The transformed larvæ then reenter the body cavity and finally the majority of them reach the interior of the labium (fig. 120). A few enter the legs and antennæ, and the abdomen, but these are wanderers which, it is possible, may likewise ultimately reach the labium, where they await the opportunity to enter their human host. It was formerly supposed that when the infested mosquito punctured the skin of man, the mature larvæ were injected into the circulation. The manner in which this occurred was not obvious, for when the insect feeds it inserts only the stylets, the labium itself remaining on the surface of the skin. Fülleborn has cleared up the question by showing that at this time the filariæ escape and, like the hookworm, actively bore into the skin of their new host. [Illustration: 120. Filaria in the muscles and labium of Culex. After Blanchard.] Once entered, they migrate to the lymphatics and there quickly become sexually mature. The full grown females measure 85-90 mm. in length by .24-.28 mm. in diameter, while the males are less than half this size, being about 40 mm. by .1 mm. Fecundation occurs and the females will be found filled with eggs in various stages of development, for they are normally viviparous. _Filaria philippinensis_ is reported by Ashburn and Craig (1907) as a common blood filaria in the Philippine Islands. As they describe it, it differs from _Filaria bancrofti_ primarily in that it does not exhibit periodicity. Its development has been found to occur in _Culex quinquefasciatus_, where it undergoes metamorphosis in about fourteen or fifteen days. There is doubt as to the species being distinct from _bancrofti_. Several other species occur in man and are thought to be transferred by various insects, among which have been mentioned Tabanidæ and tsetse-flies, but there is no experimental proof in support of such conjectures. _Filaria immitis_ is a dangerous parasite of the dog, the adult worm living in the heart and veins of this animal. It is one of the species which has been clearly shown to undergo its development in the mosquito, particularly in _Anopheles maculipennis_ and _Aedes calopus_ (= Stegomyia). The larval form occurs in the peripheral blood, especially at night. When taken up by mosquitoes they differ from _Filaria bancrofti_ in that they undergo their development in the Malpighian tubules rather than in the thoracic muscles. In about twelve days they have completed their growth in the tubules, pierce the distal end, and pass to the labium. This species occurs primarily in China and Japan, but is also found in Europe and in the United States. It is an especially favorable species for studying the transformations in the mosquito. [Illustration: 121. Dracunculus medinensis; female; mouth; embryo. After Bastian and Leuckart.]