The Plague Fleas 559 



of the serums of experimentally immunized animals for hypo- 

 dermatic injection into man was tried soon after the discovery of 

 the plague bacillus. Kitasato's experiments first showed that it 

 was possible to bring about immunity against the disease, and 

 Yersin, working in India, and Fitzpatrick, in New York, have 

 successfully immunized large animals (horses, sheep, and goats). 

 The serum of the immunized animals contains specific agglutinins 

 and bacteriolysins as well as an antitoxin, capable not only of pre- 

 venting the disease, but also of curing it in mice and guinea-pigs 

 and probably in man. 



Study of plague serums has been conducted by Yersin, Calmette 

 and Borrel,* but their value as a prophylactic lacks demonstration. 



Wyssokowitsch and Zabolotny,f used 96 monkeys in the 

 study of the value of the "plague serums," and found that when 

 treatment was begun within two days from the time of inoculation 

 the animals could be saved, even though symptoms of the disease 

 were marked. After the second day the treatment could be relied 

 upon. The dose necessary was 20 cc. of a serum having a potency 

 of 1 : 10. If too little serum was given, the course of the disease was 

 retarded and^the animal improved for a time, then suffered a re- 

 lapse, and died in from thirteen to seventeen days. The serum also 

 produced immunity, but of only ten to fourteen days' duration. 

 Immunity lasting three weeks was conferred by inoculating a monkey 

 with an agar-agar culture heated to 6o°C. If too large a dose of 

 such a culture was given, however, the animal was enfeebled and 

 remained susceptible. 



THE PLAGUE FLEAS 



Fleas were formerly classed as a suborder of the Diptera, or two-winged in- 

 sects, and because they had no wings, were known as Aphaniptera. At the 

 present time they constitute an order by themselves, the Siphonaptera. 



Every flea undergoes a complete metamorphosis. It begins its life history as 

 a minute, oval, pearly-white egg measuring about 0.6 mm. in length, that falls 

 from the body of the female to the floor or ground. The eggs of fleas are not 

 cemented to the hairs like those of lice, but drop to the ground where the larva 

 lives. More or less eggs are therefore always scattered about where dogs, cats, 

 rats, mice or other animals that harbor fleas are to be found, and more or less 

 larvse and pupae are likewise to be found in such places. In the course of from 

 five to ten days, a minute, active caterpillar-like larva emerges from the egg to 

 feed upon such organic matter as it may find for the six to eight weeks of this 

 stage. During the larval period the skin is shed three or four times. When full 

 grown, the larva empties its alimentary canal, spins itself a tiny silken cocoon, 

 sometipes including minute bits of rubbish or grains of sand in its structure, 

 sheds its skin for the last time, and becomes a pupa. As such it is inactive for 

 from two to eight weeks, according to external conditions of temperature and 

 moisture, then opens the cocoon and emerges from the pupa shell, a perfect in- 

 sect—the flea proper. 



The adult fleas, both males and females, have soft exoskeletons at first, but 

 soon they harden, through the formation of chitin, to the weU-known tough and 

 brittle armor. 



The male differs from the female in being smaller and in its shorter abdomen. 



* "Ann. de I'lnst. Pasteur," 1895, ix, 589. 

 t Loc. cit. 



