there. It is possible, too, that the Leishmania parasite of kala- azar is communicated from man to man in India, as W. S. Patton thinks, by the bedbug, and in the Mediterranean region, as others suppose, by fleas.
A specific infection acquired by a blood-sucking arthropod in feeding is not always passed on by means of the infected saliva: it may be communicated in its juices and excreta. It is in this way, as numerous observers have ascertained, that body lice infected by imbibing the blood of sufferers from typhus fever, relapsing fever, and trench fever usually transmit the respective virus to a fresh host who, in seeking relief from their presence by scratching, may crush them or rub their infected excrement into his broken skin ; in the case of typhus the insect is said to transmit infection by its bite also. The part played by the fleas of rats and other rodents in epidemics of bubonic plague is similar, except that the original foci of infection are infected rodents. From these the fleas absorb the plague bacillus, which, as shown by the Indian Plague Commission, multiplies in their stomach and is voided with undiminished viru- lence in their excreta. Bacot and Martin have also shown that a flea may sometimes get its stomach so stuffed with the prolific bacilli that it may regurgitate some of them when biting a fresh victim.
A specific infection acquired by a bloodsucking arthropod is not always terminated in the individual : it may be bequeathed to the offspring. This has been shown by Dutton and Todd to be the course of events with the tick Ornithodorus moubata, which propagates the spirillum of the African variety of relapsing fever. Hereditary transmission of an acquired infection is said by Nicolle and other authorities to take place in lice with the spirillum of relapsing fever. Another aspect of the same phenomenon occurs in the case of the tick, Dermatocentor venustus, that serves as intermediary for the virus of the typhus-like disease known as Rocky Mountain spotted fever. Here, according to Ricketts, it is the minute lar- val tick that becomes infected, perhaps from natural foci of the infection in ground squirrels and various other local rodents, per- haps by inheritance. The larva, however, has to undergo many vicis- situdes before it is transformed into an adult; accommodation has to be found on three different hosts, and in the case of Dermatocen- tor venustus these vicissitudes may be spun out for two years before the larva reaches the adult stage in which the virus subsisting in it can become infective to man. These inherited and postponed in- fections render preventive measures correspondingly tedious and difficult, even when there are no natural foci of infection to be reck- oned with. The noxa of the exanthematous disease known as Japa- nese river fever, attributed to the bite of the " harvest-mite " larva of a velvet mite, is thought to be inherited by that larva.
Finally, a specific infection can be transmitted from man to man by the intermediation of an arthropod that is neither parasitic nor predaceous. This was shown by Fedshenko to be the case with the guinea-worm (Dracunculus medinensis) , a subcutaneous parasite of man (and other animals) in certain warm parts of the globe. The embryos of the worm after being set free are ingested by and undergo a definite larval development in fresh-water copepod Crustacea of the genus Cyclops, and man is infected by swallowing the infected Cyclops in unfiltered water. The ribbon-worm (Dibothriocephalus latus) is also thus fostered and disseminated by copepod Crustacea, as has recently been explained by Janicki and Rosen; only in this case the infected copepods are first swallowed by a fresh-water fish in which the larval ribbon-worms develop still further before they get to man. In the Far East several species of fresh-water crabs harbour the cercaria stage of the lung fluke.
Contaminative Arthropoda. It is necessary to discriminate be- tween insects which, like Anopheles and Glossina, supply the in- dispensable frame for a particular pathogenic microorganism, and insects which, like house-flies, are casual but not necessary agents in the promiscuous dispersal of microbes of many kinds. Among in- sects of the latter sort, indiscriminately pollutive but not specifically infective, all common house pests, such as cockroaches, crickets, ants, domestic beetles and flies, must be included. By far the most dangerous of them perhaps the most insidiously dangerous of all the insects that come into the medical purview is the common house-fly, Musca domestica, by reason of its ubiquity, its fecundity, its persistent activity, and, above all, by the profane impartiality with which it distributes its attentions, now upon the purgamenta and faeculenta of the community, and now upon the kitchen, the larder and the refreshment-table. A house-fly may carry contamina- tion on its feet, and so is said to spread purulent ophthalmia, an- thrax and perhaps other infections. Or, after feeding upon infected excrement it may void the contents of its crop or its intestine into food or drink, and in this way house-flies have played their part in outbreaks of epidemic diarrhcea, typhoid fevers, bacillary dysentery and, perhaps, cholera, and may also disseminate eggs of intestinal worms and cysts of intestinal protozoa. It must not, however, be supposed that house-flies are always common carriers of all the germs they may pick up: much, fortunately, depends upon coexistent circumstances, and all germs cannot stand such rough and pre- carious transport.
