The Collaborative International Dictionary of English v.0.48:

Cucullus \Cu*cul"lus\, n.; pl. Cuculli. [L., a hood.]
   1. (Bot.) A hood-shaped organ, resembling a cowl or monk's
      hood, as certain concave and arched sepals or petals.
      [Webster 1913 Suppl.]

   2. (Zool.) A color marking or structure on the head somewhat
      resembling a hood.
      [Webster 1913 Suppl.]