An Orillian lung maggot was an undesirable creature.
In 2373, Neelix told Tosin that he would have no problem dying if it would get rid of an Orillian lung maggot like Tosin. (VOY: "Fair Trade")
Presumably this is a larva (a juvenile form that metamorphoses into an adult) of a creature hailing from or discovered by "Orill".