It is possible that the beetles, moths, mites, etc., which in all parts of the world infest grain, meal, biscuit and dried provisions of all kinds, may at times so befoul and vitiate those commodities as to make them harmful as food though not actually spoiling them
for certain markets. Beyond the fact that larvae of the meal moth and the meal beetle may be infected with the larvae of a tapeworm, nothing very definite is known at present on this point.
Venomous Arthropoda. Some of the insects and ticks mentioned on other grounds might be included here also. The bite of ticks in particular is notoriously venomous, and may cause fever and temporary paralysis by the toxic properties of the saliva ; so also may that of some of the gadflies, particularly those of the genus Chrysops. Scorpions, bees, wasps, etc., all have special venom glands, the secretion of which is variously neurotoxic and haemolytic like that of snakes, and have special organs for injecting the secretion. The venom of some spiders is known to be seriously toxic even to man : that of Lathrodectes is particularly so, and that of the South American " Podadora " (Glyptocranium gastracanthoides) is said by Escomel to be sometimes fatal to man. In West Africa the larva (known locally as " Fura ") of a tiger-beetle is said by Pollard to inflict a bite having effects almost as severe as the sting of a scorpion. Many species of lepidopterous caterpillars are liberally provided with spines or finely barbed hairs having venomous properties, and numer- ous kinds of insects, besides the well-known blister beetles, can eject irritant and vesicant secretions. But from the entomological stand- point these venomous arthropoda are not important.
Process of Research. Having surveyed the field, it remains to consider the economic aspects of medical entomology.
It is plain that the actual discovery of the pathogenic capa- bilities of any particular arthropod is most likely to be made by the medical or pathological specialist the history of the subject entirely confirms this assumption. The treatment of pathological effects is even more plainly and exclusively a medical matter. But, once the pathogenetic significance of a species has been established, it becomes the first concern of medical entomology to unravel the biological history of that species in every detail, however apparently trivial, and to investigate every circumstance that may be supposed to influence its noxious powers, with the object of circumventing its activity or of restricting its existence in propinquity to man.
Where the harmful species is a specific parasite of man, or like the yellow-fever mosquito constantly haunts domiciles, its biology is usually easy to follow, and its control, in an educated and convinced community, should not be difficult. But where it is a free ranger, like the tsetse fly and many species of Anopheles, the investigation of its biology may be extremely difficult.
An entomological investigation must comprehend every stage of the creature's existence, from the egg to the procreant adult. It must include not only its natural affinities, specific characters and anatomical structure, but also its distribution and seasonal prevalence, HO habits, hiding-places and hours of work and rest; its powers and usual range of locomotion, and its propensity to extend its range at any season; its fecundity, sexual instincts and manner of reproduction; its times and places of breeding, method of dispensing its eggs and providing for its larvae; and its length of life in every stage of its existence. It must also follow up, in each 'separate stage, the general conditions of exist- ence, such as food preferences, meteorological requirements and means of withstanding vicissitudes of season and climate, adapta- tions for transport and dispersal, and all the circumstances of the organic environment natural shelters, direct and indirect help-givers in the struggle for existence, parasites, enemies and rivals; for, as Darwin explained so well, the species of a fauna do not stand alone in nature, but all hang together in most com- plicated interdependence. Furthermore, the investigation must embrace the varying circumstances meteorological, seasonal, etc. which in the case of a specifically infective species influence its reception and retention of infection.
With some assured knowledge of the bionomy of a harmful species, it becomes the practical work of medical entomology to consider how its harmful activities can be forestalled or it itself entirely banished from the vicinity of man whether by clearing off everything that can shelter the adult ; or by abolishing or restricting or periodically devastating its breeding-places; or by cultivating its parasites and natural enemies; or by depriving it of its ultimate food resources; or by direct attack with insecti- cides and other destructive appliances; or by screens and de- fensive apparatus; or by educational propaganda. All these principles have their application, which must be decided with regard to local conditions and resources.
