free rider vs gadfly

free rider

noun
  • Someone who obtains goods or services legally without paying. 

  • A worker at an open shop who is able to enjoy the benefits negotiated by a union without being a member of the union. 

  • One who obtains benefit from a public good without paying for it directly. 

gadfly

noun
  • A person who takes without giving back; a bloodsucker. 

  • Synonym of gadabout (“a person who restlessly moves from place to place, seeking amusement or the companionship of others”) 

  • Any dipterous (“two-winged”) insect or fly of the family Oestridae (commonly known as a botfly) or Tabanidae (horsefly), noted for irritating animals by buzzing about them, and biting them to suck their blood; a gadbee. 

  • A person or thing that irritates or instigates. 

  • A person who upsets the status quo by posing novel or upsetting questions, or attempts to stimulate innovation by being an irritant. 

How often have the words free rider and gadfly occurred in a corpus of books? (source: Google Ngram Viewer )