« PreviousContinue »
furrow is to prevent the spilling back into the host's wound of any blood already in the pharynx. The tonguelike process also appears to play an essential part in the mechanism of ejection of salivary fluid into the blood as it is being ingested. Since the salivary fluid is discharged into the distal region of the buccal canal, it is assumed to reach the wound in the host (and thus might transmit disease causing organisms contained in it).
As stated above, Bertram's study has considerable practical value, but must be read in its entirety to be fully appreciated. It should be noted that Snodgrass (1948) refers to the tonguelike process as the labrum in his noncommittal review of Bertram's findings and conclusions,
Alimentary canal: We now return to Christophers' (1906) study of O. Savignyi, and it is interesting to note that he found the pharynx to open into the floor of the "mouth" (buccal canal) in contrast to Bertram's observation on 0. moubata, mentioned above. At any rate, the pharynx leads to a narrow, straight esophagus. The latter, after perforating the central ganglion, enters the enormous saccular midgut, which, with its diverticula, forms the great bulk of body contents. Posteriorly, an extremely fine canal, which appears to be a functionless rudiment, joins the midgut with the rectal ampulla (but in 0. moubata even this is absent and the alimentary canal ends in a completely closed sac separated from the rectal ampulla) (see below).
Esophagus and proventricular fold. The esophagus, a short, straight tube perforating the central ganglion in its course from pharynx to alimentary sac, is lined with a layer of clear columnar cells with small nuclei. The irregular outlines of these cells are mutually adapted to one another in a dovetailing arrangement. At the juncture of the esophagus and large blood sac there is a small solid organ. This organ, in section, consists of a thick fold of epithelium of the same general character as that in the esophagus but of more columnar and less irregular cells, In the fold are some thick circular bands of muscular tissue and outside are longitudinal fibers passing from the esophagus to the gut. The epithelium of the fold passes imperceptibly into that of the esophagus, but ends abruptly on reaching the wall of the alimentary sac. The organ is very similar to, though
still more rudimentary than, the proventricular fold in the mos quito and probably has a similar function. The importance of this fold in the trypanosome infection of Culex makes its pres. ence in ticks of concern in connection with spirochete infection.
Alimentary sac and its diverticula. These organs, when fresh ly distended with blood, form smooth, dark red, lobulated masses. As the amount of blood diminishes, the diverticula become almost black in color and exhibit innumerable small lobulations.
In the young, unengorged tick, the long, narrow diverticula show active pulsatile movements that probably have no effect in drawing blood from the host but serve to distribute fluid to different parts of the sac.
From the entrance of the esophagus and extending posteriad to the neighborhood of the rectum there is a large central reser. voir. This reservoir extends anteriorly a little beyond the entrance of the esophagus so that this latter is situated upon the ventral surface of the sac. From the ventral surface poste riorly a conical tag passes ventrally behind the spermatheca to wards the rectum. The basal part of this tag contains blood, but as it narrows it becomes a clear tube of capillary character. This portion of the canal appears functionally inactive and can play no part in the passage of matter from the sac to the rectum. In 0. savignyi, therefore, the alimentary system is practically a cIosed one I but in o. moubata it is entirely closed; there is no passage between the small intestine and the rectal ampulla (Enigk and Grittner 1952)].
From the central reservoir a number of blind diverticula originate, the disposition and extent of which are constant in all ticks. There is an anterior, a lateral, and a posterior series. The anterior series, found only in Ornithodoros, is absent in Rhipicephalus and Hyalomma. It consists of a single, small median di verticulum extending anteriorly so as to lie over the gene's organ. The three pairs of lateral diverticula arise together at about the level of the entrance of the esophagus. The anterior lateral di. verticulum is short, the median one is somewhat longer, and the posterior one is large and long. The anterior and middle branches divide into two or usually three blind pouches. The posterior
branch divides into two branches that curve to the ventral surface. The more anterior of the branches ends beside the common genital duct. The most posterior surrounds the anus and ends a short distance anterior of this structure. Irregularities in diverticula arrangement are sometimes seen. The median lateral diverticulum may be large and give rise to the anterior of the two ventral branches.
The various sulci and prominences on the surface of Ornithodoros have relation to these alimentary diverticula. On the dorsum, the transverse sulcus limits posteriorly the central alimentary sac. Ventrally the region between the coxae supports upon its inner sur. face, with which the viscus is in actual contact, the caecal ends of the posterior lateral diverticula. The lesser prominences correspond in nearly every case with a particular diverticulum and the sulci with the intervals between two diverticula.
Structure of the alimentary sac.
