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