The second molt occurs at twelve days. Third-stage larvae resemble miniature adults. During the next two to three days, they increase in length, break out of the Malpighian tubules, migrate through the body to the head, and accumulate in the mouthparts. Thus, in two to three weeks, a microfilaria transforms into an infective larva. This infective larva cannot develop further in the mosquito.
Further development can only take place in a dog.
Infective larvae are found primarily in the proboscis, or mouth parts. As the infective mosquito feeds on a dog, the infective larvae emerge from the timp 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 a host. The infective larvae penetrate the skin through the puncture wound that remains after the mosquito withdraws her mouthparts. Fourth-stage larvae migrate through subcutaneous tissue and muscle toward the upper abdomen and thoracic cavity.