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vances they become more and more replete with material and increase in size until readily visible to the naked eye as red granules already noted. In early stages of digestion, cells packed with chromatin bodies and superficially resembling macrophages, the nature of which is not clear, may be seen.

Although a prominent part in digestion is taken by the free cells just alluded to, epithelium lining the diverticula also takes an active part in the process. The swollen and vacuolated portion of the large projecting cells is crowded with products of digestion very much as is that of the free cells. Smaller cells lying nearer the basement membrane are also, as a rule, packed with fine black granules, though they rarely contain the large granules seen in the other cells.

The intensely black and opaque globules are highly characteristic of digestion in the tick and undoubtedly represent the ultimate condition to which blood remaining in the gut is reduced by the digestive process. These globules probably represent only the portion of food not assimilable, for in Ornithodoros ticks, which may be kept alive for long periods without food, the diverticula contain, after some weeks, an inky black material consisting entirely of these granules.

As diverticula contents are digested, the muscle fibres, which in the fully distended organ slightly indent the surface, sink more and more into the body of the viscus. The wall between the fibres becomes ballooned and eventually forms flasklike pockets with only a narrow opening connecting with the lumen. The epithelium is, as a rule, present in the pockets, though it is generally more noticeable on the ridges formed by the contracted muscular fibres. Remains of ingested blood, in the form of black granules, are present both in the pockets and in the lumen. Ticks examined months after a meal still have the diverticula loaded with the black material.

Waste matter is not passed into the rectum and any remnant of food not "absorbed must remain in "the"diverticula unti Tcleath of the tick.TTHETEtho: "by WHICH absorption takes pracETHEST5t been ascertained. Black pigment is not detected in the tissue cells or in the body cavity. Note that excess fluid in the blood is excreted by the coxal organ during and following feeding so that a large amount of blood can be rapidly ingested; this is elucidated in the section on the coxal organ below.

Wigglesworth (1943) confirmed that in O. moubata, blood (haemoglobin) is absorbed by swollen epithelial cells of the wall of the large stomach and its diverticula. In these cells, which detach and remain free in the lumen, blood pigment is converted into black globules that are ultimately discharged into the gut cavity. Similar dark granules are dispersed through smaller cells of the gut wall, but no black pigment can be seen in other tissues or in the body cavity. The haemoglobin is digested more or less to protohaematin and is demonstrable in the tick's haemolymph probably as alkaline haematin. The gut contents are reddish brown with black haematin deposits. No free iron can be detected in the gut lumen or cells, or in other tissues, and no nephrocytes containing haemoglobin derivatives can be found.

The type of host from which O. moubata draws a blood meal may be identified by the precipitin test more than six months following feeding (laboratory studies at 20°C. and eighty percent relative humidity) (Weitz and Buxton 1953), or even for twelve months (fowl blood meal, kept at 30°C., ticks also fed on mouse) (Gozony, Hindle, and Ross 1914).

Rectal Excretion

As stated above, O. moubata has no passage between the small intestine and the rectal ampulla, and defecation does not occur. Excretion of water ("urination") from the malpighian tubules takes place only after the first nymphal stage has been reached and a blood meal has been absorbed; this excretion is viscous and dries within a few hours. In the weeks following the first excretion only a slight amount of water is irregularly excreted (but can be produced through various stimuli). This pattern is similar in each developmental stage after the larva.

Variations in tick excretion and a comparison of this function according to species, morphology, number of hosts, size, duration of development, quantity of blood ingested, and transmission of disease organisms to vertebrate hosts have been analyzed by Enigk and Grittner (1952).

Coxal Organ Morphology and Function*

Inasmuch as the volume of the blood meal is from two to six times the tick's original body weight and engorgement is usually completed in about half an hour, the tick must have a means of reducing the total intake volume and of preserving the internal medium while feeding. For this purpose coxal organs function as ionic (chloride) regulators and for ultrarapid excretion of a large volume of water during ingestion of blood. Coxal discharge, which commences about fifteen minutes after the tick has begun to feed, continues till completion of the meal and intermittently for about an hour afterwards. (See also Lavoipierre and Riek 1955).

Malpighian tubules do not function until about an hour after feeding is completed, and the amount of water they excrete is limited.

Chloride regulation. About half the ingested water is excreted in COX uid. The mean haemolymph chloride concentration before feeding is l.00% and after feeding 0.96% NaCl; that of coxal fluid is 0.80% NaCl. These values are similar to those determined by Boné (1943).

