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XL] Once introduced into the human body, the filaria finds its way into the lymphatics. Arrived in one of these, it attains sexual maturity, fecundation is effected, and in due course new generations of larval filariæ are poured into the lymph. These, passing through the glands—if such should intervene—by way of the thoracic duct and left subclavian vein or by the lymphatics of the upper part of the body, finally appear in the circulation.

Such is the life-history of Filaria bancrofti; man is its definitive host, the mosquito its intermediary host. It is manifest that filarial periodicity is an

Fig. 108.—Stages of larval form of F. bancrofti, from the thoracic muscles of Culex fatigans. (After Looss.)

adaptation of the habits of the parasite to the nocturnal habits of the mosquito. It is also manifest that the purpose of the "sheath" with which it is provided while circulating in the human host is to muzzle the young filaria and prevent its breaking through the blood-vessels, and thereby missing its chance of gaining access to the mosquito. The cephalic armature is adapted for piercing the walls of the mosquito's stomach, and for enabling the parasite to travel through the tissues of the insect.

Anyone desirous of working out for himself the metamorphosis of the filaria in the mosquito can readily do so, even in the absence of a suitable human subject, by setting Anopheles mosquitoes to bite filariated dogs. In most tropical countries a large proportion of the dogs harbour Filaria (Dirofilaria)