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 filarie 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 filarie 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 bancrofti 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 filarie 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 filarie 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 Rodenwaldt 
in their investigations with filarie 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 filarie 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 filarlal embryos has been very 
ably discussed by Dr. Ernst Rodenwaldt, based on the investigations of Filleborn and himself. 
The subject is a complex one and we recommend to those sufficiently interested the perusal of the 
original paper. 