The structure of the sac and its diverticula is identical. The cavity is lined by a single layer of large cells resting upon a thin basement membrane. Externally, very large single muscular fibres, arranged circularly and longitudinally, form an open meshwork with square meshes as in the mosquito. The lining epithelial cells are large with reticular protoplasm and large vesicular nuclei, some of which project freely into the lumen. Such cells are especially large and have their inner portions much swollen and vacuolated; they may contain dense black globules as well as red cells in various stages of intracellular digestion. In addition to large projecting cells, smaller cells, whose nuclei are situated nearer to the basement membrane, are present. Practically all cells of the sac contain small black granules, evidently derived from the digestion of the blood in the lumen. In undistended diverticula, the epithelium may form a more or less continuous lining of the tube, but in the distended tube the cells become very un evenly distributed, being almost absent in some places and in others forming very striking projecting masses.
Rectum and malpighian tubules. The rectum, which lies im mediately behind the spermatheca, or the white gland in the male, is an irregular sac having several capacious but short saccular dilatations. (In O. savignyi) it receives the rudiment of the
intestine and the two malpighian tubules (but in 0. moubata the rectum receives only the malpighian tubules and there is no con nection between intestine and rectal ampulla). Its walls are extremely thin and consist of a single layer of flattened cells. It contains a white fluid identical with the secretion of mal. pighian tubules. The rectum therefore does not serve as an ad junct to the alimentary canal, but functions as an excretory receptable. The white matter passed per anum by ticks also can not, strictly speaking, be regarded as feces.
The malpighian tubules are important because of their great length and functional activity and because of the frequency with which such organs are utilized by parasites in other animals. They consist of two long, fine white or transparent tubules arising on either side from the rectum, and after a complicated course among the viscera, ending blindly in the anterior portion of the body. These tubules come in relation with almost every important organ in the body and drain every quarter of the body cavity. The tubes in young ticks are of an even calibre through out and contain small quantities of secretion only. They are often swollen in aged ticks to form sacs similar to, but smaller than, the rectum. After oviposition the tubes and rectum may be greatly distended with characteristic white fluid that is evident externally as patches of lighter color. It is probable that the appearance is that described as a "fungus" in these ticks by Well man (1906A ,D,1907B).
Feeding habits of 0. moubata have been discussed in the section on the life cycle of this tick. Certain aspects of digestion and excretion of fluids are of considerable practical importance for they concern, at least, the fate of ingested spirochete param sites of relapsing fever (Borrelia spp.) and the discharge of these organisms onto or into a new vertebrate host when the tick subsequently feeds.
The volume of a blood meal is from two to six times the tickis original body weight (Lees 1946B). During ingestion, the cuticle stretches to accomodate this huge amount of fluid. Engorgement is completed in about half an hour. In order to reduce this tremendous volume ingested so rapidly, fluid is discharged from the coxal or
gans while the tick is yet feeding. Rectal discharge is very slight. These two methods of excretion are discussed in separate sections below.
Digestion in o. moubata apparently is much like that in o. savignyi, as described by Christophers, abstracted below.
Twenty-four hours after a meal the greatly distended di. verticula contain a soft coagulum from which a considerable amount of fluid blood may drain. Blood corpuscles are apparently un changed. Scattered through the fluid are numbers of intensely black, globular granules measuring from su to .5u or less in diameter. In sections these granules are collected especially at the periphery of the (fresh) blood, but they are also present in large numbers scattered throughout the mass. The black gran ules are derived from a previous meal, and there is therefore a considerable degree of mixture between the new blood and the contents of the diverticula prior to the meal.
Diverticula examined at some considerable time after diges tion show a number of reddish granules lying in the still partial ly fluid blood. These are free from attachments and when washed out fall to the bottom of the dish or among the viscera. Each is an entire cell containing a well-marked nucleus. Films of the sac contents made twenty-four hours after a blood meal show cells derived from the epithelium of the sac in addition to the host's leucocytes. Many of these are evidently the smaller un distended cells, previously noted as lying near the basement mem brane, now detached in preparation of the specimen. They contain a large circular or oval nucleus and finely reticular or partial ly vacuolated protoplasm. Similar cells, but larger and with portions of the vacuolated protoplasm stored with black granules, are also seen. In addition to these cells of the sac epithelium, there are other large, dark staining, circular cells with rather small nucleus. Their substance is markedly vacuolated and crowded with matter that they evidently have engulfed, blood corpuscles, black granules, chromatin fragments, etc. In section specimens made even six hours after the ingestion of blood, they appear lying apparently in isolated positions far removed from the sac walls. These probably function as wandering digestive cells. Their relam tion to the epithelium of the sac is not clear. As digestion ada