[Ill.u.s.tration: 82. Young larva of Dermatobia cyaniventris. After Surcouf.]
_Dermatobia cyaniventris_--This fly (fig. 83) is widely distributed throughout tropical America, and in its larval stage is well known as a parasite of man. The larvae (figs. 81 and 82) which are known as the "ver macaque," "torcel," "ver moyocuil" or by several other local names, enter the skin and give rise to a boil-like swelling, open at the top, and comparable with the swelling produced by the warble fly larvae, in cattle. They cause itching and occasional excruciating pain. When mature, nearly an inch in length, they voluntarily leave their host, drop to the ground and complete their development. The adult female is about 12 mm. in length. The face is yellow, the frons black with a grayish bloom; antennae yellow, the third segment four times as long as the second, the arista pectinate. The thorax is bluish black with grayish bloom; the abdomen depressed, brilliant metallescent blue with violet tinge. The legs are yellowish, the squamae and wings brownish.
The different types of larvae represented in figure 81 were formerly supposed to belong to different species but Blanchard regards them as merely various stages of the same species. It is only very recently that the early stage and the method by which man becomes infested were made known.
[Ill.u.s.tration: 83. Dermatobia cyaniventris (1). After Manson.]
[Ill.u.s.tration: 84. Mosquito carrying eggs of Dermatobia cyaniventris.
After Surcouf.]
About 1900, Blanchard observed the presence of packets of large-sized eggs under the abdomen of certain mosquitoes from Central America; and in 1910, Dr. Morales, of Costa Rica, declared that the Dermatobia deposited its eggs directly under the abdomen of the mosquito and that they were thus carried to vertebrates. Dr. Nunez Tovar observed the mosquito carriers of the eggs and placing larvae from this source on animals, produced typical tumors and reared the adult flies. It remained for Surcouf (1913) to work out the full details. He found that the Dermatobia deposits its eggs in packets covered by a very viscid substance, on leaves. These become attached to mosquitoes of the species _Janthinosoma lutzi_ (fig. 84) which walk over the leaves. The eggs which adhere to the abdomen, remain attached and are thus transported.
The embryo develops, but the young larva (fig. 82) remains in the egg until it has opportunity to drop upon a vertebrate fed upon by the mosquito.
Muscidae
The following MUSCIDae, characterized elsewhere, deserve special mention under our present grouping of parasitic species. Other important species will be considered as facultative parasites.
_Stomoxys calcitrans_, the stable-fly, or the biting house-fly, is often confused with _Musca domestica_ and therefore is discussed especially in our consideration of the latter species as an accidental carrier of disease. Its possible relation to the spread of infantile paralysis is also considered later.
The _tsetse flies_, belonging to the genus _Glossina_, are African species of blood-sucking Muscidae which have attracted much attention because of their role in transmitting various trypanosome diseases of man and animals. They are characterized in Chapter XII and are also discussed in connection with the diseases which they convey.
_Chrysomyia macellaria_, (= _Compsomyia_), the "screw worm"-fly is one of the most important species of flies directly affecting man, in North America. It is not normally parasitic, however, and hence will be considered with other facultative parasites in Chapter IV.
[Ill.u.s.tration: 85. Larva of Auchmeromyia luteola. After Graham-Smith.]
[Ill.u.s.tration: 86. Auchmeromyia luteola (4). After Graham-Smith.]
_Auchmeromyia luteola_, the Congo floor maggot. This is a muscid of grewsome habits, which has a wide distribution throughout Africa. The fly (fig. 86) deposits its eggs on the ground of the huts of the natives. The whitish larvae (fig. 85) on hatching are slightly flattened ventrally, and each segment bears posteriorly three foot-pads transversely arranged. At night the larvae find their way into the low beds or couches of the natives and suck their blood. The adult flies do not bite man and, as far as known, the larvae do not play any role in the transmission of sleeping sickness or other diseases.
This habit of blood-sucking by muscid larvae is usually referred to as peculiar to _Auchmeromyia luteola_ but it should be noted that the larvae of _Protocalliphora_ frequent the nests of birds and feed upon the young. Mr. A. F. Coutant has studied especially the life-history and habits of _P. azurea_, whose larvae he found attacking young crows at Ithaca, N.Y. He was unable to induce the larvae to feed on man.
[Ill.u.s.tration: 87. Cordylobia anthropophaga (3). After Fulleborn.]
[Ill.u.s.tration: 88. Larva of Cordylobia anthropophaga. After Blanchard.]
_Cordylobia anthropophaga_, (_Ochromyia anthropophaga_), or Tumbu-fly (fig. 87) is an African species whose larvae affect man much as do those of _Dermatobia cyniventris_, of Central and South America. The larva (fig. 88), which is known as "ver du Cayor" because it was first observed in Cayor, in Senegambia, develops in the skin of man and of various animals, such as dogs, cats, and monkeys. It is about 12 mm. in length, and of the form of the larvae of other muscids. Upon the intermediate segments are minute, brownish recurved spines which give to the larva its characteristic appearance. The life-history is not satisfactorily worked out, but Fuller (1914), after reviewing the evidence believes that, as a rule, it deposits its young in the sleeping places of man and animals, whether such be a bed, a board, the floor, or the bare ground. In the case of babies, the maggots may be deposited on the scalp. The minute maggots bore their way painlessly into the skin.
