other insects; and some kinds of plant bugs and leaf hoppers in
which the thirst for blood is an aberration.
Specifically Infective Arthropoda. The parasitic and predaceous forms so far enumerated are for the most part obnoxious rather than greatly dangerous, and the trouble they cause does not linger after their disappearance ; but we have now to deal with an assortment of parasitic and predaceous arthropoda, which, by propagating micro- parasites pathogenic to man, cause ill effects that outweigh and out- Fast any direct damage that they themselves can inflict. Such ar- thropoda, having severally a definite biological association with a particular disease, and being specific " nurses " or intermediary hosts of the parasite actually causing the disease, are known as " carriers " or " vectors." Their function is beautifully exemplified in Sir Ronald Ross's great discovery of the way by which the parasites of malarial fevers are maintained and disseminated a discovery which must here have precedence, not that it was his- torically the first of its kind, but because it was originally presented in a complete form and best illustrates the emergence of Medical Entomology as a distinct study.
Independently of Ross's investigations, it was known that the parasites multiplied periodically by non-sexual fission (schizogony) in the blood of malaria patients, and that they also gave issue to prospectively sexual bodies (gametocytes) , male and female, which did not become ripe until after the blood had been withdrawn from the blood vessels; and it had been discovered by MacCallum that when the ripe sexual elements (gametes') did become revealed in the drawn blood the males and females paired together to form united couples or zygotes; it was known, in short, that while the parasites multiply non-sexually in the blood of the individual human sufferer, the process, by which they are maintained as specific parasites of the human race must take place, somehow, outside the individual human host. Manson inferred, from his own earlier investigations of the filaria parasite, that it must be in the interior of some blood-sucking arthropod, probably a mosquito, and the arduous proof of this hypothesis was unravelled by Ross. Ross followed the parasites from the vertebrate host into the stomach of the mosquito, and thence into the insect's stomach wall, where the zygotes encyst and establish themselves as parasitic oocysts. Under favourable conditions the oocysts increase in size by the internal proliferation of their contents, and in about ten days ripen into sporocysts, full of minute spore elements or sporozoites. By the rupture of the sporocysts the sporozoites escape into the body cavity of the mos- quito and accumulate considerably in its salivary glands, which also lie in the body cavity, so that when the now infective insect bites a healthy person the sporozoites are injected with its saliva into that person's blood, where they grow and multiply non-sexually, and after a definite interval usually reveal their presence in a par- oxysm of malarial fever.
The mosquito thus is the medium for the necessary sexual re- production of the malaria parasites; eventually the insect not only becomes a kind of supersporocyst at whose expense the actual sporocysts and their sporozoites are developed and nourished, but also acts as the locomotor instrument for distributing the sporozoites and planting them in their future sphere of action.
So far as is known, the only mosquitos that serve as nurses for the malaria parasites of man are mosquitos of the genus Anopheles. It can hardly be supposed that these parasites originated at the outset either in man or in Anopheles mosquitos. Kindred parasites inhabit the blood of other mammals as well as of birds and reptiles and are distributed by other kinds of blood-sucking insects, and it is reasonable to believe that all may have had a common origin and, up to a certain point, a common line of evolution, and that the species now adapted to man were derived, through the inter- mediation of Anopheles mosquitos, from forms whose evolution had been hammered out quite independently of the human race. Not every one of the many species of Anopheles is susceptible to infec- tion, nor of the susceptible species are all equally so, or indifferently so under all conditions. Again, not all species of Anopheles are dis- posed to attack man, or to shelter in houses. Furthermore, although under favourable conditions the sexual development of the parasites in the mosquito is completed in about ten days, and although under ordinary circumstances the sporozoites would be discharged from the salivary glands of the infected insect in the course of a few bites, yet the sexual development may be much delayed by cold and other unfavourable conditions, and the salivary glands might remain in- fective for a long time if the insect remained inactive or torpid.
Ross's great work, which was experimentally elucidated with a common malaria parasite of birds, was the outcome of Manson's illuminative discovery of the necessary intermediation of mosquitos in propagating lymphatic filariasis, a disease, common in most tropical countries, caused by the invasion of the lymphatic system by the parasitic worm Filaria bancrofti. In certain phases of this disease the female worm gives issue to swarms of minute embryos, or microfilariae, which are carried into the patient's blood and are usually found in the superficial blood-vessels at night. Manson, in 1878, persuaded a Chinaman with microfilariae in his blood to suffer him- self to be bitten by some common house mcsquitcs during sleep, and on examining the replete insects at intervals during a number of days following their infective repast he traced the ingested micro- filariae through definite stages of larval development in the insects'
stomach and muscles. Subsequent observers, of whom Dr. G. C. Low was the first to publish his investigations, discovered that the larval filariae find their way into the proboscis of the mosquito, whence on occasion offered they escape into the skin of a fresh human victim. More than a dozen species of mosquitos are now known to be capable of acting as intermediary host to the larvae of the worm of lymphatic filariasis.
