The Collaborative International Dictionary of English v.0.48:

Aphis \A"phis\, n.; pl. Aphides. [NL.] (Zool.)
   A genus of insects belonging to the order Hemiptera and
   family Aphid[ae], including numerous species known as plant
   lice and green flies.
   [1913 Webster]

   Note: Besides the true males and females, there is a race of
         wingless asexual individuals which have the power of
         producing living young in rapid succession, and these
         in turn may produce others of the same kind for several
         generations, before sexual individuals appear. They
         suck the sap of plants by means of a tubular proboscis,
         and owing to the wonderful rapidity of their
         reproduction become very destructive to vegetation.
         Many of the Aphid[ae] excrete honeydew from two tubes
         near the end of the body.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Aphis
    n 1: type genus of the Aphididae: injurious to fruit trees and
         vegetables [syn: Aphis, genus Aphis]