DEVELOPMENT IX MOSQUITO 311 



The movements, however, to the end of the ripening stage of the larva remain 

 slow and very circumscribed. Generally the larvae are found in extended 

 position in the muscle fibers, these latter disintegrating into a fine granular sub- 

 stance which serves as nourishment for the larvae. A variable proportion of the 

 larvae die during this stage. 



When the larvae have reached a certain size (given by various investigators as 

 from 1.006 to 1.8 mm.) they leave the muscles and return to the general body- 

 cavity. They now are very active and generally make their way to the anterior 

 end of the mosquito, penetrating from the thorax into the head and from thence 

 into the labium. Occasionally filarias go astray and get into the abdomen ; they 

 have also been found in the hollows within the legs and palpi. 



Lebredo gives the following synopsis of the filarial development within the 

 mosquito : 



" 1. The insect sucks the blood of a patient infected with filaria. 



" 2, The embryo loses its sheath in the stomach of the mosquito, ecdysis. 



" 3. Migration from the stomach to the thorax. This migration always takes 

 place through the gastric wall, since both orifices of the gastric dilatation are 

 completely closed when the stomach is full. The embryo leaves its sheath in the 

 gastric contents or caught in the wall of the stomach, where it is left at the 

 moment of exit. 



" 4. The embryo rests in the thorax, and goes through the following trans- 

 formations : 



" (a) JSTarrowing and invagination of the tail. 



"(b) Invagination continues and the embryo grows shorter and wider. 



" (c) Widening and shortening continue, and the invaginated portion forms 

 a hyaline appendix. 



" (d) Period of growth and formation of the three lobes." 



Then follows the escape of the fully developed larvae into the general body- 

 cavity. This is the period of its greatest activity which generally results in its 

 gaining the interior of the labium. Then the larva awaits the next step in its 

 evolution, the transfer to its vertebrate host. 



It has been frequently observed that the proportion of embryo filariae con- 

 tained in the blood within the mosquito's stomach is much greater than in the 

 same quantity taken directly from the circulation. Various explanations have 

 been offered, but Fiillebom has shown that this difference is only an apparent 

 one, being due to the dehydration of the blood within the mosquito's stomach. 

 At the same time this thickening of the blood makes it viscid and this serves, 

 as it were, to hold the sheath when the embryo moults, as it does at this time. 



The time necessary for the development of the larval filaria to its final active 

 condition is variable. Manson first gave a period of seven days but no such rapid 

 development has been observed by others and he himself admits that there may 

 have i)een an error in his observation. James, in India, states the period to be 

 from 13-14 days; Bancroft, in Australia, never saw them before the 16th or 

 17th day, and the period was extended to 35 days in cold weather. The govern- 

 ing factor is evidently the temperature. Looss found that in the Egyptian 

 winter some of the larvae were not fully developed until the 41st day after the 



