Microfilaria is an early stage in the life cycle of certain parasitic nematodes such as Wuchereria bancrofti which causes a disease known as filariasis. The adult worms live in a tissue or the circulatory system of vertebrates. They release microfilariae into the bloodstream of the vertebrate host. The microfilariae are taken up by blood-feeding arthropod vectors, most commonly by female Culex mosquito. When a mosquito bites a healthy individual, the microfilariae are released into the bloodstream. So, the correct answer is 'Culex'.