Morphology of coxal organs. The flaskshaped coxal organs, which elaborate the bulk o uid, consist of an outer filtration

chamber with an inner tubule system leading to the external opening and of a small organ with glandular structure, the so-called accessory gland. The filtration chamber, which communicates with the tubules of only one point, is highly folded into an elaborate series of pockets and fingers that closely invest the tubules;

*Chiefly from Lees (1946B). See other remarks in section on life cycle. It should also be mentioned, for practical significance in relation to disease, that Lees found that O. delanoëi acinus and 0. keri have coxal organs differing from those of Q. moubata, and # in these species coxal fluid is liberated only after cessation of ingestion. It should also be noted that what Lees and others have called "coxal gland" is rather a coxal organ (Burgdorfer 1951) because it excretes fluid rather than secreting fluid, and the filter chamber histologically has no glandular structure.

numerous small muscle fibres inserted in these pockets pass outwards from the organ to attachments on the body wall. The histology of the two regions is entirely different. The filtration membrane is only one or two microns thick and its cellular origin is much obscured. The tubule walls, from five to thirty microns thick, are composed of cells with a dense, deeply-staining cytoplasm and are richly supplied with tracheae.

Function. The production of coxal fluid is under muscular control. It is believed that contraction of coxal organ muscles enlarges the filtration chamber and sets up a sufficient pressure difference across the membrane to initiate filtration into the organ. In subsequent passage of fluid down the tubules, threshold substances such as chloride are reabsorbed. That the coxal fluid is primarily an ultrafiltrate of the haemolymph is suggested by (a) the rapid passage of dyes and even haemoglobin into coxal fluid after injection into the haemolymph, and (b) the very high rate of fluid liberation. Serum albumin sometimes passes into coxal fluid after injection, but casein (and normal haemolymph proteins) are fully retained (Lees' summary).

Boné (1943) proposed somewhat different explanations concerning coxal organ function. Lees further indicates that Patton and Evans' (1929) opinions regarding the functions of the coxal organs are incorrect. An earlier work on the same subject is that of von Künssberg (1911).

The small accessory coxal glands have an unknown function. Rapid engorgement in argasid ticks is allowed by passive cuticular stretching. In ixodid ticks new cuticle is produced to allow for volume of intake and engorgement is much more slowly accomplished.

When specimens of O. moubata and other argasid ticks that

possess coxal organs are warmed or irritated they exude from these organs a clear fluid. This may possibly serve in part as a defensive mechanism although the actual reason remains to be determined. Coxal organ discharge has been observed and reported, highly inaccurately, by Remy (1922A, and for Argas reflexus, 1921 and 1922B), who believed the exudate to be haemolym containing haemocytes. Lees (1946B) has shown that these structures are actually small, globular clusters of refractive granules, possibly derived from

partial regression of the salivary glands during molting or from granule-bearing cells in the accessory organs. The density of these granules in the coxal fluid of newly molted but unfed ticks is much greater than in the fluid of engorged ticks, in which the granules are more widely dispersed in the greater amount of fluid.

Spiracular Morphology and Function

Argasid spiracles have been described by Robinson and Davidson (1913), Cunliffe (1921), Mellanby (1935) and Browning (1954A). The last two workers paid particular attention to the spiracular structure and function of C. roubata.

As described by Mellanby, the spiracle consists externally of a semicircular cribiform plate inserted into a smooth macula of thickened skin, with a slitlike ostium between these. The thin external layer of the plate is supported by rodlike pedicles. The external layer was stated to be pierced by minute pores opening into the tracheal atrium, which is a tube connected to the ostium. Muscular attachments of the macula allow opening and closing of the ostium.

If it were true that the external layer is pierced by pores, it would appear that there is no way for the tampan to close off the direct connection between the external air and the internal body tracheae. Since tampans show remarkable ability to withstand desiccation in the laboratory and in nature, Browning (1954A) was led to investigate the spiracle anew. He found that a surface view of the spiracular plate gives the impression of being porous. On examination of transverse sections these "pores" are shown to be expanded distal junctures of branching pillars (pedicels) arising from a basal, underlying layer of sclerotized endocuticle. These pillars support the very thin outer membrane, which is, however, not porous but continuous. The cavity between the basal cuticle and outer membrane and ramifying between the pillars is continuous between the atrium and the spiracle. From surface view the hard maculum can be seen between the inner curves of the crescent of the spiracular plate. The macula encloses a slitlike aperture, or ostium, connecting the atrium of the trachea with the outside air. The argasid spiracular plate functions to provide a pad against which the macula can impinge when depressed

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