The Basics
Mosquitoes and sand flies bite their prey to feed on their blood. In both insects, only the females bite in order to gain blood protein so they are able to produce eggs. A bite from a mosquito or sand fly turns into an itchy welt because the female injects saliva into the victim as she feeds. The saliva works to thin the blood and prevent it from clotting during the feeding. This saliva causes an autoimmune reaction that causes the bite to itch and swell.
Mosquitoes
There are well over 2,000 species of mosquitoes worldwide. These flying insects feed on the blood of both birds and mammals. Many factors attract and cause mosquitoes to bite humans, including the carbon dioxide we exhale, moisture, lactic acid and perspiration. Mosquitoes are also attracted by things like deodorants, detergents, movement and body heat. People wearing dark colors are more likely to be bitten, because dark colors absorb more heat. Mosquitoes commonly bite in the evening and nighttime; however these bugs can bite at any time of day.
Sand Flies
Sand flies are also called biting midges. These insects are found living in and around aquatic and semi-aquatic habitats including mud and wet organic debris. During windy weather they remain inactive, but typically female sand flies emerge to find victims in the early morning hours and at dusk. Sand flies also will feed on a wide variety of animals, including humans. When biting humans, sand flies are known to attack in large numbers. They will commonly bite the face, hands or scalp of their victims, but they will also bite any area of exposed skin.
Bites
Both sand flies and mosquitoes produce a red, itchy bump after biting humans; however some humans will experience more violent allergic reactions than others. A mosquito bite may also burn and turn into a large welt. Sand fly bites typically occur in clusters. Their bites are very painful. A bite from a small sand fly can be more painful that the bite of a large mosquito. Sand fly bites can also produces rashes and cause fevers in their victims depending on the tolerance to the bite.
Diseases
While most bites from a mosquito or sand fly are simply an itchy annoyance, these bugs are known in some areas to transmit diseases. Mosquitoes are known to transmit malaria and yellow fever through their bites. On the other hand, sand flies are able to transmit ailments like Carrion's disease, pappataci fever, filarial worms and conditions like leishmaniasis, which is likened to malaria.