The next stages in the development of Filaria nocturna occur in mosquitoes, a fact suggested almost simultaneously by Bancroft and Manson in 1877, and first demonstrated by the latter very soon thereafter. The experiments were first carried out with Culex quinquefasciatus (= fatigans) as a host, but it is now known that a number of species of mosquitoes, both anopheline and culicine, may serve equally well.
When the blood of an infested individual is sucked up and reaches the stomach of such a mosquito, the larvæ, by very active movements, escape from their sheaths and within a very few hours actively migrate to the body cavity of their new host and settle down primarily in the thoracic muscles. There in the course of sixteen to twenty days they undergo a metamorphosis of which the more conspicuous features are the formation of a mouth, an alimentary canal and a trilobed tail. At the same time there is an enormous increase in size, the larvæ which measured .3 mm. in the blood becoming 1.5 mm. in length. This developmental period may be somewhat shortened in some cases and on the other hand may be considerably extended. The controlling factor seems to be the one of temperature.
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.
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.