Part 12
The aspiratory pharynx extends from the connection of the hypopharynx with the mandibles to the œsophageal commissure. In a general way it first passes upward and then turns, passing backward. Its roof is formed by the continuation of the ventral surface of the epipharynx, while its floor is formed by the hypopharynx below and above by the chitinous layer which is continuous with the œsophagus. The anterior end of this particular portion curves strongly downward, where it is attached to the upper portion of the hypopharynx by a membranous ligament. In a general way it may be divided into a vertical and longitudinal portion. The longitudinal portion expands laterally, so that its capacity is greatly increased when dilated. Into the floor of this longitudinal portion empties the vertical part of the aspiratory pharynx, and at the junction of the two there seems to be a valvular arrangement, preventing blood from escaping after it has entered the upper part of the aspiratory canal. The œsophagus starts at the œsophageal commissure and ends in the gizzard. It is not expanded as in some insects, forming a gullet, but is practically the same diameter throughout its entire extent. It is lined with chitin, surrounded by a delicate basement membrane.
The gizzard is a mushroom-shaped organ, opening into the stomach and receiving the contents of the œsophagus and the aspiratory pharynx. From its anterior concave inner surface project a number of finger-like processes that arise from a basement membrane. They are lined with chitin, and each one near its base contains an elongated nucleus. These processes reach to the center of the gizzard and in a general way point towards the opening into the stomach. The gizzard is surrounded by circular bands of muscle fibers. Its function is not entirely understood. Wagner[205] has pointed out that these processes may act as whips to defibrinate the blood. It is more probable that their action is mainly valvular, preventing regurgitation of blood from the stomach.
The stomach of a flea is large and is capable of great distention. It is composed of a layer of secretory cells, resting on a basement membrane, the organ being surrounded by muscle fibers passing in different directions. The epithelial surface is thrown into little projections like villi. As absorption occurs in the stomach, these villi, or projections of the epithelial cells, may serve to increase the absorptive surface as well as serving a glandular function. At the anterior end of the stomach are the cecal glands.
The intestine is short, receives the excretion from the Malpighian tubules, and ends in the rectum. In the rectum may be seen the so-called “rectal glands.” All of the alimentary canal, with the exception of the stomach, is lined with chitin. The stomach and the rectum are capable of peristaltic movement.
The salivary glands, four in number, two on each side of the anterior part of the stomach, are simple acinous glands, lined with a single layer of secreting cells. The lumen of the glands is large and acts as a reservoir for the salivary secretion. The ducts from these glands unite to form a single duct which passes beneath the subœsophagal ganglion and empties into the salivary pump. This duct is lined on its inner surface by a spiral arrangement of chitin, giving it a very characteristic appearance.
DESCRIPTION OF FIGURE SHOWING MOUTH PARTS.
1. Epipharynx.
2. Mandibles.
3. Rostrum, paired portion.
4. Rostrum, unpaired portion.
5. Maxilla.
6. Maxillary palpus.
7. Salivary grooves.
8. Basal element of rostrum.
9. Basal element of mandibles.
10. Salivary pump.
11. Salivary duct.
12. Vertical portion, aspiratory canal.
13. Longitudinal portion, aspiratory canal.
14. Œsophagus.
15. Œsophageal ganglia.
16. Muscles operating aspiratory canal.
17. Hypopharynx.
18. Muscles operating salivary pump.
19. Ligament connecting hypopharynx with floor of aspiratory canal.
[Illustration:
PLATE I.
MOUTH PARTS OF CERATOPHYLLUS FASCIATUS. ]
THE ACT OF BITING.
The epipharynx, or pricker, makes an opening into the skin, through which the mandibles are inserted. These organs, by means of their serrations and independent movement, then enlarge the opening as with a saw, permitting them, with the epipharynx, to pass deeper and deeper until the points of the maxilla rest upon the cutaneous surface. The labial palpi serve as a protective case when the organs are not in action. When in action they serve as a guide to the piercing organs, but are not inserted into the skin. They double up like a bow, on each side, the bend of the bow becoming greater and greater as the biting apparatus passes deeper and deeper. Mitzmain[206] has pointed out that the spring-like action of this bow may assist the flea to withdraw the mandibles and epipharynx.
