calamistrum

from The Collaborative International Dictionary of English v.0.48

Calamistrum \Cal`a*mis"trum\, n. [L., a curling iron.] (Zool.)
   A comblike structure on the metatarsus of the hind legs of
   certain spiders ({Ciniflonid[ae]}), used to curl certain
   fibers in the construction of their webs.
   [1913 Webster]