VD vs crab louse

VD

noun
  • The design diving speed of an aircraft; the highest speed which the aircraft is planned to reach during flight testing. Always higher than VNE (the speed that must not be exceeded in service). 

crab louse

noun
  • A parasitic insect, Pthirus pubis, that lives amongst the pubic hairs of humans and feeds on blood. 

How often have the words VD and crab louse occurred in a corpus of books? (source: Google Ngram Viewer )