Why Do Bloodsuckers Suck Blood?

Deer Fly - John Moore, AFPMB

Bloodsuckers don’t suck blood to be mean. When they take blood out of an animal it is because they need the blood to survive.

We call the animals that bloodsuckers take blood from “hosts.”  

Female mosquitoes, horseflies, and deer flies are bloodsuckers that need proteins and fats in the host’s blood so they can develop their eggs. When it’s time to reproduce, these bloodsuckers eat a blood meal.

This bug is a deer fly. Its mouthparts let it poke and suck blood. Wouldn't those eyes make cool sunglasses??