WordNet sense 1 (a person who has a nasty or unethical character undeserving of respect):

insect, worm, dirt ball

WordNet sense 2 (any of several small insects especially aphids that feed by sucking the juices from plants):

plant louse

WordNet sense 3 (wingless insect with mouth parts adapted for biting; mostly parasitic on birds):

bird louse, biting louse

WordNet sense 4 (wingless usually flattened bloodsucking insect parasitic on warm-blooded animals):

sucking louse