Mosquitoes are types of flies with a tube-like mouth. Most are about 1/8 to 3/4 inches in size. They feed on the blood of mammals, birds, reptiles and amphibians. Only females are able to suck blood. The saliva of a mosquito is what causes the itchy rash or red bump that most humans get when bit. Mosquitoes can transmit such illnesses as malaria, west nile virus, yellow fever and filariasis.