Yellow fever is another disease which, so far as is known, is com- municated from man to man exclusively by mosquitos of one species, namely, the notorious Stegomyia fasciata. The connexion between this characteristically house-haunting insect and the disease was inferred by Dr. Finlay of Havana in 1881, and was experimentally demonstrated by the U.S. Commission under Dr. Walter Reed in 1900. Here again, as in malaria and filariasis, the specific parasite of the disease must undergo a definite development within the insect, since the insect can become infected only if it feeds on a patient during the first three days of fever, and does not become infective until at least 12 days after so feeding.
Another disease spread in this way by mosquitoes is dengue fever. The actual virus of the disease has not yet been demonstrated, but there is good proof that it is transmitted from sick to healthy per- sons by two of the commonest house-haunting species of the tropics, namely Stegomyia fasciata and Cu'.ex fatigans.
Mosquitos may, possibly, spread other infections mechanically. Indeed they are known to be one of several kinds of flies that mechanically transport the eggs of the " macaw worm " (Derma- tobia hominis) to its host. But such mechanical work is something different from the constant and indispensable biological accommoda- tion which mosquitos alone provide for the parasites of filariasis, malarial fevers, and yellow fever, outside the human body.
Almost as notorious in recent history as the Stegomyia and Anoph- eles that at one time threatened to stop the construction of the Panama Canal are the tsetse flies, which, as intermediary hosts of the trypanosome parasites of sleeping-sickness, have depopulated certain parts of tropical Africa. These bloodthirsty insects, which constitute the genus Glossina, are akin to the stable-fly (Stomoxys), the house-fly, and the blow-fly, all being included in the great family of Muscidae. Except for one species, Glossina tachinoides, which ranges into the extreme south-western corner of Arabia, the entire genus Glossina is restricted to the torrid regions of the African continent. The trypanosome animalcules, which, so far as the species that cause disease in man and domestic animals in Africa are concerned, are associated mainly with tsetse flies, occur in the state of nature in the blood of all classes of vertebrate animals in most parts of the world; many, if not all, of the trypanosomes living naturally in the blood of wil J animals appear to be harmless to their hosts, and it is perhaps because the trypanosomes that get into man and domestic animals, by the agency mainly of biting insects, are trypanosomes out of their proper place, that they are hurtful.
Recent investigations, beginning with the discovery made by Sir David Bruce in 1895 that the destructive " tsetse-fly disease " of domestic animals is due to a trypanosome parasite in the blood, have established the facts that there are two varieties of the human trypanosome disease known as sleeping-sickness, one predominant in equatorial Africa and transmitted mainly by Glossina palpalis, the other predominant in south-eastern tropical Africa and trans- mitted mainly by Glossina morsitans; that in both cases the re- spective trypanosome undergoes definite stages of development in the fly's gut and at last settlas itself and continues to proliferate in the fly's salivary glands; and that a fly once infected remains infec- tive by its saliva for the rest of its life. Thus in a general way the part taken by tsetse flies in spreading sleeping-sickness is similar to that taken by Anopheles in spreading malaria. But there is this difference: first, that the development of the trypanosome parasite in the tsetse fly is not actually known to have any sexual interpreta- tion; and secondly, that although in epidemics of sleeping-sickness the fly derives its infection from human sufferers, yet in other circumstances it may possibly become infected from a wild animal, since trypanosomes morphologically indistinguishable from those that cause disease in man in Africa have been observed living naturally in some of the big game animals and also in the wild tsetse flies. The existence of these natural foci of possible though not demon- strated infection must enormously increase the difficulty of con- trolling sleeping-sickness. On the other hand, the laboratory ex- periments of Bruce and others have shown that only a small per- centage of tsetse flies fed on infected blood take the infection. Finally it should be mentioned that there are some who still believe that in certain circumstances a tsetse fly may transfer infection from man to man mechanically on its proboscis.
In the instances reviewed so far a blood-sucking insect mos- quito or tsetse fly acquires a specific infection while feeding, in- cubates it for a definite term during which infectivity is latent, and then at length transmits it, in the act of feeding, to another sus- ceptible host. In this manner also Conorhinus megistus and other Reduviid bugs have been shown to transmit the trypanosome of Chagas's disease in South America, and in like manner the midge Phlebotomus papatasii is believed to transmit the virus of phlebot- omus fever. In the same way, probably, as Manson suggested, the Tabanid flies Chryspps dimidiata and salacea in West Africa foster and transmit a filarial worm, Filaria loa, that is troublesome to man
