During the second larval stage, the internal organs of the worms are formed. The second molt occurs at 11- 12 days, resulting in third stage larvae which resemble miniature adults. During the next 2-3 days, they increase in length, break out of the Malpighian tubules, migrate through the body to the head and accumulate in the mouthparts. These third stage larvae are now called infective larvae. Thus, in 2-3 weeks, microfilariae transform into infective larva.
Further development can only take place when mosquitoes feed on a dog.
Infective larvae are concentrated in the proboscis, or mouth parts of the mosquito. As the infected mosquito feeds on a dog, the infective larvae emerge from the tip of the proboscis and on to the skin of the animal. A drop of mosquito blood protects the larvae from drying prior to their entry into the host. The infective larvae penetrate the skin through the puncture wound that remains after the mosquito withdraws her mouthparts. After penetrating the skin, the larvae stay close to the entry site and grow very little during the next few days.