Source : Webster's Revised Unabridged Dictionary (1913)

Daddy longlegs \Dad"dy long"legs`\
   1. (Zo["o]l.) An arachnidan of the genus {Phalangium}, and
      allied genera, having a small body and four pairs of long
      legs; -- called also {harvestman}, {carter}, and
      {grandfather longlegs}.

   2. (Zo["o]l.) A name applied to many species of dipterous
      insects of the genus {Tipula}, and allied genera, with
      slender bodies, and very long, slender legs; the crane
      fly; -- called also {father longlegs}.

Source : WordNet®

daddy longlegs
     n 1: long-legged slender flies that resemble large mosquitoes but
          do not bite [syn: {crane fly}]
     2: spider-like arachnid with a small rounded body and very long
        thin legs [syn: {harvestman}, {Phalangium opilio}]