During the process of penetration the salivary pump receives saliva from the salivary glands and pumps it down, through the channel in the mandibles, into the wound. It will be seen that the hypopharynx, being attached above by a membranous ligament and connected intimately with the mandibles below, moves downward with these organs as they pass through the skin. At the same time the muscles attached to its under surface and the salivary pump contract, enlarging the lumen of the pump. When the mandibles are retracted the salivary pump collapses, thereby forcing the saliva out with the movement upward of the mandibles. At the proper time the muscles operating the aspiratory pharynx contract, drawing the canal open and aspirating blood through the canal made by the approximation of the epipharynx and mandibles and into the aspiratory pharynx. When full, the muscles relax from before backward and the pharynx, by means of the elastic reaction of its chitinous lining, contracts and forces the blood backward through the gizzard and into the stomach. It has already been pointed out that the finger-like processes in the gizzard probably act as valves to prevent regurgitation from the stomach.
HOW THE FLEA INFECTS ITS HOST.
The exact method by which the flea can transmit plague from animal to animal has, in our opinion, never been satisfactorily explained. There have been several explanations offered: First, that the rat may eat the flea. Miller[207] has found that the _Hepatazoon perniciosum_ is transmitted from rat to rat through the rat eating the mite, _Lelaps echidninus_, which acts as the intermediate host. We know, however, that when a rat is fed on plague material a mesenteric bubo is the rule, while in naturally infected rats a mesenteric bubo is a rare condition. This, then, negatives the possibility of plague being contracted through eating the flea.
Another explanation is that the infection comes from the saliva injected at the time of biting. We have already stated that after repeated examinations, both by dissecting out the salivary glands and by serial sections of the entire flea, plague bacilli have never been demonstrated in these glands or anywhere outside of the alimentary tract.
Another explanation has been advanced, that the bacillus is introduced by the contaminated mandibles. It is not possible to exclude this as a means of infection, although the Indian Plague Commission made numerous investigations and was unable to demonstrate the bacillus on the mandibles.
The possibility of infection taking place by regurgitation from the stomach has also been considered. As the stomach is guarded by the finger-like processes in the gizzard which seem to act as competent valves, and as the movement of the blood aspirated by reason of the mechanism already explained is in a backward direction, it would seem improbable that there is any regurgitation from the stomach.
The most plausible explanation that has been advanced has been based on an observation that blood-sucking insects at the time of biting frequently eject a drop of blood from the rectum. We know that the rectum may contain numerous plague bacilli, and it is supposed that this blood ejected in the vicinity of the bite is either brought in contact with the slight wound by the feet or mandibles of the flea itself or is rubbed in as a result of scratching. Verjbitski has shown that an emulsion of the feces of fleas or any plague material when placed upon the bitten part before the expiration of twenty-four hours is sufficient to give the animal plague. After twenty-four hours the animals did not develop plague, it being supposed that the slight wound in the skin made by the biting apparatus had healed. It is probable that this ejection of blood is purely accidental and does not necessarily occur at the time of biting, but it is likely that the insect had just previously had a full meal, which had been digested and passed into the rectum. In the many biting experiments done by McCoy and Mitzmain they report never having seen this ejection of rectal contents taking place. It might also be stated that where they used plague-infected fleas none of the animals developed plague after being bitten.
ENUMERATION OF FLEAS THAT HAVE BEEN FOUND ON RATS.
Various writers have reported the following fleas taken off rats:
Family SARCOPSYLLIDÆ Taschenberg.
Genus DERMATOPHILUS.
1. _Dermatophilus cæcata_ Enderlein.—Seventeen specimens (females) were found by Doctor Enderlein on the skin behind the ears of a specimen of _Mus rattus_ from Saopaulo, Brazil.