As many as forty parasites have been found in one individual and one author has reported finding more than three hundred in a spaniel puppy.
Though their attacks are at times extremely painful, it is seldom that any serious results follow.
THE SIPHONAPTERA OR FLEAS
The SIPHONAPTERA, or fleas (fig. 89) are wingless insects, with highly chitinized and laterally compressed bodies. The mouth-parts are formed for piercing and sucking. Compound eyes are lacking but some species possess ocelli. The metamorphosis is complete.
This group of parasites, concerning which little was known until recently, has a.s.sumed a very great importance since it was learned that fleas are the carriers of bubonic plague. Now over four hundred species are known. Of these, several species commonly attack man. The most common hominoxious species are _Pulex irritans_, _Xenopsylla cheopis_, _Ctenocephalus canis_, _Ctenocephalus felis_, _Ceratophyllus fasciatus_ and _Dermatophilus penetrans_, but many others will feed readily on human blood if occasion arises.
[Ill.u.s.tration: 89. Xenopsylla cheopis, male (25). After Jordan and Rothschild.]
We shall treat in this place of the general biology and habits of the hominoxious forms and reserve for the systematic section the discussion of the characteristics of the different genera.
The most common fleas infesting houses in the Eastern United States are the cosmopolitan dog and cat fleas, _Ctenocephalus canis_ (fig. 90) and _C. felis_. Their life cycles will serve as typical. These two species have until recently been considered as one, under the name _Pulex serraticeps_. See figure 92.
[Ill.u.s.tration: 90. Dog flea (15). After Howard.]
The eggs are oval, slightly translucent or pearly white, and measure about .5 mm. in their long diameter. They are deposited loosely in the hairs of the host and readily drop off as the animal moves around.
Howard found that these eggs hatch in one to two days. The larvae are elongate, legless, white, worm-like creatures. They are exceedingly active, and avoid the light in every way possible. They cast their first skin in from three to seven days and their second in from three to four days. They commenced spinning in from seven to fourteen days after hatching and the imago appeared five days later. Thus in summer, at Washington, the entire life cycle may be completed in about two weeks.
(cf. fig. 91, 92).
Strickland"s (1914) studies on the biology of the rat flea, _Ceratophyllus fasciatus_, have so important a general bearing that we shall cite them in considerable detail.
[Ill.u.s.tration: 91. Larva of Xenopsylla cheopis. After Bacot and Ridewood.]
He found, to begin with, that there is a marked inherent range in the rate of development. Thus, of a batch of seventy-three eggs, all laid in the same day and kept together under the same conditions, one hatched in ten days; four in eleven days; twenty-five in twelve days; thirty-one in thirteen days; ten in fourteen days; one in fifteen days; and one in sixteen days. Within these limits the duration of the egg period seems to depend mainly on the degree of humidity. The incubation period is never abnormally prolonged as in the case of lice, (Warburton) and varying conditions of temperature and humidity have practically no effect on the percentage of eggs which ultimately hatch.
The same investigator found that the most favorable condition for the larva is a low temperature, combined with a high degree of humidity; and that the presence of rubbish in which the larva may bury itself is essential to its successful development. When larvae are placed in a bottle containing either wood-wool soiled by excrement, or with feathers or filter paper covered with dried blood they will thrive readily and pupate. They seem to have no choice between dried blood and powdered rat feces for food, and also feed readily on flea excrement. They possess the curious habit of always devouring their molted skins.
[Ill.u.s.tration: 92. Head and p.r.o.notum of (_a_) dog flea; (_b_) of cat flea; (_c_) of hen flea. After Rothschild. (_d_) Nycteridiphilus (Ischnopsyllus) hexactenus. After Oudemans.]
An important part of Strickland"s experiments dealt with the question of duration of the pupal stage under the influence of temperature and with the longevity and habits of the adult. In October, he placed a batch of freshly formed coc.o.o.ns in a small dish that was kept near a white rat in a deep gla.s.s jar in the laboratory. Two months later one small and feeble flea had emerged, but no more until February, four months after the beginning of the experiment. Eight coc.o.o.ns were then dissected and seven more found to contain the imago fully formed but in a resting state. The remainder of the batch was then placed at 70 F. for one night, near a white rat. The next day all the coc.o.o.ns were empty and the fleas were found on the white rat.
Thus, temperature greatly influences the duration of the pupal period, which in _Ceratophyllus fasciatus_ averages seventeen days. Moreover, when metamorphosis is complete a low temperature will cause the imago to remain within the coc.o.o.n.
s.e.xually mature and ovipositing fleas, he fed at intervals and kept alive for two months, when the experiment was discontinued. In the presence of rubbish in which they could bury themselves, unfed rat fleas were kept alive for many months, whereas in the absence of any such substratum they rarely lived a month. In the former case, it was found that the length of life is influenced to some degree by the temperature and humidity. In an experiment carried out at 70 F. and 45 per cent humidity, the fleas did not live for more than four months, while in an experiment at 60 F. and 70 per cent humidity they lived for at least seventeen months. There was no indication that fleas kept under these conditions sucked moisture from surrounding objects, and those kept in bell jars, with an extract of flea-rubbish on filter paper, did not live any longer than those which were not so supplied.
