310 MOSQUITOES OF NORTH AMERICA 



stitute a very curious adaptation of the parasite to the nocturnal habits of the 

 mosquito which serves as an intermediary host.* 



Manson discovered the periodicity of the embryos. He placed a patient whose 

 blood contained filarias in a room where mosquitoes were plentiful. After the 

 patient had gone to bed a light was placed beside him, and the door left open for 

 the mosquitoes to enter. Later, when many mosquitoes had entered, the light 

 was put out and the door was closed. In the morning mosquitoes filled with 

 blood were captured, and the blood in their stomachs was examined and found 

 to contain more filari^ than an equal quantity of blood taken directly from the 

 patient. Since then the periodicity of the filarial embryos in the peripheral 

 circulation has been abundantly verified. In fact, as with one species of Filaria 

 the embryos appear at night while with another they are only in evidence in the 

 daytime and with still others there is no periodicity apparent, this phenomenon 

 is largely relied upon for determination of the parasite. The periodicity is, to a 

 certain degree at least, determined by the habits of the host. In the case of 

 Filaria hancrofti the appearance of the embryos in the peripheral circulation 

 seems to be determined by the decreased action of the heart during sleep, rather 

 than by the time of day, Mackenzie succeeded in reversing the time of appear- 

 ance of the filarise by inducing his patients to sleep by day and keeping them 

 active at night, Manson found that when a patient sleeps alternately, sometimes 

 by night, sometimes by day, the periodicity of the filarise disappears altogether. 



In the case of Filaria perstans and F. demarquayi there is no marked peri- 

 odicity, and the absence of periodicity is given as one of the characteristics of 

 F. philippinensis. The African Filaria diurna, which is believed by Manson and 

 others to be the embryo form of Filaria loa, is present in the peripheral circula- 

 tion only in the daytime. Numerous experiments with mosquitoes of a great 

 variety of species, to determine the transmitter of Filaria diurna, have all proved 

 negative and, while a number of diurnal blood-sucking insects have been sus- 

 pected, the actual transmitter remains unknown. Fiilleborn and Eodenwaldt 

 in their investigations with filarise of dogs from Italy found that there were two 

 forms (one of them probably Filaria immitis) one of which showed periodicity 

 while the other did not. 



EVOLUTION IN THE MOSQUITO. 



Later Manson ascertained that when these embryos are taken into the stomach 

 of the mosquito they cast their envelope, penetrate the wall of the stomach and 

 thus enter the general body cavity. The embryos then make their way into the 

 muscles of the thorax and lodge within the muscle bundles. They now loose 

 their activity and shorten and thicken very appreciably. It is only after they 

 have become fixed and immovable that they actually begin to grow. According 

 to Looss and others, when the larval filarise have reached a length of about 0.24 

 mm., and 0.03 mm, thickness, they begin to show signs of returning activity. 



* Recently the question of the cause of the periodicity of the filarial embryos has been very 

 ably discussed by Dr. Ernst Rodenwaldt, based on the investigations of Fiilleborn and himself. 

 The subject is a complex one and we recommend to those suflBciently interested the perusal of the 

 original paper. 