Genus ECHIDNOPHAGA Olliff.
2. _Echidnophaga gallinacea_ Westwood.—Tiraboschi has found this flea on the _Mus rattus_ in Italy.
3. _Echidnophaga rhynchopsylla_ Tiraboschi.—This flea has been taken in Italy from _Mus rattus_ and _Mus alexandrinus_. It has been described by Rothschild under the name of _Echidnophaga murina_.
Family PULICIDÆ Taschenberg.
Genus CERATOPHYLLUS Curtis.
4. _Ceratophyllus fasciatus_ Bosc.—This is the common rat flea of Europe and the United States. It has also been found in Cape Town, Australia, and is occasionally found on rats in India.
5. _Ceratophyllus londiniensis_ Rothschild.—This flea has been taken off mice in England; off rats in Italy (_Ceratophyllus italicus_ Tiraboschi) and has been found once on _Mus rattus_ in San Francisco, Cal.
6. _Ceratophyllus acutus_ Baker.—This is the common flea of the California ground squirrel; and has been taken off _Mus norvegicus_ in San Francisco, Cal.
7. _Ceratophyllus anisus_ Rothschild.—This flea has been described by Rothschild from Yokohama, Japan, taken off _Felis_ sp. One specimen was found in San Francisco, Cal., taken off _Mus norvegicus_.
8. _Ceratophyllus niger_ Fox.—This flea is commonly found in San Francisco, Cal., in chicken yards and sparrows’ nests and has also been found on rats, _Mus norvegicus_, and on man.
9. _Ceratophyllus consimilis_ Wagner.
10. _Ceratophyllus lagomys_ Wagner.
11. _Ceratophyllus mustelæ_ Wagner.
12. _Ceratophyllus penicilliger_ Grube.
These fleas have been taken off _Mus norvegicus_ in Europe.
Genus PULEX Linn.
13. _Pulex irritans_ Linn.—This flea is widely distributed throughout the world, and while essentially the human flea has been found on many different species of animals and has frequently been encountered on rats. A very large number of specimens have been taken off rats in San Francisco, Cal.
Genus LŒMOPSYLLA Rothschild.
14. _Lœmopsylla cheopis_ Rothschild.—This is the common rat flea in tropical and subtropical countries. It has also been found in seaports of the temperate zone, where it has been brought by ship rats. Ninety-eight per cent of the rat fleas in India are of this species. It has been found in Australia, where it was described by Tidswell under the name of _Pulex pallidus_. In the Philippine Islands, where it was described by Hertzog as the _Pulex philippinensis_. It has been found in Brazil, where it was described by Baker as _Pulex brasiliensis_, and Tiraboschi has found it in Italy, where it has been described as the _Pulex murinus_. This flea has been frequently found on man in India.
Genus CTENOCEPHALUS Kolenati.
15. _Ctenocephalus canis_ Curtis.—This is the common dog flea found in many parts of the world and is frequently taken off rats.
16. _Ctenocephalus felis_ Bouché.—This is the common cat flea and is also a widely distributed species. Frequently taken off rats.
Genus CTENOPSYLLUS Kolenati.
17. _Ctenopsyllus musculi_ Dugés.—In England this flea is commonly found on the domestic mouse. It has a wide distribution and has been found on rats and mice in Europe, South Africa, India, Australia, Mexico, and other places, and has been taken off _Mus norvegicus_, _Mus rattus_, and _Mus musculus_ in San Francisco, Cal.
Genus NEOPSYLLA Wagner.
18. _Neopsylla bidentatiformis_ Wagner.—This flea has been taken off _Mus norvegicus_ in the Crimea.
Genus HOPLOPSYLLUS Baker.
19. _Hoplopsyllus anomalus_ Baker.—This is one of the common groundsquirrel fleas of California and has been found on _Mus norvegicus_ in San Francisco and Palo Alto, Cal. That these squirrel fleas are occasionally found on rats is interesting from the fact that plague has been demonstrated both in rats and the ground squirrel in California.