Curiously enough, although the rat is the normal host of _Ceratophyllus fasciatus_, it was found that when given the choice these fleas would feed upon man in preference to rats. However, none of the fleas laid eggs unless they fed on rat blood.
The experiments of Strickland on copulation and oviposition in the rat flea showed that fleas do not copulate until they are s.e.xually mature and that, at least in the case of _Ceratophyllus fasciatus_, the reproductive organs are imperfectly developed for some time (more than a week) after emerging from the pupa. When mature, copulation takes place soon after the fleas have fed on their true host--the rat--but not if they have fed on a facultative host only, such as man. Copulation is always followed by oviposition within a very short time.
The effect of the rat"s blood on the female with regard to egg-laying, Strickland concludes, is stimulating rather than nutritive, as fleas that were without food for many months were observed to lay eggs immediately after one feed. Similarly, the male requires the stimulus of a meal of rat"s blood before it displays any copulatory activity.
Mitzmain (1910) has described in detail the act of biting on man, as observed in the squirrel flea, _Ceratophyllus acutus_. "The flea when permitted to walk freely on the arm selects a suitable hairy s.p.a.ce where it ceases abruptly in its locomotion, takes a firm hold with the tarsi, projects its proboscis, and prepares to puncture the skin. A puncture is drilled by the p.r.i.c.king epipharynx, the saw-tooth mandibles supplementing the movement by lacerating the cavity formed. The two organs of the rostrum work alternately, the middle piece boring, while the two lateral elements execute a sawing movement. The mandibles, owing to their basal attachments, are, as is expressed by the advisory committee on plague investigations in India (_Journal of Hygiene_, vol.
6, No. 4, p. 499), "capable of independent action, sliding up and down but maintaining their relative positions and preserving the lumen of the aspiratory channel." The labium doubles back, the V-shaped groove of this organ guiding the mandibles on either side."
"The action of the proboscis is executed with a forward movement of the head and a lateral and downward thrust of the entire body. As the mouth-parts are sharply inserted, the abdomen rises simultaneously. The hind and middle legs are elevated, resembling oars. The forelegs are doubled under the thorax, the tibia and tarsi resting firmly on the epidermis serve as a support for the body during the feeding. The maxillary palpi are retracted beneath the head and thorax. The labium continues to bend, at first acting as a sheath for the sawing mandibles, and as these are more deeply inserted, it bends beneath the head with the elasticity of a bow, forcing the mandibles into the wound until the maxillae are embedded in the skin of the victim. When the proboscis is fully inserted, the abdomen ceases for a time its lateral swinging."
"The acute pain of biting is first felt when the mandibles have not quite penetrated and subsequently during each distinct movement of the abdomen. The swinging of the abdomen gradually ceases as it becomes filled with blood. The sting of the biting gradually becomes duller and less sensitive as feeding progresses. The movements of the elevated abdomen grow noticeably feebler as the downward thrusts of the springy bow-like labium becomes less frequent."
"As the feeding process advances one can discern through the translucent walls of the abdomen a constant flow of blood, caudally from the pharynx, accompanied by a peristaltic movement. The end of the meal is signified in an abrupt manner. The flea shakes its entire body, and gradually withdraws its proboscis by lowering the abdomen and legs and violently twisting the head."
"When starved for several days the feeding of the rat fleas is conducted in a rather vigorous manner. As soon as the proboscis is buried to the full length the abdomen is raised and there ensues a gradual lateral swaying motion, increasing the alt.i.tude of the raised end of the abdomen until it a.s.sumes the perpendicular. The flea is observed at this point to gain a better foothold by advancing the fore tarsi, and then, gradually doubling back the abdomen, it turns with extreme agility, nearly touching with its dorsal side the skin of the hand upon which it is feeding. Meanwhile, the hungry parasite feeds ravenously."
"It is interesting to note the peculiar nervous action which the rodent fleas exhibit immediately when the feeding process is completed or when disturbed during the biting. Even while the rostrum is inserted to the fullest the parasite shakes its head spasmodically; in a twinkling the mouth is withdrawn and then the flea hops away."
A habit of fleas which we shall see is of significance in considering their agency in the spread of bubonic plague, is that of ejecting blood from the a.n.u.s as they feed.
Fleas are famous for their jumping powers, and in control measures it is of importance to determine their ability along this line. It is often stated that they can jump about four inches, or, according to the Indian Plague Commission _Xenopsylla cheopis_ cannot hop farther than five inches. Mitzmain (1910) conducted some careful experiments in which he found that the human flea, _Pulex irritans_, was able to jump as far as thirteen inches on a horizontal plane. The mean average of five specimens permitted to jump at will was seven and three-tenths inches.
The same species was observed to jump perpendicularly to a height of at least seven and three-fourths inches. Other species were not able to equal this record.