The Collaborative International Dictionary of English v.0.48:

Culex \Cu"lex\, n. [L., a gnat.] (Zool.)
   A genus of mosquitoes of the family Culicidae, to which
   most of the North American species belong. Some members of
   this genus are exceedingly annoying, as Culex sollicitans,
   which breeds in enormous numbers in the salt marshes of the
   Atlantic coast, and Culex pipiens, the common house
   mosquito, breeding very widely in the fresh waters of North
   America. (For characters distinguishing these from the
   malaria mosquitoes, see Anopheles, above). The yellow-fever
   mosquito is now placed in another genus, Stegomyia.

   Note: This genus formerly (ca. 1900) included the gnat as
         well as the mosquito.
         [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

Culex
    n 1: type genus of the Culicidae: widespread genus of mosquitoes
         distinguished by holding the body parallel to the resting
         surface [syn: Culex, genus Culex]