Why can assassin bugs inflict painful bites?

Assassin bugs (family Reduviidae) can inflict painful bites because they are equipped with specialized piercing-sucking mouthparts designed to inject enzymes into their prey.1

These insects use their short, three-segmented beaks to pierce the victim and pump in venomous saliva. This saliva contains lytic substances that serve to paralyze the prey and externally digest its tissues, allowing the bug to liquefy and consume the body fluids. When humans handle these insects or feel threatened by them, the bugs may use these same mouthparts defensively, resulting in a painful bite often accompanied by localized swelling.12