"one great spider lived for centuries in the tower of the old Spanish church"
by cm
St Eustace interior
Creative Commons AttributionSt Eustace interior - Credit: Commodore Gandalf Cunningham

Van Helsing seems to be half-remembering a story from many decades before:

The sexton of the church of St Eustace, at Paris, amazed to find frequently a particular lamp extinct early, and yet the oil consumed only, sat up several nights to discover the cause. At length he detected that a spider of surprising size came down the cord to drink the oil. A still more extraordinary instance of the same kind occurred during the year 1751, in the Cathedral of Milan. A vast spider was observed there, which fed on the oil of the lamps... This spider, of four pounds weight, was sent to the Emperor of Austria, and placed in the Imperial Museum.

-- The Atheneum magazine, 1821

 

St Eustace
Creative Commons AttributionSt Eustace - Credit: spacejulien, Flickr