Genus HYSTRICHOPSYLLA Taschenberg.
20. _Hystrichopsylla tripectinata_ Tiraboschi.—Reported by Tiraboschi from _Mus musculus_ in Italy.
Genus CTENOPTHALMUS Kolenati.
21. _Ctenopthalmus agyrtes_ Heller.—Taken off _Mus Norvegicus_ in England.
_The results of the identification of 19,768 fleas in San Francisco and Oakland, Cal._
SAN FRANCISCO, 1908. Host: MUS NORVEGICUS. ───────────┬───────────┬───────────┬───────────┬───────────┬─────────── Month. │ C. │L. cheopis.│ P. │ Cten. │ Cten. │fasciatus. │ │ irritans. │ musculi. │ felis, │ │ │ │ │ Cten. │ │ │ │ │ canis. ───────────┼─────┬─────┼─────┬─────┼─────┬─────┼─────┬─────┼─────┬───── „ │Male.│ Fe- │Male.│ Fe- │Male.│ Fe- │Male.│ Fe- │Male.│ Fe- │ │male.│ │male.│ │male.│ │male.│ │male. ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── April to │1,343│2,510│ 485│ 837│ 31│ 76│ 78│ 211│ 16│ 31 July 31 │ │ │ │ │ │ │ │ │ │ August │ 489│ 883│ 145│ 228│ 156│ 206│ 27│ 90│ 17│ 22 September │ 543│1,180│ 655│ 930│ 339│ 387│ 33│ 109│ 46│ 119 October │ 254│ 435│ 509│ 652│ 59│ 64│ 9│ 45│ 6│ 18 November │ 129│ 252│ 256│ 288│ 52│ 69│ 20│ 54│ 6│ 6 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │2,758│5,260│2,050│2,935│ 637│ 802│ 167│ 509│ 91│ 196 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── Host: MUS RATTUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── │ 23│ 43│ 3│ 3│ 0│ 0│ 17│ 16│ 1│ 0 │ 4│ 7│ 1│ 0│ 9│ 16│ 3│ 3│ 0│ 0 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │ 27│ 50│ 4│ 3│ 9│ 16│ 20│ 19│ 1│ 0 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── Host: MUS MUSCULUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── │ 4│ 10│ 1│ 0│ 0│ 0│ 2│ 10│ 0│ 0 │ 11│ 10│ 1│ 6│ 4│ 4│ 0│ 3│ 1│ 1 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │ 15│ 20│ 2│ 6│ 4│ 4│ 2│ 13│ 1│ 1 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── OAKLAND, CAL., 1909. Host: MUS NORVEGICUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── February │ 135│ 304│ 166│ 178│ 1│ 1│ 229│ 506│ 1│ 3 March │ 253│ 456│ 167│ 215│ 2│ 5│ 125│ 243│ 1│ 2 April │ 227│ 479│ 105│ 129│ 0│ 1│ 62│ 143│ 0│ 1 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │ 615│1,239│ 438│ 522│ 3│ 7│ 416│ 892│ 2│ 6 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── Host: MUS ALEXANDRINUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── April │ 1│ 5│ 0│ 0│ 0│ 0│ 0│ 0│ 0│ 0 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────
This does not include a few other specimens of different species taken from _Mus rattus_ and _Mus norvegicus_, which have been included under the heading of “Enumeration of fleas which have been found on rats.”
SYNOPSIS OF FLEAS COMMONLY FOUND ON RATS.
A. WITHOUT A COMB OF SPINES ON THE PROTHORAX OR THE HEAD. 1. Two bristles on the gena, an ocular bristle placed below the eye, an oral bristle placed just above root of maxilla. Mesothorax not divided by an internal incrassation. Claspers in male forming prominent hump, claw like and covered by hairy flap. An irregular row of about 10 teeth on inner _Pulex side of hind coxa irritans_.
