The Collaborative International Dictionary of English v.0.48:

Dog day \Dog" day`\ or Dogday \Dog"day`\
   One of the dog days.
   [1913 Webster]

   Dogday cicada (Zool.), a large American cicada (Cicada
      pruinosa), which trills loudly in midsummer.
      [1913 Webster] Dog days

The Collaborative International Dictionary of English v.0.48:

Cicada \Ci*ca"da\ (s[i^]*k[=a]"d[.a]), n.; pl. E. Cicadas
   (-d[.a]z), L. Cicad[ae] (-d[=e]). [L.] (Zool.)
   Any species of the genus Cicada or of the family
   Cicadidae. They are large hemipterous insects, with nearly
   transparent wings. The male makes a shrill sound by peculiar
   organs in the under side of the abdomen, consisting of a pair
   of stretched membranes, acted upon by powerful muscles. A
   noted American species (Cicada septendecim) is called the
   seventeen year locust. Another common species is the
   dogday cicada.

   Syn: cicala. [1913 Webster]

WordNet (r) 3.0 (2006):

dog-day cicada
    n 1: its distinctive song is heard during July and August [syn:
         dog-day cicada, harvest fly]