze, 1942, Fig. 7) are identical with the neck cells which remain after the type B cells have degenerated (Fig. 13 B). Their sensory function has never been demonstrated.

The true functions of the dermal glands in Ixodidae are therefore in doubt. Clearly the glands of types A and B do not secrete the waterproofing layer for these glands only develop as the tick becomes engorged, whereas the true waterproofing layer is completed before moulting. Neither do they secrete the moulting fluid for nearly all of them undergo involution before the epidermis has separated from the old cuticle and long before any moulting fluid appears. The period of hypertrophy as the tick feeds coincides with a growth phase undergone by all the epidermal cells which at this time are concerned with the renewed synthesis of cuticular material. However, it is uncertain whether A and B cells actually secrete the inner part of the duct which is continued through the thickening cuticle, or whether this, like the outer, amber-coloured part of the duct, is laid down by the group of small neck cells. Type C cells may secrete, or participate in secreting, the moulting fluid. It is noteworthy that degeneration of the dermal glands also occurs in the adult Rhodnius where the products of involution form little patches of ‘secretion ‘near the openings of the ducts.

The greatly increased transpiration which follows abrasion of the cuticle shows that ticks owe their powers of resisting desiccation primarily to a superficial waterproofing layer. The visible products secreted during recovery from abrasion leave little doubt that the waterproofing agents are in all cases true waxes, even in the less resistant species, such as Ixodes ricinus. The waterproofing system therefore closely resembles that of the majority of insects (Wigglesworth, 1945).

The waterproofing layer in ticks forms a physical barrier to evaporation and as such is an essential feature in their water economy. But in unfed ticks the passive retention of water is assisted by active secretion. This faculty depends on the normal functioning of the epidermal cells and is displayed when the tick is exposed to any humidity below saturation; under these conditions the tick can resist water loss more effectively than it can by virtue of the wax layer alone—as is shown, for example, by increased transpiration if the tick is killed (Lees, 1946). Although the active and passive mechanisms operate towards the same ends in retaining water, it is worth noting that if the waterproofing layer in the living tick is interrupted by abrasion, secretion by itself is incapable of preventing rapid water loss.

If the humidity to which the living tick is exposed is higher than the ^‘equilibrium humidity^’, secretory activity results in water being taken up through the cuticle (Lees, 1946). As the previous work had indicated, the waterproofing layer, although effectively preventing water loss does not hinder the uptake of water in the reverse direction. Indeed after injury to the cuticle by abrasion, water uptake by the whole epidermis is entirely upset for a time—a phenomenon which may perhaps be likened to wound shock. And after recovery of the full powers of secretion, water uptake proceeds at the same rate irrespective of whether the wax layer is still incomplete or whether it has been fully repaired. The mechanism of uptake is unknown. Even if some part of the epicuticle serves as a condensing system, water withdrawn from moist air will still have to pass through the wax and polyphenol layers before reaching the pore canals (assuming that the pore canals penetrate the cuticulin layer). In Omithodorus water is probably taken up through the cement layer as well.

The tick epicuticle, like that of insects (Wigglesworth, 1947), is evidently a complex laminated structure. In the family Argasidae there are four components comprising the cuticulin, polyphenol, wax and cement layers. In the Ixodidae only the three inner layers are represented, the outermost covering of cement being absent. A further difference concerns the type of wax secreted. Without exception the waxes of the argasid group have higher critical temperatures and confer a more perfect degree of impermeability than the ixodid waxes. No doubt both the structure of the epicuticle and the nature of the wax will influence the ease of penetration through the cuticle of such substances as insecticidal materials. The simultaneous presence of a high melting point wax and a cement covering in Omithodorus may account for the resistance of this species to nearly all contact insecticides and to injurious solvents, such as xylenol (Robinson, 1942). On the other hand, as is well known, the Ixodidae are relatively susceptible to insecticides, even to aqueous solutions of poisons such as sodium arsenite (Burtt, 1945).

The deposition of the different layers in the epicuticle follows the general pattern described in Rhodnius (Wigglesworth, 1947). The cuticulin and polyphenol layers are secreted first, the wax afterwards. There is a difference in timing, however. The wax layer is laid down precociously in ticks, deposition being well advanced by the time moulting fluid becomes abundant. All three layers are secreted by the epidermal cells. In Ixodidae the dermal glands appear to contribute nothing of functional significance to the structure of the cuticle. In Omithodorus moubata, on the other hand, the cement is probably secreted by the dermal glands and is poured over the surface of the wax layer shortly after emergence. The general parallelism between the structure and development of the epicuticles in Omithodorus and Rhodnius is remarkable: the identity of the epicuticular layers, their mode of deposition, and even details such as the delayed deposition of wax over the muscle insertions, are very similar. That there are differences in the chemistry of the outermost layers is suggested, however, by the lack of effect in Omithodorus of detergents which in Rhodnius are very efficient in attacking the cement and wax layers.

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 lxodidae ranging from 32 (Ixodes ricinus) to 45°C. (Hyalomma savignyi)-, and in Argasidae from 63 (Omithodorus moubata) to 75° C. (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 lxodidae 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 lxodidae. 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 moulting tick relatively early in development and may be nearly complete by the time moulting fluid is abundant. In Ornithodorus moubata the cement is poured out by the dermal glands shortly after emergence. In lxodidae 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.

The present work was begun at the London School of Hygiene and Tropical Medicine and I wish to thank Prof. P. A. Buxton, F.R.S., both for hospitality received and for assistance in obtaining cultures. I am also indebted to the following who have kindly supplied material : Dr R. A. Cooley, Rocky Mountain Laboratory, U.S.A.; Dr Carroll N. Smith, U.S. Department of Agriculture; Prof. R. M. Gordon, Liverpool School of Tropical Medicine; Dr B. Feldman-Muhsam, Hebrew University, Jerusalem; and Mr D. J. Lewis, Wad Medani, Sudan.