2. Two bristles on gena, an ocular placed in front of and just above middle of eye, an oral bristle placed just above root of maxilla. Mesothorax divided by internal incrassation, claspers not forming prominent hump, not claw like, not covered by hairy flap. A regular row of about six _Lœmopsylla teeth on inner side of hind coxa, cheopis_.
AA. WITH A COMB OF SPINES ON PROTHORAX BUT NOT ON HEAD. 3. Three bristles on lower genal row, upper genal row represented by three or four small bristles running along anterior margin of antennal groove. Eye present, about five hairs on second joint of antenna, not as long as third joint. Maxillary palpi not as long as labial palpi. Labial palpi reach to apex of fore coxa. Spines on posterior tibia in pairs of about five groups. Head of male _Ceratophyllus flattened on top, fasciatus_.
AAA. WITH A COMB OF SPINES ON THE PROTHORAX AND ON THE HEAD. 4. Eye present, seven spines on lower margin of gena. _Ctenocephalus Spines on posterior border of tibia in pairs canis_ or _felis_.
5. Eye absent, four spines on hind margin of gena. Spines on posterior tibia single and in a close _Ctenopsyllus set row musculi_.
DESCRIPTION OF PLATE II. ────────────────────────────────────────────┬────────────────────── Fig. 1. Clasping organs of male. │P Process. „ │F Finger. „ │M Manubrium. „ │IX St Ninth Sternite. ────────────────────────────────────────────┼────────────────────── Fig. 2. Head of female. │ ────────────────────────────────────────────┼────────────────────── Fig. 3. Terminal abdominal segments, female.│8 T Eighth Tergite. „ │8 St Eighth Sternite. „ │10 T Tenth Tergite. „ │10 St Tenth Sternite. „ │Sp Spermatheca. „ │Sty Stylet. ────────────────────────────────────────────┼────────────────────── Fig. 4. Hind tibia. │
CERATOPHYLLUS FASCIATUS Bosc.
[Plate II.]
_Head._—Evenly and gently rounded in the female, flattened on top in the male. Frontal notch distinct. Eye present, placed low down in head. Gena acutely pointed posteriorly. Maxilla triangular. Maxillary palpi not as long as the labial palpi. Labial palpi reach to apex of anterior coxa, 5-jointed. Antennal groove in the male reaches to top of head, in the female to within one-third. There are 3 bristles on the lower genal row, the middle of which is the smallest, while the upper genal row is represented by 3 small bristles, extending along the edge of the antennal groove. In the male the lowermost bristle is frequently paired. There are several fine hairs above the eye. The occiput contains the normal row of apical bristles, the lowest of which is the largest. There is one bristle back of the middle of the antennal groove and a number of fine hairs along the posterior margin of the antennal groove. The antenna is 3-jointed, the first joint contains a row of about 5 very short fine hairs, while the second joint contains about 5 not as long as the third joint.
[Illustration:
PLATE II.
CERATOPHVLLUS FASCIATUS, BOSC. ]
_Thorax._—The pronotum has one row of about 10 bristles, and a ctenidium composed of about 16 or 18 spines. The mesonotum has a posterior row of about 10 long bristles and there is an anterior row of more numerous smaller ones. The metanotum has also a posterior row of about 10 large bristles, with an anterior row of more numerous smaller ones, while still anterior to this there are 5 or 6 still smaller bristles. The metathorax contains 8 or 10 bristles which are small anteriorly, larger posteriorly. On the sternum of the metathorax there are 2 large bristles, while the episternum has 3 smaller ones. On the epimerum are 2 bristles placed anteriorly and 3 or 4 posteriorly, one of which is on the apical margin.
_Abdomen._—The first stigma is nearly in line with those of the other abdominal segments. There are two rows of bristles on the abdominal tergites, a posterior of about 12 or 14 and an anterior of smaller, less numerous bristles. The antipygidial bristles in the female are 3 in number on each side, of which the middle is the longest, and the inner one the smallest. The male has but 2 antipygidial bristles on each side. The sternites from the third to the sixth have a single row of about 10 bristles, while the seventh has about 12. The metanotum has 2 teeth on each side, as have the first and second abdominal tergites. The third and fourth abdominal tergites have 1 tooth on each side.
