The Collaborative International Dictionary of English v.0.48:

Hairstreak \Hair"streak`\ (-str[=e]k`), n.
   A butterfly of the genus Thecla; as, the green hairstreak
   (Thecla rubi).
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thecla \Thec"la\, n.
   Any one of many species of small delicately colored
   butterflies belonging to Thecla and allied genera; --
   called also hairstreak, and elfin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hairstreak
    n 1: small butterflies having striped markings under the wings
         [syn: hairstreak, hairstreak butterfly]