314 MOSQUITOES OF NORTH AMERICA 



Lebredo shows by his experiments that it is the heat from the body of the 

 prospective host that stimulates the filarias to leave the proboscis of the mosquito. 



" A mosquito is selected in which the filarige have reached their final stage of 

 development, and are lodged in the labium. It is best to select an insect 28 to 

 30 days, or more, after infection. After removal of the wings and legs the insect 

 is placed alive upon a slide and irrigated with a very weak salt solution. No 

 cover-glass is employed. Under the microscope it is possible to see, through the 

 transparent walls of the labium, the actively moving worms within that 

 structure. 



" If no pressure is used, these movements may be watched for hours without 

 observing the escape of a single worm. 



" Usually all the setse are enclosed within the labium, thus preventing a clear 

 view of the movements of the filaria. In order to obviate this difficulty I press 

 very lightly with the side of a needle upon the base of the proboscis, and then 

 with the point of a needle inserted between the setse and the labium, I pry them 

 further apart. The filarige can now be distinctly seen actively moving and 

 agitating the two tracheal tubes. The worms occupy usually the proximal third 

 of the labium ; sometimes they extend further out. 



" The slide is now placed near the flame of a Bunsen burner. This must be 

 done very carefully in order to avoid fatal over-heating to the insect and the 

 parasite. Watching the preparation, an unusual activity in the movements of 

 the filarise is now to be seen, and the cephalic end particularly becomes agitated 

 as if seeking a point of exit. This cannot be found laterally because of the 

 chitinous covering, and the filaria advances toward the anterior extremity of 

 the labium. If the liquid is allowed to cool, the movements become slower and 

 even cease altogether. Upon warming carefully again, and adding more tepid 

 fluid, the movements are revived until the worms reach the point of the labium. 

 If now the application of heat is stopped, we find that the worms appear to feel 

 around with their cephalic extremity, but fail to break out. 



" I am sure that there is no natural orifice at the terminal end of the labium, 

 because if there were, the filaria, having reached this point, would find no ob- 

 stacle to its exit. And, furthermore, a careful study of the extremity fails to 

 discover any orifice. 



" If the heat is kept up carefully and steadily we can see the embryo seeking 

 the point, making pressure there, and finally perforating the cuticle at a certain 

 point. 



" This perforation is made quite suddenly, for we see the cephalic end jump 

 out, as it were ; the rest of the worm following slowly by serpentine movements. 

 As soon as it reaches the fluid the serpentine movements continue, but the worm 

 ceases to be able to advance. This remarkable difference in the results obtained 

 by the movement indicates that the soft tissues at this stage are the natural 

 element of the worm, and that it is prepared to move on into the human tissues 

 directly from the mosquito. In water, on the other hand, the filaria not only 

 cannot live, but cannot even move from place to place. 



" In their exit the worms follow the regular order in which they occur in the 

 labium and head. We have seen two worms making their exit at the same time. 



" Occasionally, after two embryos have been started out, it has been necessary 

 to warm the liquid again in order to bring other worms down from the head to 

 the labium. Of course, after the first embryo has broken out, the others find 

 their way out with greater readiness." 