_Legs._—The fore coxæ are normally clothed. The fore femur has on the outer side 11 or 12 fine bristles irregularly disposed, while on the mid femur there is a row of about 3 to 5 bristles on the inner surface. The hind coxa has no patch of spines on the inner side, while on the inner surface of the hind femur there is a row of about 5 to 7 bristles. The spines on the posterior tibia are in pairs of six groups, while on the outer surface there is a row of about 7 bristles. None of the apical bristles of the tarsi are as long as the next succeeding joint. The fifth tarsal joints on all the legs have 5 lateral spines.
Length of joints of tarsi:
Mid tarsi (♂) 8 7 4½ 3 7 Hind tarsi 18 11 7 4 8 Mid tarsi (♀) 8 7 5 3½ 7 Hind tarsi 21 13 8 5 8
_Modified segments._—(♂). The manubrium of the claspers is straight and narrow, while the process extends upward as a short, blunt cone, where at the tip there are several fine hairs. The lower margin is evenly and gently rounded. The finger is short, extending but a little above the process. It is concave on its anterior surface and convex on its posterior, and from the posterior margin there are 2 large and 2 small bristles alternating. Two long heavy bristles arise from the process below the insertion of the finger. The ninth sternite is broad, with a deep sinus in its posterior border. Its lateral surface contains numerous fine hairs, these hairs being somewhat larger just beneath the sinus. Along the dorsal border of the tenth sternite there are 3 heavy bristles in line. At the tip of the tenth tergite there is one heavy bristle. Besides these heavy bristles in this segment there are numerous fine hairs.
(♀) The eighth tergite contains just anterior to the sensory plate about 12 small hairs while just beneath the sensory plate there are 2 long bristles. Lower down there is a patch of about 6 bristles and on the apical margin 4 to 6. The stylet is short, cylindrical, slightly larger at the base than at the tip, where there is a long bristle. On the under surface arises a fine hair. Substylar flap (tenth sternite) has along its margin numerous hairs.
DESCRIPTION OF PLATE III. ────────────────────────────────────────────┬────────────────────── Fig. 1. Clasping organs of male. │P Process. „ │M Manubrium. „ │F Finger. „ │IX St Ninth Sternite. ────────────────────────────────────────────┼────────────────────── Fig. 2. Head of female. │ ────────────────────────────────────────────┼────────────────────── Fig. 3. Terminal abdominal segments, female.│8 T Eighth Tergite. „ │8 St Eighth Sternite. „ │10 T Tenth Tergite. „ │10 St Tenth Sternite. „ │Sp Spermatheca. ────────────────────────────────────────────┼────────────────────── Fig. 4. Hind coxa inner surface. │
LŒMOPSYLLA CHEOPIS Rothschild.
[Plate III.]
_Head._—Abruptly rounded. Flattened on top in ♂. Eye present. No ctenidia on head. Antennal groove in the ♀ reaches to within one-third of the top of the head. In ♂ reaches to top of head. Gena obtusely pointed posteriorly. Maxilla triangular. Maxillary palpi are not as long as labial palpi. Labial palpi reach to apex of fore coxæ, 4-jointed. Anterior edge of antennal groove overlapped by chitinous flap. On posterior edge of antennal groove are a number of small bristles, these being most distinct in the male. The first antennal joint in the male contains 4 or 5 hairs at its outer edge, while transversely there is a row of several fine hairs. The second joint has a row of fine hairs not as long as the third joint. Divisions marking separations of third joint most pronounced on dorsal edge. Two bristles on gena. The oral bristle placed low down just above the base of the maxilla; the ocular bristle in front and just above the middle of the eye. Six bristles on the posterior margin of the occiput on each side with 2 back of the antennal groove.
[Illustration:
PLATE III.
LŒMOPSYLLA CHEOPIS, ROTHSCHILD. ]