94 GNATS OR MOSQUITOES — CHAPTER VI 



of the hypophai-ynx that the saHvary secretion, with its 

 contained mahirial germs, can be injected into the tissues 

 of the vertebrate alternative host. 



In the case of filariasis the route of infection is different. 

 Taken as a family, tha filar uc are essentially parasites of the 

 intercellular lymph spaces, and hence are most commonly 

 found in such situations as the cellular tissues, and the 

 great serous sacs, such as the cavities of the peritoneum and 

 pleura, and that stage of the parasite life-cycle of the filaria 

 inhabiting as an adult the human cellular tissue, which is 

 passed in the mosquito, forms no exception to this general 

 rule. 



Necessarily it is along the groove of the labrum that 

 the embryonic filarlcc find their way, along with the in- 

 gested blood into the mosquito's stomach ; but once there, 

 they hasten to reach a more congenial situation, by boring 

 through the walls of the organ into the cellular tissue around 

 it, and arc then free to wander in its interstices all over 

 the body, including that of the sheath of the proboscis or 

 labium, which is a comparatively thick fleshy organ, con- 

 taining between its inner and outer walls a considerable 

 amount of cellular tissue supporting the tracheae, muscles, 

 and nerves, with which it is liberally supplied. Now, as 

 Grassi has recently pointed out, filarUc, though not usually 

 very destructive parasites, do cause a certain amount of 

 irritation and effusion ; and the result of the intrusion of 

 fi,laruc into the substance of the labium will be to raise a 

 tense swelling of the delicate inner covering of the organ. 



As already pointed out, when the mosquito bites, the 

 labium is sharply doubled into a loop, and if the organ be 

 swelled, owing to the presence in its substance of filar ice, 

 the result of this doubling up will be to rupture its delicate 

 integument, and so set free the contained filaria. Once 

 free they can scarcely fail to be carried by capillarity 

 among the blades of the lancets, and so introduced with 

 them into the cellular tissues of the new victim in which 

 they propose to grow to maturity. 



Passing through an opening between the trabeculae 

 which form the framework of the head, and to which it is 



