PS to: When A.S brought up the associations to the word "arethusoides" he mentioned a water nymph named Arethusa and he also added information about butterflies and orchids. I can now offer another tidbit related to Rack thanks to his initial diligence:  According to a few sources the "Arethusa bulbosa's bruised bulbs (are) useful for the tooth ache, and in cataplasms for tumors. [ Cf. Schoepf" and Medical Flora, or Manual of the Medical Botany of the United States of North America, Vol. 2, 1830, written by C. S. Rafinesque." // Data on this kind of medical application for the orchid's crushed bulbs could have been part of VN's knowledge about orchids - and he was, most certainly, equally aware of the mythological Arethusa.  If these elements inspired him to compound Dr. Fitzbishop's explanation for Philip Rack's afflictions, he must have juggled with their almost hidden associations to dental problems and, therefore, they must carry no further connections to the plot. He'd be playing like Van, perhaps, who saw himself as "ein unverbesserlicher Witzbold." 

J.M: The links to the information I sent unfortunately didn't refer to the medicinal use of the "Arethusa" (significantly named: Dragon's Mouth, Swamp Pink, Bog Rose...). Here are two new ones:

http://www.orchidspecies.com/arethbulbosa.htm
Common Name Dragon's Mouth - Swamp Pink - Bog Rose. Found in North America south to South Carolina in wet acid meadows in mosses with subterranean corm-like bulbs carrying grass-like, linear-lanceolate leaves that blooms in the late spring and early summer on a terminal, thin, erect inflorescence carrying a solitary, fragrant flower and is a cold growing, water loving terrestrial./ In recent times this orchid has been used as a treatment for toothache.

http://www.botany.wisc.edu/orchids/Arethusa.html

According to Luer (1975), early americans used the corms of Arethusa as a remedy for toothaches.