The bloodbug appears to be a monstrously mutated form of mosquito. Likely a byproduct of mutation caused by nuclear fallout; they have grown in size over the time period since the Great War in a manner similar to radroaches and bloatflies.
Bloodbugs appear to be roughly two feet in length from the head to the tip of the abdomen. Like their pre-war ancestors, bloodbugs possess six long double-jointed limbs, one pair of finely-veined wings and a long, pointed proboscis that extends from between two compound eyes. The proboscis is the bloodbug's primary means of assault, sinking it into prey almost like a syringe and inflicting minor-to-intermediate radiation poisoning on the victim. The proboscis can be collected by the Resident upon the bloodbug's death.
Bloodbugs latch onto their prey in an attempt to drink their blood. The insect then becomes engorged with the victim's blood, and can spit the contaminated blood back at its target for a large amount of radiation damage, which suggests that radiation is transmitted from the mutant to the victim. While these mutated mosquitoes can punch their proboscis through any kind of regular armor, it's not powerful enough to penetrate power armor. Should they find themselves faced with such heavily armored prey, their range of available attacks becomes limited to largely ineffectual stings and their acid spit, should the target be out of melee range.
Bloodbugs, like bloatflies and radscorpions, have a propensity for swarming tactics. Once they are alerted to their victim's presence, they will spread out, surround their prey and attack from multiple directions. This, coupled with their erratic evasive movement while under fire, their good camouflage and their slender silhouette, makes it difficult to keep track of and kill them all before at least some of them get into melee range. However, should the majority of the swarm be destroyed before it reaches its prey, the survivors will often retreat. Usually they will regroup and attack again, but sometimes they will flee for good without returning.
Another habit bloodbugs share with bloatflies and especially radroaches is their clinging to trees and structures, as well as walls or the ceiling while indoors and unaware of an enemy's presence. This can result in a nasty surprise when the player character enters a seemingly empty room or subway station and suddenly finds themselves in the middle of an entire bloodbug swarm.
If the Resident damages the bloodbug's blood sac, it will lose its spit ability, but the bloodbug can still suck the victim's blood. If done multiple times, the bloodbug's sac will be restored and it will regain its spit attack.