THE WAYS IN WHICH ARTHROPODS MAY AFFECT THE HEALTH OF MAN
When we consider the ways in which insects and their allies may affect the health of man, we find that we may treat them under three main groups:
A. They may be directly poisonous. Such, for example, are the scorpions, certain spiders and mites, some of the predaceous bugs, and stinging insects. Even such forms as the mosquito deserve some consideration from this viewpoint.
B. They may be parasitic, living more or less permanently on or in the body and deriving their sustenance from it.
Of the parasitic arthropods we may distinguish, first, the true parasites, those which have adopted and become confirmed in the parasitic habit. Such are the itch mites, the lice, fleas, and the majority of the forms to be considered as parasitic.
In addition to these, we may distinguish a group of accidental, or facultative parasites, species which are normally free-living, feeding on decaying substances, but which when accidentally introduced into the alimentary canal or other cavities of man, may exist there for a greater or less period. For example, certain fly larvæ, or maggots, normally feeding in putrifying meat, have been known to occur as accidental or facultative parasites in the stomach of man.
C. Finally, and most important, arthropods may be transmitters and disseminators of disease. In this capacity they may function in one of three ways; as simple carriers, as direct inoculators, or as essential hosts of disease germs.
As simple carriers, they may, in a wholly incidental manner, transport from the diseased to the healthy, or from filth to food, pathogenic germs which cling to their bodies or appendages. Such, for instance, is the relation of the house-fly to the dissemination of typhoid.
As direct inoculators, biting or piercing species may take up from a diseased man or animal, germs which, clinging to the mouth parts, are inoculated directly into the blood of the insect's next victim. It it thus that horse-flies may occasionally transmit anthrax. Similarly, species of spiders and other forms which are ordinarily perfectly harmless, may accidentally convey and inoculate pyogenic bacteria.
It is as essential hosts of disease germs that arthropods play their most important rôle. In such cases an essential part of the life cycle of the pathogenic organism is undergone in the insect. In other words, without the arthropod host the disease-producing organism cannot complete its development. As illustrations may be cited the relation of the Anopheles mosquito to the malarial parasite, and the relation of the cattle tick to Texas fever.
A little consideration will show that this is the most important of the group. Typhoid fever is carried by water or by contaminated milk, and in various other ways, as well as by the house-fly. Kill all the house-flies and typhoid would still exist. On the other hand, malaria is carried only by the mosquito, because an essential part of the development of the malarial parasite is undergone in this insect. Exterminate all of the mosquitoes of certain species and the dissemination of human malaria is absolutely prevented.
Once an arthropod becomes an essential host for a given parasite it may disseminate infection in three different ways:
1. By infecting man or animals who ingest it. It is thus, for example, that man, dog, or cat, becomes infected with the double-pored dog tapeworm, Dipylidium caninum. The cysticercoid stage occurs in the dog louse, or in the dog or cat fleas, and by accidentally ingesting the infested insect the vertebrate becomes infested. Similarly, Hymenolepis diminuta, a common tapeworm of rats and mice, and occasional in man, undergoes part of its life cycle in various meal-infesting insects, and is accidentally taken up by its definitive host. It is very probable that man becomes infested with Dracunculus (Filaria) medinensis through swallowing in drinking water, the crustacean, Cyclops, containing the larvæ of this worm.
2. By infecting man or animals on whose skin or mucous membranes the insect host may be crushed or may deposit its excrement. The pathogenic organism may then actively penetrate, or may be inoculated by scratching. The causative organism of typhus fever is thus transmitted by the body louse.
3. By direct inoculation by its bite, the insect host may transfer the parasite which has undergone development within it. The malarial parasite is thus transferred by mosquitoes; the Texas fever parasite by cattle ticks.
CHAPTER II.
ARTHROPODS WHICH ARE DIRECTLY POISONOUS
Of all the myriads of insects and related forms, a very few are of direct use to man, some few others have forced his approbation on account of their wonderful beauty, but the great hordes of them are loathed or regarded as directly dangerous. As a matter of fact, only a very small number are in the slightest degree poisonous to man or to the higher animals. The result is that entomologists and lovers of nature, intent upon dissipating the foolish dread of insects, are sometimes inclined to go to the extreme of discrediting all statements of serious injury from the bites or stings of any species.
Nevertheless, it must not be overlooked that poisonous forms do exist, and they must receive attention in a consideration of the ways in which arthropods may affect the health of man. Moreover, it must be recognized that "what is one man's meat, is another man's poison," and that in considering the possibilities of injury we must not ignore individual idiosyncrasies. Just as certain individuals may be poisoned by what, for others, are common articles of food, so some persons may be abnormally susceptible to insect poison. Thus, the poison of a bee sting may be of varying severity, but there are individuals who are made seriously sick by a single sting, regardless of the point of entry. Some individuals scarcely notice a mosquito bite, others find it very painful, and so illustrations of this difference in individuals might be multiplied.
In considering the poisonous arthropods, we shall take them up by groups. The reader who is unacquainted with the systematic relationship of insects and their allies is referred to [Chapter XII]. No attempt will be made to make the lists under the various headings exhaustive, but typical forms will be discussed.